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GeezerVison is showing github.com — gh-115103: Implement delayed free mechanism for free-threaded builds by colesbury · Pull Request #115367 · python/cpython

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@colesbury

@colesbury colesbury commented Feb 12, 2024 •
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This adds _PyMem_FreeDelayed() and supporting functions. The
_PyMem_FreeDelayed() function frees memory with the same allocator as
PyMem_Free(), but after some delay to ensure that concurrent lock-free
readers have finished.

  • Issue: Add delayed reclamation mechanism for free-threaded build (QSBR) #115103

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@colesbury
colesbury requested a review from DinoV February 12, 2024 20:47
@bedevere-app bedevere-app Bot mentioned this pull request Feb 12, 2024
Add delayed reclamation mechanism for free-threaded build (QSBR) #115103
Closed
@colesbury colesbury added skip news topic-free-threading labels Feb 12, 2024
@colesbury colesbury mentioned this pull request Feb 14, 2024
gh-115103: Implement delayed memory reclamation (QSBR) #115180
Merged
@colesbury
pythongh-115103: Implement delayed free mechanism for free-threaded b…
5e9f1dc 
…uilds

This adds `_PyMem_FreeDelayed()` and supporting functions. The
`_PyMem_FreeDelayed()` function frees memory with the same allocator as
`PyMem_Free()`, but after some delay to ensure that concurrent lock-free
readers have finished.
@colesbury
colesbury force-pushed the gh-115103-delayed-free branch from a661366 to 5e9f1dc Compare February 16, 2024 20:35
@colesbury
colesbury marked this pull request as ready for review February 16, 2024 20:53
@colesbury
colesbury requested a review from ericsnowcurrently as a code owner February 16, 2024 20:53
@bedevere-app bedevere-app Bot added the awaiting core review label Feb 16, 2024
DinoV
DinoV approved these changes Feb 16, 2024
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Looks pretty solid, let me know what you think of the comments.

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Comment thread Objects/obmalloc.c
process_queue(struct llist_node *head, struct _qsbr_thread_state *qsbr)
{
while (!llist_empty(head)) {
struct _mem_work_chunk *buf = work_queue_first(head);

@DinoV DinoV Feb 16, 2024

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Is there a reason we'e not just doing something like:
for (Py_ssize_t i = buf->rd_idx; i < buf->wr_idx; i++) {
...
}

Or the while loop like _PyMem_FiniDelayed with the polling from here added in vs just going back through the head of the list each time.

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@colesbury colesbury Feb 17, 2024

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I changed the loop to avoid grabbing the head every iteration

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Comment thread Objects/obmalloc.c Outdated
struct _mem_work_chunk *buf = work_queue_first(head);

if (buf->rd_idx == buf->wr_idx) {
llist_remove(&buf->node);

@DinoV DinoV Feb 16, 2024

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It seems like we should wait for buf->rd_idx == WORK_ITEMS_PER_CHUNK before freeing and removing?

Otherwise it seems like we could end up in a situation where we're doing some delay frees, process them all, and then quickly need to do more delayed frees and end up re-allocating this 4k buffer repeatedly.

And if this is any buffer other than the last then buf->wr_idx == WORK_ITEMS_PER_CHUNK so we'd always free all but one.

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@colesbury colesbury Feb 17, 2024

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Yeah, that's a good idea. We can do that for the thread queues but not the interpreter queue.

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels Feb 16, 2024
@colesbury
Keep last buffer in thread's queue to reduce allocations
0d12f6b
corona10
corona10 approved these changes Feb 17, 2024
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@corona10 corona10 left a comment •
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lgtm!
(I 've taken a look at QSBR mechanism due to working on list allocation while making list thread-safe)

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DinoV
DinoV approved these changes Feb 20, 2024
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LGTM!

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@colesbury
colesbury merged commit e3ad6ca into python:main Feb 20, 2024
@colesbury
colesbury deleted the gh-115103-delayed-free branch February 20, 2024 18:04
@bedevere-app bedevere-app Bot removed the awaiting merge label Feb 20, 2024
woodruffw pushed a commit to woodruffw-forks/cpython that referenced this pull request Mar 4, 2024
@colesbury @woodruffw
pythongh-115103: Implement delayed free mechanism for free-threaded b…
ad1ca5a 
…uilds (python#115367)

This adds `_PyMem_FreeDelayed()` and supporting functions. The
`_PyMem_FreeDelayed()` function frees memory with the same allocator as
`PyMem_Free()`, but after some delay to ensure that concurrent lock-free
readers have finished.
diegorusso pushed a commit to diegorusso/cpython that referenced this pull request Apr 17, 2024
@colesbury @diegorusso
pythongh-115103: Implement delayed free mechanism for free-threaded b…
b445298 
…uilds (python#115367)

This adds `_PyMem_FreeDelayed()` and supporting functions. The
`_PyMem_FreeDelayed()` function frees memory with the same allocator as
`PyMem_Free()`, but after some delay to ensure that concurrent lock-free
readers have finished.
LukasWoodtli pushed a commit to LukasWoodtli/cpython that referenced this pull request Jan 22, 2025
@colesbury @LukasWoodtli
pythongh-115103: Implement delayed free mechanism for free-threaded b…
bdbaa84 
…uilds (python#115367)

This adds `_PyMem_FreeDelayed()` and supporting functions. The
`_PyMem_FreeDelayed()` function frees memory with the same allocator as
`PyMem_Free()`, but after some delay to ensure that concurrent lock-free
readers have finished.
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@DinoV DinoV DinoV approved these changes
@corona10 corona10 corona10 approved these changes
@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently ericsnowcurrently is a code owner

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@colesbury @DinoV @corona10
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