Re: [rfc] optimise unlock_page

On Tue, 2007-05-08 at 13:40 +0200, Nick Piggin wrote:
This patch trades a page flag for a significant improvement in the unlock_page
fastpath. Various problems in the previous version were spotted by Hugh and
Ben (and fixed in this one).

Comments?

--

Speed up unlock_page by introducing a new page flag to signal that there are
page waitqueue waiters for PG_locked. This means a memory barrier and a random
waitqueue hash cacheline load can be avoided in the fastpath when there is no
contention.

I'm not 100% familiar with the exclusive vs. non exclusive wait thingy
but wake_up_page() does __wake_up_bit() which calls __wake_up() with
nr_exclusive set to 1. Doesn't that mean that only one waiter will be
woken up ?

If that's the case, then we lose because we'll have clear PG_waiters but
only wake up one of them.

Waking them all would fix it but at the risk of causing other
problems... Maybe PG_waiters need to actually be a counter but if that
is the case, then it complicates things even more.

Any smart idea ?

Ben.


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Relevant Pages

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  • Re: [rfc] optimise unlock_page
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