Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

[Linus Torvalds]

On Tue, 5 Jan 2010, Christoph Lameter wrote:
> 
> If the critical section protected by the spinlock is small then the
> delay will keep the cacheline exclusive until we hit the unlock. This
> is the case here as far as I can tell.

I hope somebody can time it. Because I think the idle reads on all the 
(unsuccessful) spinlocks will kill it.

Think of it this way: under heavy contention, you'll see a lot of people 
waiting for the spinlocks and one of them succeeds at writing it, reading 
the line. So you get an O(n^2) bus traffic access pattern. In contrast, 
with an xadd, you get O(n) behavior - everybody does _one_ acquire-for- 
write bus access.

Remember: the critical section is small, but since you're contending on 
the spinlock, that doesn't much _help_. The readers are all hitting the 
lock (and you can try to solve the O(n*2) issue with back-off, but quite 
frankly, anybody who does that has basically already lost - I'm personally 
convinced you should never do lock backoff, and instead look at what you 
did wrong at a higher level instead).

				Linus
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