kernel/git - git - PowerEL Git System

Author	SHA1	Message	Date
Jeff King	171bdaca69	sha1-lookup: handle duplicate keys with GIT_USE_LOOKUP The sha1_entry_pos function tries to be smart about selecting the middle of a range for its binary search by looking at the value differences between the "lo" and "hi" constraints. However, it is unable to cope with entries with duplicate keys in the sorted list. We may hit a point in the search where both our "lo" and "hi" point to the same key. In this case, the range of values between our endpoints is 0, and trying to scale the difference between our key and the endpoints over that range is undefined (i.e., divide by zero). The current code catches this with an "assert(lov < hiv)". Moreover, after seeing that the first 20 byte of the key are the same, we will try to establish a value from the 21st byte. Which is nonsensical. Instead, we can detect the case that we are in a run of duplicates, and simply do a final comparison against any one of them (since they are all the same, it does not matter which). If the keys match, we have found our entry (or one of them, anyway). If not, then we know that we do not need to look further, as we must be in a run of the duplicate key. Signed-off-by: Jeff King <peff@peff.net> Acked-by: Nicolas Pitre <nico@fluxnic.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	12 years ago
Junio C Hamano	1a7b1f6b9c	sha1-lookup: fix up the assertion message Signed-off-by: Junio C Hamano <gitster@pobox.com>	16 years ago
Christian Couder	96beef8c2e	sha1-lookup: add new "sha1_pos" function to efficiently lookup sha1 This function has been copied from the "patch_pos" function in "patch-ids.c" but an additional parameter has been added. The new parameter is a function pointer, that is used to access the sha1 of an element in the table. Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>	16 years ago
Junio C Hamano	12ecb01107	sha1-lookup: make selection of 'middle' less aggressive If we pick 'mi' between 'lo' and 'hi' at 50%, which was what the simple binary search did, we are halving the search space whether the entry at 'mi' is lower or higher than the target. The previous patch was about picking not the middle but closer to 'hi', when we know the target is a lot closer to 'hi' than it is to 'lo'. However, if it turns out that the entry at 'mi' is higher than the target, we would end up reducing the search space only by the difference between 'mi' and 'hi' (which by definition is less than 50% --- that was the whole point of not using the simple binary search), which made the search less efficient. And the risk of overshooting becomes very high, if we try to be too precise. This tweaks the selection of 'mi' to be a bit closer to the middle than we would otherwise pick to avoid the problem. Signed-off-by: Junio C Hamano <gitster@pobox.com>	17 years ago
Junio C Hamano	628522ec14	sha1-lookup: more memory efficient search in sorted list of SHA-1 Currently, when looking for a packed object from the pack idx, a simple binary search is used. A conventional binary search loop looks like this: unsigned lo, hi; do { unsigned mi = (lo + hi) / 2; int cmp = "entry pointed at by mi" minus "target"; if (!cmp) return mi; "mi is the wanted one" if (cmp > 0) hi = mi; "mi is larger than target" else lo = mi+1; "mi is smaller than target" } while (lo < hi); "did not find what we wanted" The invariants are: - When entering the loop, 'lo' points at a slot that is never above the target (it could be at the target), 'hi' points at a slot that is guaranteed to be above the target (it can never be at the target). - We find a point 'mi' between 'lo' and 'hi' ('mi' could be the same as 'lo', but never can be as high as 'hi'), and check if 'mi' hits the target. There are three cases: - if it is a hit, we have found what we are looking for; - if it is strictly higher than the target, we set it to 'hi', and repeat the search. - if it is strictly lower than the target, we update 'lo' to one slot after it, because we allow 'lo' to be at the target and 'mi' is known to be below the target. If the loop exits, there is no matching entry. When choosing 'mi', we do not have to take the "middle" but anywhere in between 'lo' and 'hi', as long as lo <= mi < hi is satisfied. When we somehow know that the distance between the target and 'lo' is much shorter than the target and 'hi', we could pick 'mi' that is much closer to 'lo' than (hi+lo)/2, which a conventional binary search would pick. This patch takes advantage of the fact that the SHA-1 is a good hash function, and as long as there are enough entries in the table, we can expect uniform distribution. An entry that begins with for example "deadbeef..." is much likely to appear much later than in the midway of a reasonably populated table. In fact, it can be expected to be near 87% (222/256) from the top of the table. This is a work-in-progress and has switches to allow easier experiments and debugging. Exporting GIT_USE_LOOKUP environment variable enables this code. On my admittedly memory starved machine, with a partial KDE repository (3.0G pack with 95M idx): $ GIT_USE_LOOKUP=t git log -800 --stat HEAD >/dev/null 3.93user 0.16system 0:04.09elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+55588minor)pagefaults 0swaps Without the patch, the numbers are: $ git log -800 --stat HEAD >/dev/null 4.00user 0.15system 0:04.17elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+60258minor)pagefaults 0swaps In the same repository: $ GIT_USE_LOOKUP=t git log -2000 HEAD >/dev/null 0.12user 0.00system 0:00.12elapsed 97%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+4241minor)pagefaults 0swaps Without the patch, the numbers are: $ git log -2000 HEAD >/dev/null 0.05user 0.01system 0:00.07elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+8506minor)pagefaults 0swaps There isn't much time difference, but the number of minor faults seems to show that we are touching much smaller number of pages, which is expected. Signed-off-by: Junio C Hamano <gitster@pobox.com>	17 years ago