Re: [PATCH 0/4] slab: implement byte sized indexes for the freelist of a slab

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Joonsoo,
On Mon, Sep 02, 2013 at 05:38:54PM +0900, Joonsoo Kim wrote:
>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.
>
>This introduce one likely branch to functions used for setting/getting
>objects to/from the freelist, but we may get more benefits from
>this change.
>
>Below is some numbers of 'cat /proc/slabinfo' related to my previous posting
>and this patchset.
>
>
>* Before *
># name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables [snip...]
>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 my previous posting(overload struct slab over struct page) *
># name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables [snip...]
>kmalloc-512          527    600    512    8    1 : tunables   54   27    0 : slabdata     75     75      0   
>kmalloc-256          210    210    256   15    1 : tunables  120   60    0 : slabdata     14     14      0   
>kmalloc-192         1040   1040    192   20    1 : tunables  120   60    0 : slabdata     52     52      0   
>kmalloc-96           750    750    128   30    1 : tunables  120   60    0 : slabdata     25     25      0   
>kmalloc-64          2773   2773     64   59    1 : tunables  120   60    0 : slabdata     47     47      0   
>kmalloc-128          660    690    128   30    1 : tunables  120   60    0 : slabdata     23     23      0   
>kmalloc-32         11200  11200     32  112    1 : tunables  120   60    0 : slabdata    100    100      0   
>kmem_cache           197    200    192   20    1 : tunables  120   60    0 : slabdata     10     10      0   
>
>kmem_caches consisting of objects less than or equal to 128 byte have one more
>objects in a slab. You can see it at objperslab.

I think there is one less objects in a slab after observing objperslab.

Regards,
Wanpeng Li 

>
>We can improve further with this patchset.
>
>* My previous posting + this patchset *
># name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables [snip...]
>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 12 more
>objects, so 384 bytes (12 * 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.
>
>Please let me know expert's opions :)
>Thanks.
>
>This patchset comes from a Christoph's idea.
>https://lkml.org/lkml/2013/8/23/315
>
>Patches are on top of my previous posting.
>https://lkml.org/lkml/2013/8/22/137
>
>Joonsoo Kim (4):
>  slab: factor out calculate nr objects in cache_estimate
>  slab: introduce helper functions to get/set free object
>  slab: introduce byte sized index for the freelist of a slab
>  slab: make more slab management structure off the slab
>
> mm/slab.c |  138 +++++++++++++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 103 insertions(+), 35 deletions(-)
>
>-- 
>1.7.9.5
>
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