Fwd: Improvements to the heap allocator

[Andrea Canciani]

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Andrea

---------- Forwarded message ----------
From: Andrea Canciani <ranma42 at gmail.com>
Date: Fri, Jun 23, 2017 at 9:29 AM
Subject: Improvements to the heap allocator
To: wine-devel at winehq.org

Yesterday I opened a bug report about a performance issue in the wine heap
allocator: https://bugs.winehq.org/show_bug.cgi?id=43224
Since I was also asking for some suggestions about what the preferred fix
would be, Sebastian Lackner pointed to this mailing list.

The bug report contains the source code for a program that has an
allocation pattern that is very bad (for the current wine allocator).

This appears to be a known problem, but I was unable to find any
test/benchmark that shows such degenerate behavior.
So far none of the attempts at fixing it seem to have landed. Some possible
approaches are:
1. ensure that the freelist sizes cover all (alignment rounded) sizes up to
a certain value;
2. when looking up for a free block, start from the freelist whose smallest
element is at least as big as the desired block;
3. implement more complex free block management.

#1 would fix the issue for some allocations sizes, but for bigger
allocations the problem would stay the same.

#2 would fix the performance problem at the cost of increased memory usage:
for some sizes it would be possible that after a block is allocated and
freed, the following allocation for that size does not re-use that memory
(if the free block is stored in a freelist together with smaller blocks).

#3 would increase the complexity of the code; to keep it under control, it
might be possible to re-use the red-black tree implementation already
available in wine. To avoid regressing performance, the smallest blocks
might still be handled with simple freelists (fixed as in #1), while larger
blocks are managed in a tree.

Some of the current/previous attempts at tackling this issue are:
 - https://github.com/wine-compholio/wine-staging/blob/
master/patches/ntdll-Heap_FreeLists/0001-ntdll-Improve-
heap-allocation-performance-by-using-m.patch (implements #1)
 - https://github.com/laino/wine-patches (IIUC, a combination of #1 and #3,
in which the code tries to keep the freelist length under control, but
AFAICT there is no guaranteed bound)

I would like to try and work on fixing this, but there are some decision to
make that might depend on constraints I am not aware of.
Would the solution I propose (#3, rbtree based, with some tuning for small
allocation sizes) be acceptable?
Are there any benchmarks that a new implementation should strive to improve
or at least avoid regressing?
Suggestions and alternative solutions are welcome :)

Sorry for the long message, thank you for your patience
Andrea
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