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Introduce pinned extents to contain unpurgeable pages

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Some pages (e.g., hugetlb pages) cannot be purged, and should be
prioritized for reuse.  A custom extent_alloc hook signals this by
OR'ing EXTENT_ALLOC_FLAG_PINNED into the low bits of the returned
pointer; jemalloc strips the flag bits and caches pinned extents in
a dedicated ecache_pinned, separate from the dirty/muzzy decay
pipeline.

Pinned extents do not coalesce eagerly, except for ones larger than
SC_LARGE_MINCLASS.  A prefer-small policy reuses the smallest fitting
pinned extent, to avoid unnecessary split/fragmentation.
This commit is contained in:
Bin Liu 2026-04-19 22:56:22 -07:00 • committed by Guangli Dai
parent 7638093c73
commit be2de8ccd8
22 changed files with 977 additions and 86 deletions
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27
src/eset.c
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@ -98,7 +98,10 @@ eset_insert(eset_t *eset, edata_t *edata) {
eset_stats_add(eset, pind, size);
}
edata_list_inactive_append(&eset->lru, edata);
/* Pinned extents skip LRU as they do not decay. */
if (!edata_pinned_get(edata)) {
edata_list_inactive_append(&eset->lru, edata);
}
size_t npages = size >> LG_PAGE;
/*
* All modifications to npages hold the mutex (as asserted above), so we
@ -143,7 +146,9 @@ eset_remove(eset_t *eset, edata_t *edata) {
edata_heap_first(&eset->bins[pind].heap));
}
}
edata_list_inactive_remove(&eset->lru, edata);
if (!edata_pinned_get(edata)) {
edata_list_inactive_remove(&eset->lru, edata);
}
size_t npages = size >> LG_PAGE;
/*
* As in eset_insert, we hold eset->mtx and so don't need atomic
@ -279,10 +284,15 @@ eset_fit_alignment(
* avoiding reusing and splitting large extents for smaller sizes. In practice,
* it's set to opt_lg_extent_max_active_fit for the dirty eset and SC_PTR_BITS
* for others.
*
* If prefer_small is true, return as soon as the smallest fitting bin yields a
* candidate, instead of scanning further bins for an older/lower extent.
* Useful for fragmentation control for the pinned pool.
*/
static edata_t *
eset_first_fit(
eset_t *eset, size_t size, bool exact_only, unsigned lg_max_fit) {
eset_t *eset, size_t size, bool exact_only, unsigned lg_max_fit,
bool prefer_small) {
edata_t *ret = NULL;
edata_cmp_summary_t ret_summ JEMALLOC_CC_SILENCE_INIT({0});
@ -327,6 +337,9 @@ eset_first_fit(
if (sz_large_size_classes_disabled() && pind != pind_prev) {
ret = eset_enumerate_search(eset, size, pind_prev,
/* exact_only */ false, &ret_summ);
if (prefer_small && ret != NULL) {
return ret;
}
}
for (pszind_t i =
@ -363,6 +376,9 @@ eset_first_fit(
edata_cmp_summary_get(edata))
== 0);
ret = edata;
if (prefer_small) {
return ret;
}
ret_summ = eset->bins[i].heap_min;
}
if (i == SC_NPSIZES) {
@ -376,14 +392,15 @@ eset_first_fit(
edata_t *
eset_fit(eset_t *eset, size_t esize, size_t alignment, bool exact_only,
unsigned lg_max_fit) {
unsigned lg_max_fit, bool prefer_small) {
size_t max_size = esize + PAGE_CEILING(alignment) - PAGE;
/* Beware size_t wrap-around. */
if (max_size < esize) {
return NULL;
}
edata_t *edata = eset_first_fit(eset, max_size, exact_only, lg_max_fit);
edata_t *edata = eset_first_fit(eset, max_size, exact_only, lg_max_fit,
prefer_small);
if (alignment > PAGE && edata == NULL) {
/*