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    Subject[PATCH v2 00/15] slab: overload struct slab over struct page to reduce memory usage
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    There is two main topics in this patchset. One is to reduce memory usage
    and the other is to change a management method of free objects of a slab.

    The SLAB allocate a struct slab for each slab. The size of this structure
    except bufctl array is 40 bytes on 64 bits machine. We can reduce memory
    waste and cache footprint if we overload struct slab over struct page.

    And this patchset change a management method of free objects of a slab.
    Current free objects management method of the slab is weird, because
    it touch random position of the array of kmem_bufctl_t when we try to
    get free object. See following example.

    struct slab's free = 6
    kmem_bufctl_t array: 1 END 5 7 0 4 3 2

    To get free objects, we access this array with following index pattern.
    6 -> 3 -> 7 -> 2 -> 5 -> 4 -> 0 -> 1 -> END

    If we have many objects, this array would be larger and be not in the same
    cache line. It is not good for performance.

    We can do same thing through more easy way, like as the stack.
    This patchset implement it and remove complex code for above algorithm.
    This makes slab code much cleaner.

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

    * Before *
    # 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

    * After *
    # 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

    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.


    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):

    238,309,671 cache-misses ( +- 0.40% )

    12.010172090 seconds time elapsed ( +- 0.21% )

    * After *

    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% )

    cache-misses are reduced by this patchset, roughly 5%.
    And elapsed times are also improved by 3.1% to baseline.

    I think that this patchsets deserve to be merged, since it reduces memory usage and
    also improves performance. :)
    Please let me know expert's opinion.

    Thanks.

    This patchset is based on v3.12-rc5.

    Joonsoo Kim (15):
    slab: correct pfmemalloc check
    slab: change return type of kmem_getpages() to struct page
    slab: remove colouroff in struct slab
    slab: remove nodeid in struct slab
    slab: remove cachep in struct slab_rcu
    slab: overloading the RCU head over the LRU for RCU free
    slab: use well-defined macro, virt_to_slab()
    slab: use __GFP_COMP flag for allocating slab pages
    slab: change the management method of free objects of the slab
    slab: remove kmem_bufctl_t
    slab: remove SLAB_LIMIT
    slab: replace free and inuse in struct slab with newly introduced
    active
    slab: use struct page for slab management
    slab: remove useless statement for checking pfmemalloc
    slab: rename slab_bufctl to slab_freelist

    include/linux/mm_types.h | 24 +-
    include/linux/slab.h | 9 +-
    include/linux/slab_def.h | 4 +-
    mm/slab.c | 565 ++++++++++++++++++----------------------------
    4 files changed, 244 insertions(+), 358 deletions(-)

    --
    1.7.9.5



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