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Remove best fit

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This option saves a few CPU cycles, but potentially adds a lot of
fragmentation - so much so that there are workarounds like
max_active.  Instead, let's just drop it entirely.  It only made
a difference in one service I tested (.3% cpu regression), while
many services saw a memory win (also small, less than 1% mem P99)
This commit is contained in:
Dave Watson 2019-03-07 11:14:31 -08:00
parent b62d126df8
commit 5679751208
1 changed files with 8 additions and 32 deletions
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40
src/extent.c
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@ -441,30 +441,6 @@ extents_fit_alignment(extents_t *extents, size_t min_size, size_t max_size,
return NULL; return NULL;
} }
/* Do any-best-fit extent selection, i.e. select any extent that best fits. */
static extent_t *
extents_best_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
size_t size) {
pszind_t pind = sz_psz2ind(extent_size_quantize_ceil(size));
pszind_t i = (pszind_t)bitmap_ffu(extents->bitmap, &extents_bitmap_info,
(size_t)pind);
if (i < SC_NPSIZES + 1) {
/*
* In order to reduce fragmentation, avoid reusing and splitting
* large extents for much smaller sizes.
*/
if ((sz_pind2sz(i) >> opt_lg_extent_max_active_fit) > size) {
return NULL;
}
assert(!extent_heap_empty(&extents->heaps[i]));
extent_t *extent = extent_heap_first(&extents->heaps[i]);
assert(extent_size_get(extent) >= size);
return extent;
}
return NULL;
}
/* /*
* Do first-fit extent selection, i.e. select the oldest/lowest extent that is * Do first-fit extent selection, i.e. select the oldest/lowest extent that is
* large enough. * large enough.
@ -487,12 +463,15 @@ extents_first_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
/* /*
* In order to reduce fragmentation, avoid reusing and splitting * In order to reduce fragmentation, avoid reusing and splitting
* large extents for much smaller sizes. * large extents for much smaller sizes.
*
* Only do check for dirty extents (delay_coalesce).
*/ */
if ((sz_pind2sz(i) >> opt_lg_extent_max_active_fit) > size) { if (extents->delay_coalesce &&
(sz_pind2sz(i) >> opt_lg_extent_max_active_fit) > size) {
size_ok = false; size_ok = false;
} }
if (size_ok && if (size_ok &&
(ret == NULL || extent_snad_comp(extent, ret) < 0)) { (ret == NULL || extent_snad_comp(extent, ret) < 0)) {
ret = extent; ret = extent;
} }
if (i == SC_NPSIZES) { if (i == SC_NPSIZES) {
@ -505,10 +484,8 @@ extents_first_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
} }
/* /*
* Do {best,first}-fit extent selection, where the selection policy choice is * Do first-fit extent selection, where the selection policy choice is
* based on extents->delay_coalesce. Best-fit selection requires less * based on extents->delay_coalesce.
* searching, but its layout policy is less stable and may cause higher virtual
* memory fragmentation as a side effect.
*/ */
static extent_t * static extent_t *
extents_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents, extents_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
@ -521,8 +498,7 @@ extents_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
return NULL; return NULL;
} }
extent_t *extent = extents->delay_coalesce ? extent_t *extent =
extents_best_fit_locked(tsdn, arena, extents, max_size) :
extents_first_fit_locked(tsdn, arena, extents, max_size); extents_first_fit_locked(tsdn, arena, extents, max_size);
if (alignment > PAGE && extent == NULL) { if (alignment > PAGE && extent == NULL) {