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bpo-37257: obmalloc: stop simple arena thrashing #14039

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Merged
merged 4 commits into from
Jun 13, 2019
Merged

bpo-37257: obmalloc: stop simple arena thrashing #14039

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12c8880
first tab - and, with luck, the last ;-)
tim-one Jun 13, 2019
5e2e881
📜🤖 Added by blurb_it.
blurb-it[bot] Jun 13, 2019
328dfbe
Update Objects/obmalloc.c
tim-one Jun 13, 2019
7423ae8
Update Objects/obmalloc.c
tim-one Jun 13, 2019
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2019-06-13-02-27-12.bpo-37257.IMxDvT.rst
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@@ -0,0 +1 @@
Python's small object allocator (``obmalloc.c``) now allows (no more than) one empty arena to remain available for immediate reuse, without returning it to the OS. This prevents thrashing in simple loops where an arena could be created and destroyed anew on each iteration.
9 changes: 7 additions & 2 deletions Objects/obmalloc.c
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Expand Up @@ -1758,7 +1758,12 @@ pymalloc_free(void *ctx, void *p)
/* All the rest is arena management. We just freed
* a pool, and there are 4 cases for arena mgmt:
* 1. If all the pools are free, return the arena to
* the system free().
* the system free(). Except if this is the last
* arena in the list, keep it to avoid thrashing:
* keeping one wholly free arena in the list avoids
* pathological cases where a simple loop would
* otherwise provoke needing to allocate and free an
* arena on every iteration. See bpo-37257.
* 2. If this is the only free pool in the arena,
* add the arena back to the `usable_arenas` list.
* 3. If the "next" arena has a smaller count of free
Expand All @@ -1767,7 +1772,7 @@ pymalloc_free(void *ctx, void *p)
* nfreepools.
* 4. Else there's nothing more to do.
*/
if (nf == ao->ntotalpools) {
if (nf == ao->ntotalpools && ao->nextarena != NULL) {
/* Case 1. First unlink ao from usable_arenas.
*/
assert(ao->prevarena == NULL ||
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