kernel/git - git - PowerEL Git System

Commit Graph

Author	SHA1	Message	Date
Taylor Blau	0f533c7284	pack-bitmap: read multi-pack bitmaps This prepares the code in pack-bitmap to interpret the new multi-pack bitmaps described in Documentation/technical/bitmap-format.txt, which mostly involves converting bit positions to accommodate looking them up in a MIDX. Note that there are currently no writers who write multi-pack bitmaps, and that this will be implemented in the subsequent commit. Note also that get_midx_checksum() and get_midx_filename() are made non-static so they can be called from pack-bitmap.c. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	3 years ago
Taylor Blau	1d7f7f242c	pack-bitmap-write.c: free existing bitmaps When writing a new bitmap, the bitmap writer code attempts to read the existing bitmap (if one is present). This is done in order to quickly permute the bits of any bitmaps for commits which appear in the existing bitmap, and were also selected for the new bitmap. But since this code was added in `341fa34887` (pack-bitmap-write: use existing bitmaps, 2020-12-08), the resources associated with opening an existing bitmap were never released. It's fine to ignore this, but it's bad hygiene. It will also cause a problem for the multi-pack-index builtin, which will be responsible not only for writing bitmaps, but also for expiring any old multi-pack bitmaps. If an existing bitmap was reused here, it will also be expired. That will cause a problem on platforms which require file resources to be closed before unlinking them, like Windows. Avoid this by ensuring we close reused bitmaps with free_bitmap_index() before removing them. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Taylor Blau	3ba3d0621b	pack-bitmap-write.c: gracefully fail to write non-closed bitmaps The set of objects covered by a bitmap must be closed under reachability, since it must be the case that there is a valid bit position assigned for every possible reachable object (otherwise the bitmaps would be incomplete). Pack bitmaps are never written from 'git repack' unless repacking all-into-one, and so we never write non-closed bitmaps (except in the case of partial clones where we aren't guaranteed to have all objects). But multi-pack bitmaps change this, since it isn't known whether the set of objects in the MIDX is closed under reachability until walking them. Plumb through a bit that is set when a reachable object isn't found. As soon as a reachable object isn't found in the set of objects to include in the bitmap, bitmap_writer_build() knows that the set is not closed, and so it now fails gracefully. A test is added in t0410 to trigger a bitmap write without full reachability closure by removing local copies of some reachable objects from a promisor remote. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	8380dcd700	oid_pos(): access table through const pointers When we are looking up an oid in an array, we obviously don't need to write to the array. Let's mark it as const in the function interfaces, as well as in the local variables we use to derference the void pointer (note a few cases use pointers-to-pointers, so we mark everything const). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	45ee13b942	hash_pos(): convert to oid_pos() All of our callers are actually looking up an object_id, not a bare hash. Likewise, the arrays they are looking in are actual arrays of object_id (not just raw bytes of hashes, as we might find in a pack .idx; those are handled by bsearch_hash()). Using an object_id gives us more type safety, and makes the callers slightly shorter. It also gets rid of the word "sha1" from several access functions, though we could obviously also rename those with s/sha1/hash/. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Martin Ågren	bc62692757	hash-lookup: rename from sha1-lookup Change all remnants of "sha1" in hash-lookup.c and .h and rename them to reflect that we're not just able to handle SHA-1 these days. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Martin Ågren	7a7d992d0d	sha1-lookup: rename `sha1_pos()` as `hash_pos()` Rename this function to reflect that we're not just able to handle SHA-1 these days. There are a few instances of "sha1" left in sha1-lookup.[ch] after this, but those will be addressed in the next commit. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	f077b0a986	pack-bitmap-write: better reuse bitmaps If the old bitmap file contains a bitmap for a given commit, then that commit does not need help from intermediate commits in its history to compute its final bitmap. Eject that commit from the walk and insert it into a separate list of reusable commits that are eventually stored in the list of commits for computing bitmaps. This helps the repeat bitmap computation task, even if the selected commits shift drastically. This helps when a previously-bitmapped commit exists in the first-parent history of a newly-selected commit. Since we stop the walk at these commits and we use a first-parent walk, it is harder to walk "around" these bitmapped commits. It's not impossible, but we can greatly reduce the computation time for many selected commits. \| runtime (sec) \| peak heap (GB) \| \| \| \| \| from \| with \| from \| with \| \| scratch \| existing \| scratch \| existing \| -----------+---------+----------+---------+----------- last patch \| 88.478 \| 53.218 \| 2.157 \| 2.224 \| this patch \| 86.681 \| 16.164 \| 2.157 \| 2.222 \| Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	45f4eeb291	pack-bitmap-write: relax unique revwalk condition The previous commits improved the bitmap computation process for very long, linear histories with many refs by removing quadratic growth in how many objects were walked. The strategy of computing "intermediate commits" using bitmasks for which refs can reach those commits partitioned the poset of reachable objects so each part could be walked exactly once. This was effective for linear histories. However, there was a (significant) drawback: wide histories with many refs had an explosion of memory costs to compute the commit bitmasks during the exploration that discovers these intermediate commits. Since these wide histories are unlikely to repeat walking objects, the benefit of walking objects multiple times was not expensive before. But now, the commit walk before computing bitmaps is incredibly expensive. In an effort to discover a happy medium, this change reduces the walk for intermediate commits to only the first-parent history. This focuses the walk on how the histories converge, which still has significant reduction in repeat object walks. It is still possible to create quadratic behavior in this version, but it is probably less likely in realistic data shapes. Here is some data taken on a fresh clone of the kernel: \| runtime (sec) \| peak heap (GB) \| \| \| \| \| from \| with \| from \| with \| \| scratch \| existing \| scratch \| existing \| -----------+---------+----------+---------+----------- original \| 64.044 \| 83.241 \| 2.088 \| 2.194 \| last patch \| 45.049 \| 37.624 \| 2.267 \| 2.334 \| this patch \| 88.478 \| 53.218 \| 2.157 \| 2.224 \| Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	341fa34887	pack-bitmap-write: use existing bitmaps When constructing new bitmaps, we perform a commit and tree walk in fill_bitmap_commit() and fill_bitmap_tree(). This walk would benefit from using existing bitmaps when available. We must track the existing bitmaps and translate them into the new object order, but this is generally faster than parsing trees. In fill_bitmap_commit(), we must reorder thing somewhat. The priority queue walks commits from newest-to-oldest, which means we correctly stop walking when reaching a commit with a bitmap. However, if we walk trees interleaved with the commits, then we might be parsing trees that are actually part of a re-used bitmap. To avoid over-walking trees, add them to a LIFO queue and walk them after exploring commits completely. On git.git, this reduces a second immediate bitmap computation from 2.0s to 1.0s. On linux.git, we go from 32s to 22s. On chromium's fork network, we go from 227s to 198s. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	449fa5ee06	pack-bitmap-write: ignore BITMAP_FLAG_REUSE The on-disk bitmap format has a flag to mark a bitmap to be "reused". This is a rather curious feature, and works like this: - a run of pack-objects would decide to mark the last 80% of the bitmaps it generates with the reuse flag - the next time we generate bitmaps, we'd see those reuse flags from the last run, and mark those commits as special: - we'd be more likely to select those commits to get bitmaps in the new output - when generating the bitmap for a selected commit, we'd reuse the old bitmap as-is (rearranging the bits to match the new pack, of course) However, neither of these behaviors particularly makes sense. Just because a commit happened to be bitmapped last time does not make it a good candidate for having a bitmap this time. In particular, we may choose bitmaps based on how recent they are in history, or whether a ref tip points to them, and those things will change. We're better off re-considering fresh which commits are good candidates. Reusing the existing bitmap _is_ a reasonable thing to do to save computation. But only reusing exact bitmaps is a weak form of this. If we have an old bitmap for A and now want a new bitmap for its child, we should be able to compute that only by looking at trees and that are new to the child. But this code would consider only exact reuse (which is perhaps why it was eager to select those commits in the first place). Furthermore, the recent switch to the reverse-edge algorithm for generating bitmaps dropped this optimization entirely (and yet still performs better). So let's do a few cleanups: - drop the whole "reusing bitmaps" phase of generating bitmaps. It's not helping anything, and is mostly unused code (or worse, code that is using CPU but not doing anything useful) - drop the use of the on-disk reuse flag to select commits to bitmap - stop setting the on-disk reuse flag in bitmaps we generate (since nothing respects it anymore) We will keep a few innards of the reuse code, which will help us implement a more capable version of the "reuse" optimization: - simplify rebuild_existing_bitmaps() into a function that only builds the mapping of bits between the old and new orders, but doesn't actually convert any bitmaps - make rebuild_bitmap() public; we'll call it lazily to convert bitmaps as we traverse (using the mapping created above) Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	089f751360	pack-bitmap-write: build fewer intermediate bitmaps The bitmap_writer_build() method calls bitmap_builder_init() to construct a list of commits reachable from the selected commits along with a "reverse graph". This reverse graph has edges pointing from a commit to other commits that can reach that commit. After computing a reachability bitmap for a commit, the values in that bitmap are then copied to the reachability bitmaps across the edges in the reverse graph. We can now relax the role of the reverse graph to greatly reduce the number of intermediate reachability bitmaps we compute during this reverse walk. The end result is that we walk objects the same number of times as before when constructing the reachability bitmaps, but we also spend much less time copying bits between bitmaps and have much lower memory pressure in the process. The core idea is to select a set of "important" commits based on interactions among the sets of commits reachable from each selected commit. The first technical concept is to create a new 'commit_mask' member in the bb_commit struct. Note that the selected commits are provided in an ordered array. The first thing to do is to mark the ith bit in the commit_mask for the ith selected commit. As we walk the commit-graph, we copy the bits in a commit's commit_mask to its parents. At the end of the walk, the ith bit in the commit_mask for a commit C stores a boolean representing "The ith selected commit can reach C." As we walk, we will discover non-selected commits that are important. We will get into this later, but those important commits must also receive bit positions, growing the width of the bitmasks as we walk. At the true end of the walk, the ith bit means "the ith _important_ commit can reach C." MAXIMAL COMMITS --------------- We use a new 'maximal' bit in the bb_commit struct to represent whether a commit is important or not. The term "maximal" comes from the partially-ordered set of commits in the commit-graph where C >= P if P is a parent of C, and then extending the relationship transitively. Instead of taking the maximal commits across the entire commit-graph, we instead focus on selecting each commit that is maximal among commits with the same bits on in their commit_mask. This definition is important, so let's consider an example. Suppose we have three selected commits A, B, and C. These are assigned bitmasks 100, 010, and 001 to start. Each of these can be marked as maximal immediately because they each will be the uniquely maximal commit that contains their own bit. Keep in mind that that these commits may have different bitmasks after the walk; for example, if B can reach C but A cannot, then the final bitmask for C is 011. Even in these cases, C would still be a maximal commit among all commits with the third bit on in their masks. Now define sets X, Y, and Z to be the sets of commits reachable from A, B, and C, respectively. The intersections of these sets correspond to different bitmasks: * 100: X - (Y union Z) * 010: Y - (X union Z) * 001: Z - (X union Y) * 110: (X intersect Y) - Z * 101: (X intersect Z) - Y * 011: (Y intersect Z) - X * 111: X intersect Y intersect Z This can be visualized with the following Hasse diagram: 100 010 001 \| \ / \ / \| \| \/ \/ \| \| /\ /\ \| \| / \ / \ \| 110 101 011 \___ \| ___/ \ \| / 111 Some of these bitmasks may not be represented, depending on the topology of the commit-graph. In fact, we are counting on it, since the number of possible bitmasks is exponential in the number of selected commits, but is also limited by the total number of commits. In practice, very few bitmasks are possible because most commits converge on a common "trunk" in the commit history. With this three-bit example, we wish to find commits that are maximal for each bitmask. How can we identify this as we are walking? As we walk, we visit a commit C. Since we are walking the commits in topo-order, we know that C is visited after all of its children are visited. Thus, when we get C from the revision walk we inspect the 'maximal' property of its bb_data and use that to determine if C is truly important. Its commit_mask is also nearly final. If C is not one of the originally-selected commits, then assign a bit position to C (by incrementing num_maximal) and set that bit on in commit_mask. See "MULTIPLE MAXIMAL COMMITS" below for more detail on this. Now that the commit C is known to be maximal or not, consider each parent P of C. Compute two new values: * c_not_p : true if and only if the commit_mask for C contains a bit that is not contained in the commit_mask for P. * p_not_c : true if and only if the commit_mask for P contains a bit that is not contained in the commit_mask for P. If c_not_p is false, then P already has all of the bits that C would provide to its commit_mask. In this case, move on to other parents as C has nothing to contribute to P's state that was not already provided by other children of P. We continue with the case that c_not_p is true. This means there are bits in C's commit_mask to copy to P's commit_mask, so use bitmap_or() to add those bits. If p_not_c is also true, then set the maximal bit for P to one. This means that if no other commit has P as a parent, then P is definitely maximal. This is because no child had the same bitmask. It is important to think about the maximal bit for P at this point as a temporary state: "P is maximal based on current information." In contrast, if p_not_c is false, then set the maximal bit for P to zero. Further, clear all reverse_edges for P since any edges that were previously assigned to P are no longer important. P will gain all reverse edges based on C. The final thing we need to do is to update the reverse edges for P. These reverse edges respresent "which closest maximal commits contributed bits to my commit_mask?" Since C contributed bits to P's commit_mask in this case, C must add to the reverse edges of P. If C is maximal, then C is a 'closest' maximal commit that contributed bits to P. Add C to P's reverse_edges list. Otherwise, C has a list of maximal commits that contributed bits to its bitmask (and this list is exactly one element). Add all of these items to P's reverse_edges list. Be careful to ignore duplicates here. After inspecting all parents P for a commit C, we can clear the commit_mask for C. This reduces the memory load to be limited to the "width" of the commit graph. Consider our ABC/XYZ example from earlier and let's inspect the state of the commits for an interesting bitmask, say 011. Suppose that D is the only maximal commit with this bitmask (in the first three bits). All other commits with bitmask 011 have D as the only entry in their reverse_edges list. D's reverse_edges list contains B and C. COMPUTING REACHABILITY BITMAPS ------------------------------ Now that we have our definition, let's zoom out and consider what happens with our new reverse graph when computing reachability bitmaps. We walk the reverse graph in reverse-topo-order, so we visit commits with largest commit_masks first. After we compute the reachability bitmap for a commit C, we push the bits in that bitmap to each commit D in the reverse edge list for C. Then, when we finally visit D we already have the bits for everything reachable from maximal commits that D can reach and we only need to walk the objects in the set-difference. In our ABC/XYZ example, when we finally walk for the commit A we only need to walk commits with bitmask equal to A's bitmask. If that bitmask is 100, then we are only walking commits in X - (Y union Z) because the bitmap already contains the bits for objects reachable from (X intersect Y) union (X intersect Z) (i.e. the bits from the reachability bitmaps for the maximal commits with bitmasks 110 and 101). The behavior is intended to walk each commit (and the trees that commit introduces) at most once while allocating and copying fewer reachability bitmaps. There is one caveat: what happens when there are multiple maximal commits with the same bitmask, with respect to the initial set of selected commits? MULTIPLE MAXIMAL COMMITS ------------------------ Earlier, we mentioned that when we discover a new maximal commit, we assign a new bit position to that commit and set that bit position to one for that commit. This is absolutely important for interesting commit-graphs such as git/git and torvalds/linux. The reason is due to the existence of "butterflies" in the commit-graph partial order. Here is an example of four commits forming a butterfly: I J \|\ /\| \| \/ \| \| /\ \| \|/ \\| M N \ / \|/ Q Here, I and J both have parents M and N. In general, these do not need to be exact parent relationships, but reachability relationships. The most important part is that M and N cannot reach each other, so they are independent in the partial order. If I had commit_mask 10 and J had commit_mask 01, then M and N would both be assigned commit_mask 11 and be maximal commits with the bitmask 11. Then, what happens when M and N can both reach a commit Q? If Q is also assigned the bitmask 11, then it is not maximal but is reachable from both M and N. While this is not necessarily a deal-breaker for our abstract definition of finding maximal commits according to a given bitmask, we have a few issues that can come up in our larger picture of constructing reachability bitmaps. In particular, if we do not also consider Q to be a "maximal" commit, then we will walk commits reachable from Q twice: once when computing the reachability bitmap for M and another time when computing the reachability bitmap for N. This becomes much worse if the topology continues this pattern with multiple butterflies. The solution has already been mentioned: each of M and N are assigned their own bits to the bitmask and hence they become uniquely maximal for their bitmasks. Finally, Q also becomes maximal and thus we do not need to walk its commits multiple times. The final bitmasks for these commits are as follows: I:10 J:01 \|\ /\| \| \ _____/ \| \| /\____ \| \|/ \ \| M:111 N:1101 \ / Q:1111 Further, Q's reverse edge list is { M, N }, while M and N both have reverse edge list { I, J }. PERFORMANCE MEASUREMENTS ------------------------ Now that we've spent a LOT of time on the theory of this algorithm, let's show that this is actually worth all that effort. To test the performance, use GIT_TRACE2_PERF=1 when running 'git repack -abd' in a repository with no existing reachability bitmaps. This avoids any issues with keeping existing bitmaps to skew the numbers. Inspect the "building_bitmaps_total" region in the trace2 output to focus on the portion of work that is affected by this change. Here are the performance comparisons for a few repositories. The timings are for the following versions of Git: "multi" is the timing from before any reverse graph is constructed, where we might perform multiple traversals. "reverse" is for the previous change where the reverse graph has every reachable commit. Finally "maximal" is the version introduced here where the reverse graph only contains the maximal commits. Repository: git/git multi: 2.628 sec reverse: 2.344 sec maximal: 2.047 sec Repository: torvalds/linux multi: 64.7 sec reverse: 205.3 sec maximal: 44.7 sec So in all cases we've not only recovered any time lost to switching to the reverse-edge algorithm, but we come out ahead of "multi" in all cases. Likewise, peak heap has gone back to something reasonable: Repository: torvalds/linux multi: 2.087 GB reverse: 3.141 GB maximal: 2.288 GB While I do not have access to full fork networks on GitHub, Peff has run this algorithm on the chromium/chromium fork network and reported a change from 3 hours to ~233 seconds. That network is particularly beneficial for this approach because it has a long, linear history along with many tags. The "multi" approach was obviously quadratic and the new approach is linear. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	928e3f42ad	pack-bitmap-write: rename children to reverse_edges The bitmap_builder_init() method walks the reachable commits in topological order and constructs a "reverse graph" along the way. At the moment, this reverse graph contains an edge from commit A to commit B if and only if A is a parent of B. Thus, the name "children" is appropriate for for this reverse graph. In the next change, we will repurpose the reverse graph to not be directly-adjacent commits in the commit-graph, but instead a more abstract relationship. The previous changes have already incorporated the necessary updates to fill_bitmap_commit() that allow these edges to not be immediate children. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Derrick Stolee	6dc5ef759f	pack-bitmap-write: fill bitmap with commit history The current implementation of bitmap_writer_build() creates a reachability bitmap for every walked commit. After computing a bitmap for a commit, those bits are pushed to an in-progress bitmap for its children. fill_bitmap_commit() assumes the bits corresponding to objects reachable from the parents of a commit are already set. This means that when visiting a new commit, we only have to walk the objects reachable between it and any of its parents. A future change to bitmap_writer_build() will relax this condition so not all parents have their bits set. Prepare for that by having 'fill_bitmap_commit()' walk parents until reaching commits whose bits are already set. Then, walk the trees for these commits as well. This has no functional change with the current implementation of bitmap_writer_build(). Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	010e5eacfb	pack-bitmap-write: pass ownership of intermediate bitmaps Our algorithm to generate reachability bitmaps walks through the commit graph from the bottom up, passing bitmap data from each commit to its descendants. For a linear stretch of history like: A -- B -- C our sequence of steps is: - compute the bitmap for A by walking its trees, etc - duplicate A's bitmap as a starting point for B; we can now free A's bitmap, since we only needed it as an intermediate result - OR in any extra objects that B can reach into its bitmap - duplicate B's bitmap as a starting point for C; likewise, free B's bitmap - OR in objects for C, and so on... Rather than duplicating bitmaps and immediately freeing the original, we can just pass ownership from commit to commit. Note that this doesn't always work: - the recipient may be a merge which already has an intermediate bitmap from its other ancestor. In that case we have to OR our result into it. Note that the first ancestor to reach the merge does get to pass ownership, though. - we may have multiple children; we can only pass ownership to one of them However, it happens often enough and copying bitmaps is expensive enough that this provides a noticeable speedup. On a clone of linux.git, this reduces the time to generate bitmaps from 205s to 70s. This is about the same amount of time it took to generate bitmaps using our old "many traversals" algorithm (the previous commit measures the identical scenario as taking 63s). It unfortunately provides only a very modest reduction in the peak memory usage, though. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	4a9c581729	pack-bitmap-write: reimplement bitmap writing The bitmap generation code works by iterating over the set of commits for which we plan to write bitmaps, and then for each one performing a traditional traversal over the reachable commits and trees, filling in the bitmap. Between two traversals, we can often reuse the previous bitmap result as long as the first commit is an ancestor of the second. However, our worst case is that we may end up doing "n" complete complete traversals to the root in order to create "n" bitmaps. In a real-world case (the shared-storage repo consisting of all GitHub forks of chromium/chromium), we perform very poorly: generating bitmaps takes ~3 hours, whereas we can walk the whole object graph in ~3 minutes. This commit completely rewrites the algorithm, with the goal of accessing each object only once. It works roughly like this: - generate a list of commits in topo-order using a single traversal - invert the edges of the graph (so have parents point at their children) - make one pass in reverse topo-order, generating a bitmap for each commit and passing the result along to child nodes We generate correct results because each node we visit has already had all of its ancestors added to the bitmap. And we make only two linear passes over the commits. We also visit each tree usually only once. When filling in a bitmap, we don't bother to recurse into trees whose bit is already set in the bitmap (since we know we've already done so when setting their bit). That means that if commit A references tree T, none of its descendants will need to open T again. I say "usually", though, because it is possible for a given tree to be mentioned in unrelated parts of history (e.g., cherry-picking to a parallel branch). So we've accomplished our goal, and the resulting algorithm is pretty simple to understand. But there are some downsides, at least with this initial implementation: - we no longer reuse the results of any on-disk bitmaps when generating. So we'd expect to sometimes be slower than the original when bitmaps already exist. However, this is something we'll be able to add back in later. - we use much more memory. Instead of keeping one bitmap in memory at a time, we're passing them up through the graph. So our memory use should scale with the graph width (times the size of a bitmap). So how does it perform? For a clone of linux.git, generating bitmaps from scratch with the old algorithm took 63s. Using this algorithm it takes 205s. Which is much worse, but _might_ be acceptable if it behaved linearly as the size grew. It also increases peak heap usage by ~1G. That's not impossibly large, but not encouraging. On the complete fork-network of torvalds/linux, it increases the peak RAM usage by 40GB. Yikes. (I forgot to record the time it took, but the memory usage was too much to consider this reasonable anyway). On the complete fork-network of chromium/chromium, I ran out of memory before succeeding. Some back-of-the-envelope calculations indicate it would need 80+GB to complete. So at this stage, we've managed to make things much worse. But because of the way this new algorithm is structured, there are a lot of opportunities for optimization on top. We'll start implementing those in the follow-on patches. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
René Scharfe	7744a5d692	pack-bitmap-write: use hashwrite_be32() in write_hash_cache() Call hashwrite_be32() instead of open-coding it. This is shorter and easier to read. Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>	4 years ago
Jeff King	3a37876b5d	pack-objects: drop packlist index_pos optimization Once upon a time, the code to add an object to our packing list in pack-objects all lived in a single function. It computed the position within the hash table once, then used it to check if the object was already present, and if not, to add it. Later, in `2834bc27c1` (pack-objects: refactor the packing list, 2013-10-24), this was split into two functions: packlist_find() and packlist_alloc(). We ended up with an "index_pos" variable that gets passed through several functions to make it from one to the other. The resulting code is rather confusing to follow. The "index_pos" variable is sometimes undefined, if we don't yet have a hash table. This works out in practice because in that case packlist_alloc() won't use it at all, since it will have to create/grow the hash table. But it's hard to verify that, and it does cause gcc 9.2.1's -Wmaybe-uninitialized to complain when compiled with "-flto -O3" (rightfully, since we do pass the uninitialized value as a function parameter, even if nobody ends up using it). All of this is to save computing the hash index again when we're inserting into the hash table, which I found doesn't make a measurable difference in the program runtime (which is not surprising, since we're doing all kinds of other heavyweight things for each object). Let's just drop this index_pos variable entirely, simplifying the code (and pleasing the compiler). We might be better still refactoring this custom hash table to use one of our existing implementations (an oidmap, or a kh_oid_map). I stopped short of that here, but this would be the likely first step towards that anyway. Reported-by: Stephan Beyer <s-beyer@gmx.net> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Jeff King	d2bc62b1fa	pack-bitmap: convert khash_sha1 maps into kh_oid_map All of the users of our khash_sha1 maps actually have a "struct object_id". Let's use the more descriptive type. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Jeff King	3df28caefb	pack-objects: convert packlist_find() to use object_id We take a raw hash pointer, but most of our callers have a "struct object_id" already. Let's switch to taking the full struct, which will let us continue removing uses of raw sha1 buffers. There are two callers that do need special attention: - in rebuild_existing_bitmaps(), we need to switch to nth_packed_object_oid(). This incurs an extra hash copy over pointing straight to the mmap'd sha1, but it shouldn't be measurable compared to the rest of the operation. - in can_reuse_delta() we already spent the effort to copy the sha1 into a "struct object_id", but now we just have to do so a little earlier in the function (we can't easily convert that function's callers because they may be pointing at mmap'd REF_DELTA blocks). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Jeff King	05805d7411	pack-bitmap-write: convert some helpers to use object_id A few functions take raw hash pointers, but all of their callers actually have a "struct object_id". Let's retain that extra type as long as possible (which will let future patches extend that further, and so on). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
brian m. carlson	0dd4924891	pack-bitmap: replace sha1_to_hex Replace the uses of sha1_to_hex in the pack bitmap code with hash_to_hex to allow the use of SHA-256 as well. Rename a few variables since they are no longer limited to SHA-1. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
brian m. carlson	0f4d6cada8	pack-bitmap: make bitmap header handling hash agnostic Increase the checksum field in struct bitmap_disk_header to be GIT_MAX_RAWSZ bytes in length and ensure that we hash the proper number of bytes out when computing the bitmap checksum. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Nguyễn Thái Ngọc Duy	7c14112741	pack-*.c: remove the_repository references Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
brian m. carlson	825544a351	pack-bitmap-write: use GIT_MAX_RAWSZ for allocation Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Nguyễn Thái Ngọc Duy	2abf350385	revision.c: remove implicit dependency on the_index Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	6 years ago
Derrick Stolee	6404355657	commit.h: remove method declarations These methods are now declared in commit-reach.h. Remove them from commit.h and add new include statements in all files that require these declarations. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Jonathan Tan	f3c23db2d7	pack-bitmap: add free function Add a function to free struct bitmap_index instances, and use it where needed (except when rebuild_existing_bitmaps() is used, since it creates references to the bitmaps within the struct bitmap_index passed to it). Note that the hashes field in struct bitmap_index is not freed because it points to another field within the same struct. The documentation for that field has been updated to clarify that. Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Jonathan Tan	3ae5fa0768	pack-bitmap: remove bitmap_git global variable Remove the bitmap_git global variable. Instead, generate on demand an instance of struct bitmap_index for code that needs to access it. This allows us significant control over the lifetime of instances of struct bitmap_index. In particular, packs can now be closed without worrying if an unnecessarily long-lived "pack" field in struct bitmap_index still points to it. The bitmap API is also clearer in that we need to first obtain a struct bitmap_index, then we use it. This patch raises two potential issues: (1) memory for the struct bitmap_index is allocated without being freed, and (2) prepare_bitmap_git() and prepare_bitmap_walk() can reuse a previously loaded bitmap. For (1), this will be dealt with in a subsequent patch in this patch set that also deals with freeing the contents of the struct bitmap_index (which were not freed previously, because they have global scope). For (2), current bitmap users only load the bitmap once at most (note that pack-objects can use bitmaps or write bitmaps, but not both at the same time), so support for reuse has no effect - and future users can pass around the struct bitmap_index * obtained if they need to do 2 or more things with the same bitmap. Helped-by: Stefan Beller <sbeller@google.com> Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Stefan Beller	cbd53a2193	object-store: move object access functions to object-store.h This should make these functions easier to find and cache.h less overwhelming to read. In particular, this moves: - read_object_file - oid_object_info - write_object_file As a result, most of the codebase needs to #include object-store.h. In this patch the #include is only added to files that would fail to compile otherwise. It would be better to #include wherever identifiers from the header are used. That can happen later when we have better tooling for it. Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Johannes Schindelin	033abf97fc	Replace all die("BUG: ...") calls by BUG() ones In `d8193743e0` (usage.c: add BUG() function, 2017-05-12), a new macro was introduced to use for reporting bugs instead of die(). It was then subsequently used to convert one single caller in `588a538ae5` (setup_git_env: convert die("BUG") to BUG(), 2017-05-12). The cover letter of the patch series containing this patch (cf 20170513032414.mfrwabt4hovujde2@sigill.intra.peff.net) is not terribly clear why only one call site was converted, or what the plan is for other, similar calls to die() to report bugs. Let's just convert all remaining ones in one fell swoop. This trick was performed by this invocation: sed -i 's/die("BUG: /BUG("/g' $(git grep -l 'die("BUG' \*.c) Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Stefan Beller	0df8e96566	cache.h: add repository argument to oid_object_info Add a repository argument to allow the callers of oid_object_info to be more specific about which repository to handle. This is a small mechanical change; it doesn't change the implementation to handle repositories other than the_repository yet. As with the previous commits, use a macro to catch callers passing a repository other than the_repository at compile time. Signed-off-by: Stefan Beller <sbeller@google.com> Reviewed-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Nguyễn Thái Ngọc Duy	06af3bba41	pack-objects: move in_pack_pos out of struct object_entry This field is only need for pack-bitmap, which is an optional feature. Move it to a separate array that is only allocated when pack-bitmap is used (like objects[], it is not freed, since we need it until the end of the process) Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Nguyễn Thái Ngọc Duy	fd9b1baef8	pack-objects: turn type and in_pack_type to bitfields An extra field type_valid is added to carry the equivalent of OBJ_BAD in the original "type" field. in_pack_type always contains a valid type so we only need 3 bits for it. A note about accepting OBJ_NONE as "valid" type. The function read_object_list_from_stdin() can pass this value [1] and it eventually calls create_object_entry() where current code skip setting "type" field if the incoming type is zero. This does not have any bad side effects because "type" field should be memset()'d anyway. But since we also need to set type_valid now, skipping oe_set_type() leaves type_valid zero/false, which will make oe_type() return OBJ_BAD, not OBJ_NONE anymore. Apparently we do care about OBJ_NONE in prepare_pack(). This switch from OBJ_NONE to OBJ_BAD may trigger fatal: unable to get type of object ... Accepting OBJ_NONE [2] does sound wrong, but this is how it is has been for a very long time and I haven't time to dig in further. [1] See `5c49c11686` (pack-objects: better check_object() performances - 2007-04-16) [2] `21666f1aae` (convert object type handling from a string to a number - 2007-02-26) Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Derrick Stolee	cfe83216e4	csum-file: refactor finalize_hashfile() method If we want to use a hashfile on the temporary file for a lockfile, then we need finalize_hashfile() to fully write the trailing hash but also keep the file descriptor open. Do this by adding a new CSUM_HASH_IN_STREAM flag along with a functional change that checks this flag before writing the checksum to the stream. This differs from previous behavior since it would be written if either CSUM_CLOSE or CSUM_FSYNC is provided. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Derrick Stolee	f2af9f5e02	csum-file: rename hashclose() to finalize_hashfile() The hashclose() method behaves very differently depending on the flags parameter. In particular, the file descriptor is not always closed. Perform a simple rename of "hashclose()" to "finalize_hashfile()" in preparation for functional changes. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
brian m. carlson	abef9020e3	sha1_file: convert sha1_object_info* to object_id Convert sha1_object_info and sha1_object_info_extended to take pointers to struct object_id and rename them to use "oid" instead of "sha1" in their names. Update the declaration and definition and apply the following semantic patch, plus the standard object_id transforms: @@ expression E1, E2; @@ - sha1_object_info(E1.hash, E2) + oid_object_info(&E1, E2) @@ expression E1, E2; @@ - sha1_object_info(E1->hash, E2) + oid_object_info(E1, E2) @@ expression E1, E2, E3; @@ - sha1_object_info_extended(E1.hash, E2, E3) + oid_object_info_extended(&E1, E2, E3) @@ expression E1, E2, E3; @@ - sha1_object_info_extended(E1->hash, E2, E3) + oid_object_info_extended(E1, E2, E3) Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
brian m. carlson	98a3beab6a	csum-file: rename sha1file to hashfile Rename struct sha1file to struct hashfile, along with all of its related functions. The transformation in this commit was made by global search-and-replace. Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
Martin Ågren	4d01a7fa65	pack-bitmap[-write]: use `object_array_clear()`, don't leak Instead of setting the fields of rev->pending to 0/NULL, thereby leaking memory, call `object_array_clear(&rev->pending)`. In pack-bitmap.c, we make copies of those fields as `pending_nr` and `pending_e`. We never update the aliases and the original fields never change, so the aliases are not really needed and just make it harder than necessary to understand the code. While we're here, remove the aliases to make the code easier to follow. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	7 years ago
brian m. carlson	e6a492b7be	pack: convert struct pack_idx_entry to struct object_id Convert struct pack_idx_entry to use struct object_id by changing the definition and applying the following semantic patch, plus the standard object_id transforms: @@ struct pack_idx_entry E1; @@ - E1.sha1 + E1.oid.hash @@ struct pack_idx_entry *E1; @@ - E1->sha1 + E1->oid.hash Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	8 years ago
Jeff King	594fa9998c	odb_mkstemp: write filename into strbuf The odb_mkstemp() function expects the caller to provide a fixed buffer to write the resulting tempfile name into. But it creates the template using snprintf without checking the return value. This means we could silently truncate the filename. In practice, it's unlikely that the truncation would end in the template-pattern that mkstemp needs to open the file. So we'd probably end up failing either way, unless the path was specially crafted. The simplest fix would be to notice the truncation and die. However, we can observe that most callers immediately xstrdup() the result anyway. So instead, let's switch to using a strbuf, which is easier for them (and isn't a big deal for the other 2 callers, who can just strbuf_release when they're done with it). Note that many of the callers used static buffers, but this was purely to avoid putting a large buffer on the stack. We never passed the static buffers out of the function, so there's no complicated memory handling we need to change. Signed-off-by: Jeff King <peff@peff.net>	8 years ago
Jeff King	892e723afd	do not check odb_mkstemp return value for errors The odb_mkstemp function does not return an error; it dies on failure instead. But many of its callers compare the resulting descriptor against -1 and die themselves. Mostly this is just pointless, but it does raise a question when looking at the callers: if they show the results of the "template" buffer after a failure, what's in it? The answer is: it doesn't matter, because it cannot happen. So let's make that clear by removing the bogus error checks. In bitmap_writer_finish(), we can drop the error-handling code entirely. In the other two cases, it's shared with the open() in another code path; we can just move the error-check next to that open() call. And while we're at it, let's flesh out the function's docstring a bit to make the error behavior clear. Signed-off-by: Jeff King <peff@peff.net>	8 years ago
René Scharfe	9ed0d8d6e6	use QSORT Apply the semantic patch contrib/coccinelle/qsort.cocci to the code base, replacing calls of qsort(3) with QSORT. The resulting code is shorter and supports empty arrays with NULL pointers. Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>	8 years ago
Jeff King	2824e1841b	list-objects: pass full pathname to callbacks When we find a blob at "a/b/c", we currently pass this to our show_object_fn callbacks as two components: "a/b/" and "c". Callbacks which want the full value then call path_name(), which concatenates the two. But this is an inefficient interface; the path is a strbuf, and we could simply append "c" to it temporarily, then roll back the length, without creating a new copy. So we could improve this by teaching the callsites of path_name() this trick (and there are only 3). But we can also notice that no callback actually cares about the broken-down representation, and simply pass each callback the full path "a/b/c" as a string. The callback code becomes even simpler, then, as we do not have to worry about freeing an allocated buffer, nor rolling back our modification to the strbuf. This is theoretically less efficient, as some callbacks would not bother to format the final path component. But in practice this is not measurable. Since we use the same strbuf over and over, our work to grow it is amortized, and we really only pay to memcpy a few bytes. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago
Jeff King	dc06dc8800	list-objects: drop name_path entirely In the previous commit, we left name_path as a thin wrapper around a strbuf. This patch drops it entirely. As a result, every show_object_fn callback needs to be adjusted. However, none of their code needs to be changed at all, because the only use was to pass it to path_name(), which now handles the bare strbuf. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago
Jeff King	de1e67d070	list-objects: pass full pathname to callbacks When we find a blob at "a/b/c", we currently pass this to our show_object_fn callbacks as two components: "a/b/" and "c". Callbacks which want the full value then call path_name(), which concatenates the two. But this is an inefficient interface; the path is a strbuf, and we could simply append "c" to it temporarily, then roll back the length, without creating a new copy. So we could improve this by teaching the callsites of path_name() this trick (and there are only 3). But we can also notice that no callback actually cares about the broken-down representation, and simply pass each callback the full path "a/b/c" as a string. The callback code becomes even simpler, then, as we do not have to worry about freeing an allocated buffer, nor rolling back our modification to the strbuf. This is theoretically less efficient, as some callbacks would not bother to format the final path component. But in practice this is not measurable. Since we use the same strbuf over and over, our work to grow it is amortized, and we really only pay to memcpy a few bytes. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago
Jeff King	bd64516aca	list-objects: drop name_path entirely In the previous commit, we left name_path as a thin wrapper around a strbuf. This patch drops it entirely. As a result, every show_object_fn callback needs to be adjusted. However, none of their code needs to be changed at all, because the only use was to pass it to path_name(), which now handles the bare strbuf. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago
brian m. carlson	ed1c9977cb	Remove get_object_hash. Convert all instances of get_object_hash to use an appropriate reference to the hash member of the oid member of struct object. This provides no functional change, as it is essentially a macro substitution. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Jeff King <peff@peff.net>	9 years ago
brian m. carlson	f2fd0760f6	Convert struct object to object_id struct object is one of the major data structures dealing with object IDs. Convert it to use struct object_id instead of an unsigned char array. Convert get_object_hash to refer to the new member as well. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Jeff King <peff@peff.net>	9 years ago
brian m. carlson	7999b2cf77	Add several uses of get_object_hash. Convert most instances where the sha1 member of struct object is dereferenced to use get_object_hash. Most instances that are passed to functions that have versions taking struct object_id, such as get_sha1_hex/get_oid_hex, or instances that can be trivially converted to use struct object_id instead, are not converted. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Jeff King <peff@peff.net>	9 years ago

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65 Commits (726a228dfb19fae8befa0f209436f8fae7919a58)