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Author	SHA1	Message	Date
Jeff King	24d82185d2	tempfile: use list.h for linked list The tempfile API keeps to-be-cleaned tempfiles in a singly-linked list and never removes items from the list. A future patch would like to start removing items, but removal from a singly linked list is O(n), as we have to walk the list to find the predecessor element. This means that a process which takes "n" simultaneous lockfiles (for example, an atomic transaction on "n" refs) may end up quadratic in "n". Before we start allowing items to be removed, it would be nice to have a way to cover this case in linear time. The simplest solution is to make an assumption about the order in which tempfiles are added and removed from the list. If both operations iterate over the tempfiles in the same order, then by putting new items at the end of the list our removal search will always find its items at the beginning of the list. And indeed, that would work for the case of refs. But it creates a hidden dependency between unrelated parts of the code. If anybody changes the ref code (or if we add a new caller that opens multiple simultaneous tempfiles) they may unknowingly introduce a performance regression. Another solution is to use a better data structure. A doubly-linked list works fine, and we already have an implementation in list.h. But there's one snag: the elements of "struct tempfile" are all marked as "volatile", since a signal handler may interrupt us and iterate over the list at any moment (even if we were in the middle of adding a new entry). We can declare a "volatile struct list_head", but we can't actually use it with the normal list functions. The compiler complains about passing a pointer-to-volatile via a regular pointer argument. And rightfully so, as the sub-function would potentially need different code to deal with the volatile case. That leaves us with a few options: 1. Drop the "volatile" modifier for the list items. This is probably a bad idea. I checked the assembly output from "gcc -O2", and the "volatile" really does impact the order in which it updates memory. 2. Use macros instead of inline functions. The irony here is that list.h is entirely implemented as trivial inline functions. So we basically are already generating custom code for each call. But sadly there's no way in C to declare the inline function to take a more generic type. We could do so by switching the inline functions to macros, but it does make the end result harder to read. And it doesn't fully solve the problem (for instance, the declaration of list_head needs to change so that its "prev" and "next" pointers point to other volatile structs). 3. Don't use list.h, and just make our own ad-hoc doubly-linked list. It's not that much code to implement the basics that we need here. But if we're going to do so, why not add the few extra lines required to model it after the actual list.h interface? We can even reuse a few of the macro helpers. So this patch takes option 3, but actually implements a parallel "volatile list" interface in list.h, where it could potentially be reused by other code. This implements just enough for tempfile.c's use, though we could easily port other functions later if need be. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>	8 years ago
Eric Wong	ecba19531a	list: avoid incompatibility with *BSD sys/queue.h The OS X build pulls in sys/queue.h, which pollutes the preprocessor namespace with a macro generically named LIST_HEAD, and clashes with the name we use here. ref: http://mid.gmane.org/FB76544F-16F7-45CA-9649-FD62EE44B0DE@gmail.com Reported-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Eric Wong <e@80x24.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago
Eric Wong	94e99012fc	http-walker: reduce O(n) ops with doubly-linked list Using the a Linux-kernel-derived doubly-linked list implementation from the Userspace RCU library allows us to enqueue and delete items from the object request queue in constant time. This change reduces enqueue times in the prefetch() function where object request queue could grow to several thousand objects. I left out the list_for_each_entry* family macros from list.h which relied on the __typeof__ operator as we support platforms without it. Thus, list_entry (aka "container_of") needs to be called explicitly inside macro-wrapped for loops. The downside is this costs us an additional pointer per object request, but this is offset by reduced overhead on queue operations leading to improved performance and shorter queue depths. Signed-off-by: Eric Wong <e@80x24.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>	9 years ago

Author

SHA1

Message

Date

Jeff King

24d82185d2

tempfile: use list.h for linked list

The tempfile API keeps to-be-cleaned tempfiles in a
singly-linked list and never removes items from the list.  A
future patch would like to start removing items, but removal
from a singly linked list is O(n), as we have to walk the
list to find the predecessor element. This means that a
process which takes "n" simultaneous lockfiles (for example,
an atomic transaction on "n" refs) may end up quadratic in
"n".

Before we start allowing items to be removed, it would be
nice to have a way to cover this case in linear time.

The simplest solution is to make an assumption about the
order in which tempfiles are added and removed from the
list. If both operations iterate over the tempfiles in the
same order, then by putting new items at the end of the list
our removal search will always find its items at the
beginning of the list. And indeed, that would work for the
case of refs. But it creates a hidden dependency between
unrelated parts of the code. If anybody changes the ref code
(or if we add a new caller that opens multiple simultaneous
tempfiles) they may unknowingly introduce a performance
regression.

Another solution is to use a better data structure. A
doubly-linked list works fine, and we already have an
implementation in list.h. But there's one snag: the elements
of "struct tempfile" are all marked as "volatile", since a
signal handler may interrupt us and iterate over the list at
any moment (even if we were in the middle of adding a new
entry).

We can declare a "volatile struct list_head", but we can't
actually use it with the normal list functions. The compiler
complains about passing a pointer-to-volatile via a regular
pointer argument. And rightfully so, as the sub-function
would potentially need different code to deal with the
volatile case.

That leaves us with a few options:

  1. Drop the "volatile" modifier for the list items.

     This is probably a bad idea. I checked the assembly
     output from "gcc -O2", and the "volatile" really does
     impact the order in which it updates memory.

  2. Use macros instead of inline functions. The irony here
     is that list.h is entirely implemented as trivial
     inline functions. So we basically are already
     generating custom code for each call. But sadly there's no
     way in C to declare the inline function to take a more
     generic type.

     We could do so by switching the inline functions to
     macros, but it does make the end result harder to read.
     And it doesn't fully solve the problem (for instance,
     the declaration of list_head needs to change so that
     its "prev" and "next" pointers point to other volatile
     structs).

  3. Don't use list.h, and just make our own ad-hoc
     doubly-linked list. It's not that much code to
     implement the basics that we need here. But if we're
     going to do so, why not add the few extra lines
     required to model it after the actual list.h interface?
     We can even reuse a few of the macro helpers.

So this patch takes option 3, but actually implements a
parallel "volatile list" interface in list.h, where it could
potentially be reused by other code. This implements just
enough for tempfile.c's use, though we could easily port
other functions later if need be.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

8 years ago

Eric Wong

ecba19531a

list: avoid incompatibility with *BSD sys/queue.h

The OS X build pulls in sys/queue.h, which pollutes the preprocessor
namespace with a macro generically named LIST_HEAD, and clashes with
the name we use here.

ref: http://mid.gmane.org/FB76544F-16F7-45CA-9649-FD62EE44B0DE@gmail.com

Reported-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

9 years ago

Eric Wong

94e99012fc

http-walker: reduce O(n) ops with doubly-linked list

Using the a Linux-kernel-derived doubly-linked list
implementation from the Userspace RCU library allows us to
enqueue and delete items from the object request queue in
constant time.

This change reduces enqueue times in the prefetch() function
where object request queue could grow to several thousand
objects.

I left out the list_for_each_entry* family macros from list.h
which relied on the __typeof__ operator as we support platforms
without it.  Thus, list_entry (aka "container_of") needs to be
called explicitly inside macro-wrapped for loops.

The downside is this costs us an additional pointer per object
request, but this is offset by reduced overhead on queue
operations leading to improved performance and shorter queue
depths.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

9 years ago

3 Commits (3e8ed3b93e9b46eb6fe61589482751728865de57)