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Description: Make trimming logic consistent.
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Author: Carlos O'Donell
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Origin: git://sourceware.org/git/glibc.git
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Bug-RHEL: N/A
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Bug-Fedora: N/A
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Bug-Upstream: #17195
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Upstream status: committed
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commit e4bc326dbbf7328775fe7dd39de1178821363e0a
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Author: Carlos O'Donell <carlos@systemhalted.org>
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Date: Wed Oct 7 22:21:36 2015 -0400
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malloc: Consistently apply trim_threshold to all heaps (Bug 17195)
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In the per-thread arenas we apply trim_threshold-based checks
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to the extra space between the pad and the top_area. This isn't
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quite accurate and instead we should be harmonizing with the way
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in which trim_treshold is applied everywhere else like sysrtim
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and _int_free. The trimming check should be based on the size of
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the top chunk and only the size of the top chunk. The following
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patch harmonizes the trimming and make it consistent for the main
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arena and thread arenas.
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In the old code a large padding request might have meant that
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trimming was not triggered. Now trimming is considered first based
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on the chunk, then the pad is subtracted, and the remainder trimmed.
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This is how all the other trimmings operate. I didn't measure the
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performance difference of this change because it corrects what I
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consider to be a behavioural anomaly. We'll need some profile driven
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optimization to make this code better, and even there Ondrej and
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others have better ideas on how to speedup malloc.
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Tested on x86_64 with no regressions. Already reviewed by Siddhesh
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Poyarekar and Mel Gorman here and discussed here:
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https://sourceware.org/ml/libc-alpha/2015-05/msg00002.html
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Index: glibc-2.17-c758a686/malloc/arena.c
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===================================================================
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--- glibc-2.17-c758a686.orig/malloc/arena.c
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+++ glibc-2.17-c758a686/malloc/arena.c
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@@ -697,14 +697,20 @@ heap_trim(heap_info *heap, size_t pad)
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}
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/* Uses similar logic for per-thread arenas as the main arena with systrim
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- by preserving the top pad and at least a page. */
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+ and _int_free by preserving the top pad and rounding down to the nearest
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+ page. */
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top_size = chunksize(top_chunk);
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+ if ((unsigned long)(top_size) <
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+ (unsigned long)(mp_.trim_threshold))
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+ return 0;
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+
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top_area = top_size - MINSIZE - 1;
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if (top_area <= pad)
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return 0;
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+ /* Release in pagesize units and round down to the nearest page. */
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extra = ALIGN_DOWN(top_area - pad, pagesz);
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- if ((unsigned long) extra < mp_.trim_threshold)
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+ if (extra == 0)
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return 0;
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/* Try to shrink. */
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if(shrink_heap(heap, extra) != 0)
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