[patch 015/262] mm/slub: increase default cpu partial list sizes

Andrew Morton
2021-11-05T20:35:24Z

From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm/slub: increase default cpu partial list sizes

The defaults are determined based on object size and can go up to 30 for
objects smaller than 256 bytes. Before the previous patch changed the
accounting, this could have made cpu partial list contain up to 30 pages.
After that patch, only up to 2 pages with default allocation order.

Very short lists limit the usefulness of the whole concept of cpu partial
lists, so this patch aims at a more reasonable default under the new
accounting. The defaults are quadrupled, except for object size >=
PAGE_SIZE where it's doubled. This makes the lists grow up to 10 pages in
practice.

A quick test of booting a kernel under virtme with 4GB RAM and 8 vcpus
shows the following slab memory usage after boot:

Before previous patch (using page->pobjects):
Slab: 36732 kB
SReclaimable: 14836 kB
SUnreclaim: 21896 kB

After previous patch (using page->pages):
Slab: 34720 kB
SReclaimable: 13716 kB
SUnreclaim: 21004 kB

After this patch (using page->pages, higher defaults):
Slab: 35252 kB
SReclaimable: 13944 kB
SUnreclaim: 21308 kB

In the same setup, I also ran 5 times:
hackbench -l 16000 -g 16

Differences in time were in the noise, we can compare slub stats as given
by slabinfo -r skbuff_head_cache (the other cache heavily used by
hackbench, kmalloc-cg-512 looks similar). Negligible stats left out for
brevity.

Before previous patch (using page->pobjects):

Objects: 1408, Memory Total: 401408 Used : 304128

Slab Perf Counter Alloc Free %Al %Fr
--------------------------------------------------
Fastpath 469952498 5946606 91 1
Slowpath 42053573 506059465 8 98
Page Alloc 41093 41044 0 0
Add partial 18 21229327 0 4
Remove partial 20039522 36051 3 0
Cpu partial list 4686640 24767229 0 4
RemoteObj/SlabFrozen 16 124027841 0 24
Total 512006071 512006071
Flushes 18

Slab Deactivation Occurrences %
-------------------------------------------------
Slab empty 4993 0%
Deactivation bypass 24767229 99%
Refilled from foreign frees 21972674 88%

After previous patch (using page->pages):

Objects: 480, Memory Total: 131072 Used : 103680

Slab Perf Counter Alloc Free %Al %Fr
--------------------------------------------------
Fastpath 473016294 5405653 92 1
Slowpath 38989777 506600418 7 98
Page Alloc 32717 32701 0 0
Add partial 3 22749164 0 4
Remove partial 11371127 32474 2 0
Cpu partial list 11686226 23090059 2 4
RemoteObj/SlabFrozen 2 67541803 0 13
Total 512006071 512006071
Flushes 3

Slab Deactivation Occurrences %
-------------------------------------------------
Slab empty 227 0%
Deactivation bypass 23090059 99%
Refilled from foreign frees 27585695 119%

After this patch (using page->pages, higher defaults):

Objects: 896, Memory Total: 229376 Used : 193536

Slab Perf Counter Alloc Free %Al %Fr
--------------------------------------------------
Fastpath 473799295 4980278 92 0
Slowpath 38206776 507025793 7 99
Page Alloc 32295 32267 0 0
Add partial 11 23291143 0 4
Remove partial 5815764 31278 1 0
Cpu partial list 18119280 23967320 3 4
RemoteObj/SlabFrozen 10 76974794 0 15
Total 512006071 512006071
Flushes 11

Slab Deactivation Occurrences %
-------------------------------------------------
Slab empty 989 0%
Deactivation bypass 23967320 99%
Refilled from foreign frees 32358473 135%

As expected, memory usage dropped significantly with change of accounting,
increasing the defaults increased it, but not as much. The number of page
allocation/frees dropped significantly with the new accounting, but didn't
increase with the higher defaults. Interestingly, the number of fasthpath
allocations increased, as well as allocations from the cpu partial list,
even though it's shorter.

Link: https://lkml.kernel.org/r/20211012134651.11258-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/slub.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/slub.c~mm-slub-increase-default-cpu-partial-list-sizes
+++ a/mm/slub.c
@@ -4030,13 +4030,13 @@ static void set_cpu_partial(struct kmem_
if (!kmem_cache_has_cpu_partial(s))
nr_objects = 0;
else if (s->size >= PAGE_SIZE)
- nr_objects = 2;
- else if (s->size >= 1024)
nr_objects = 6;
+ else if (s->size >= 1024)
+ nr_objects = 24;
else if (s->size >= 256)
- nr_objects = 13;
+ nr_objects = 52;
else
- nr_objects = 30;
+ nr_objects = 120;

slub_set_cpu_partial(s, nr_objects);
#endif
_