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Subject[PATCH v2 0/5] slab: implement byte sized indexes for the freelist of a slab
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This patchset implements byte sized indexes for the freelist of a slab.

Currently, the freelist of a slab consist of unsigned int sized indexes.
Most of slabs have less number of objects than 256, so much space is wasted.
To reduce this overhead, this patchset implements byte sized indexes for
the freelist of a slab. With it, we can save 3 bytes for each objects.

Below is some numbers of 'cat /proc/slabinfo'.

* Before *
kmalloc-512 525 640 512 8 1 : tunables 54 27 0 : slabdata 80 80 0
kmalloc-256 210 210 256 15 1 : tunables 120 60 0 : slabdata 14 14 0
kmalloc-192 1016 1040 192 20 1 : tunables 120 60 0 : slabdata 52 52 0
kmalloc-96 560 620 128 31 1 : tunables 120 60 0 : slabdata 20 20 0
kmalloc-64 2148 2280 64 60 1 : tunables 120 60 0 : slabdata 38 38 0
kmalloc-128 647 682 128 31 1 : tunables 120 60 0 : slabdata 22 22 0
kmalloc-32 11360 11413 32 113 1 : tunables 120 60 0 : slabdata 101 101 0
kmem_cache 197 200 192 20 1 : tunables 120 60 0 : slabdata 10 10 0

* After *
kmalloc-512 521 648 512 8 1 : tunables 54 27 0 : slabdata 81 81 0
kmalloc-256 208 208 256 16 1 : tunables 120 60 0 : slabdata 13 13 0
kmalloc-192 1029 1029 192 21 1 : tunables 120 60 0 : slabdata 49 49 0
kmalloc-96 529 589 128 31 1 : tunables 120 60 0 : slabdata 19 19 0
kmalloc-64 2142 2142 64 63 1 : tunables 120 60 0 : slabdata 34 34 0
kmalloc-128 660 682 128 31 1 : tunables 120 60 0 : slabdata 22 22 0
kmalloc-32 11716 11780 32 124 1 : tunables 120 60 0 : slabdata 95 95 0
kmem_cache 197 210 192 21 1 : tunables 120 60 0 : slabdata 10 10 0

kmem_caches consisting of objects less than or equal to 256 byte have
one or more objects than before. In the case of kmalloc-32, we have 11 more
objects, so 352 bytes (11 * 32) are saved and this is roughly 9% saving of
memory. Of couse, this percentage decreases as the number of objects
in a slab decreases.

Here are the performance results on my 4 cpus machine.

* Before *

Performance counter stats for 'perf bench sched messaging -g 50 -l 1000' (10 runs):

229,945,138 cache-misses ( +- 0.23% )

11.627897174 seconds time elapsed ( +- 0.14% )

* After *

Performance counter stats for 'perf bench sched messaging -g 50 -l 1000' (10 runs):

218,640,472 cache-misses ( +- 0.42% )

11.504999837 seconds time elapsed ( +- 0.21% )

cache-misses are reduced by this patchset, roughly 5%.
And elapsed times are improved by 1%.

This patchset comes from a Christoph's idea.
https://lkml.org/lkml/2013/8/23/315

Patches are on top of my previous posting named as
"slab: overload struct slab over struct page to reduce memory usage"
https://lkml.org/lkml/2013/10/16/155

Thanks.

Joonsoo Kim (5):
slab: factor out calculate nr objects in cache_estimate
slab: introduce helper functions to get/set free object
slab: restrict the number of objects in a slab
slab: introduce byte sized index for the freelist of a slab
slab: make more slab management structure off the slab

mm/slab.c | 90 +++++++++++++++++++++++++++++++++++++++----------------------
1 file changed, 58 insertions(+), 32 deletions(-)

--
1.7.9.5



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