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    SubjectRe: [RFC] non-refcounted pages, application to slab?
    Nick Piggin a écrit :
    > If an allocator knows exactly the lifetime of its page, then there is no
    > need to do refcounting or the final put_page_zestzero (atomic op + mem
    > barriers).
    > This is probably not worthwhile for most cases, but slab did strike me
    > as a potential candidate (however the complication here is that some
    > code I think uses the refcount of underlying pages of slab allocations
    > eg nommu code). So it is not a complete patch, but I wonder if anyone
    > thinks the savings might be worth the complexity?
    > Is there any particular code that is really heavy on slab allocations?
    > That isn't mostly handled by the slab's internal freelists?

    Hi Nick

    After reading your patch, I have some crazy idea.

    The atomic op + mem barrier you want to avoid could be avoided more generally
    just by changing atomic_dec_and_test(atomic_t *v).

    If the current thread is the last referer (refcnt = 1), then it can safely set
    the value to 0 because no other CPU can be touching the value (or else there
    must be a bug somewhere, as the 'other cpu' could touch the value just after
    us and we could free an object still in use by 'other cpu'

    Something like :

    --- include/asm-i386/atomic.h.orig 2006-01-25 12:11:46.000000000 +0100
    +++ include/asm-i386/atomic.h 2006-01-25 12:13:07.000000000 +0100
    @@ -130,6 +130,13 @@
    printk("BUG: atomic counter underflow at:\n");
    +#ifdef CONFIG_SMP
    + /* avoid an atomic op if we are the last user of this atomic */
    + if (atomic_read(v) == 1) {
    + atomic_set(v, 0); /* not a real atomic op on most machines */
    + return 1;
    + }
    __asm__ __volatile__(
    LOCK_PREFIX "decl %0; sete %1"
    :"=m" (v->counter), "=qm" (c)

    Thank you

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      Last update: 2006-01-25 11:29    [W:0.022 / U:24.904 seconds]
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