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    SubjectRe: [patch] kmalloc_percpu -- 2 of 2
    Hi Andrew,

    On Wed, Dec 04, 2002 at 08:32:58PM -0800, Andrew Morton wrote:
    > Where in the kernel is such a large number of 4-, 8- or 16-byte
    > objects being used?

    Well, kernel objects may not be that small, but one would expect
    the per-cpu parts of the kernel objects to be sometimes small, often down to
    a couple of counters counting statistics.

    >
    > The slab allocator will support caches right down to 1024 x 4-byte
    > objects per page. Why is that not appropriate?

    Well, if you allocated 4-byte objects directly from the slab allocator,
    you aren't guranteed to *not* share a cache line with another object
    modified by a different cpu.

    >
    > Sorry, but you have what is basically a brand new allocator in
    > there, and we need a very good reason for including it. I'd like
    > to know what that reason is, please.

    The reason is concern about per-cpu allocation for small per-CPU
    parts (typically counters) of objects. If a driver has two counters
    counting reads and writes, you don't want to eat up a whole cacheline
    for them for each CPU per instance of the device.

    Thanks
    --
    Dipankar Sarma <dipankar@in.ibm.com> http://lse.sourceforge.net
    Linux Technology Center, IBM Software Lab, Bangalore, India.
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