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    SubjectRe: [Q] Default SLAB allocator
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    On Tue, Oct 16, 2012 at 3:44 PM, Tim Bird <tim.bird@am.sony.com> wrote:
    > On 10/16/2012 11:27 AM, Ezequiel Garcia wrote:
    >> On Tue, Oct 16, 2012 at 3:07 PM, Tim Bird <tim.bird@am.sony.com> wrote:
    >>> On 10/16/2012 05:56 AM, Eric Dumazet wrote:
    >>>> On Tue, 2012-10-16 at 09:35 -0300, Ezequiel Garcia wrote:
    >>>>
    >>>>> Now, returning to the fragmentation. The problem with SLAB is that
    >>>>> its smaller cache available for kmalloced objects is 32 bytes;
    >>>>> while SLUB allows 8, 16, 24 ...
    >>>>>
    >>>>> Perhaps adding smaller caches to SLAB might make sense?
    >>>>> Is there any strong reason for NOT doing this?
    >>>>
    >>>> I would remove small kmalloc-XX caches, as sharing a cache line
    >>>> is sometime dangerous for performance, because of false sharing.
    >>>>
    >>>> They make sense only for very small hosts.
    >>>
    >>> That's interesting...
    >>>
    >>> It would be good to measure the performance/size tradeoff here.
    >>> I'm interested in very small systems, and it might be worth
    >>> the tradeoff, depending on how bad the performance is. Maybe
    >>> a new config option would be useful (I can hear the groans now... :-)
    >>>
    >>> Ezequiel - do you have any measurements of how much memory
    >>> is wasted by 32-byte kmalloc allocations for smaller objects,
    >>> in the tests you've been doing?
    >>
    >> Yes, we have some numbers:
    >>
    >> http://elinux.org/Kernel_dynamic_memory_analysis#Kmalloc_objects
    >>
    >> Are they too informal? I can add some details...
    >
    >
    >> They've been measured on a **very** minimal setup, almost every option
    >> is stripped out, except from initramfs, sysfs, and trace.
    >>
    >> On this scenario, strings allocated for file names and directories
    >> created by sysfs
    >> are quite noticeable, being 4-16 bytes, and produce a lot of fragmentation from
    >> that 32 byte cache at SLAB.
    >
    > The detail I'm interested in is the amount of wastage for a
    > "common" workload, for each of the SLxB systems. Are we talking a
    > few K, or 10's or 100's of K? It sounds like it's all from short strings.
    > Are there other things using the 32-byte kmalloc cache, that waste
    > a lot of memory (in aggregate) as well?
    >

    A more "Common" workload is one of the next items on my queue.


    > Does your tool indicate a specific callsite (or small set of callsites)
    > where these small allocations are made? It sounds like it's in the filesystem
    > and would be content-driven (by the length of filenames)?
    >

    That's right. And, IMHO, the problem is precisely that the allocation
    size is content-driven.


    Ezequiel


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