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    SubjectRe: [PATCH] kmalloc_percpu
    Date
    In message <20030505014729.5db76f70.akpm@digeo.com> you write:
    > Rusty Russell <rusty@rustcorp.com.au> wrote:
    > >
    > > This is the kmalloc_percpu patch.
    >
    > How does it work? What restrictions does it have, and
    > what compromises were made?
    >
    > +#define PERCPU_POOL_SIZE 32768
    >
    > What's this?

    OK. It has a size restriction: PERCPU_POOL_SIZE is the maximum total
    kmalloc_percpu + static DECLARE_PER_CPU you'll get, ever. This is the
    main downside. It's allocated at boot.

    The __alloc_percpu allocator is extremely space efficient, by not
    insisting on cache-line aligning everything: __alloc_percpu(SIZE)
    overhead is sizeof(int), plus SIZE bytes (rounded up to alignment
    requirements) removed from per-cpu pool.

    The allocator is fairly slow: they're not expected to be thrown around
    like candy.

    > The current implementation of kmalloc_per_cpu() turned out to be fairly
    > disappointing because of the number of derefs which were necessary to get at
    > the data in fastpaths. How does this implementation compare?

    It uses the same method as the static ones, so it's a single addition
    of __per_cpu_offset (assuming arch doesn't override implementation).
    This is a requirement for modules to use them (which was my aim: the
    other side effects are cream).

    Hope that clarifies,
    Rusty.
    --
    Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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