Re: AIM7 40% regression with 2.6.26-rc1

[Linus Torvalds]

On Wed, 7 May 2008, Linus Torvalds wrote:
>
> Which, btw, is probably true. The BKL is normally held for short times, 
> and released (by that thread) for relatively much longer times. Which 
> is when spinlocks tend to work the best, even when they are fair (because 
> it's not so much a fairness issue, it's simply a cost-of-taking-the-lock 
> issue!)

.. and don't get me wrong: the old semaphores (and the new mutexes) should 
also have this property when lucky: taking the lock is often a hot-path 
case.

And the spinlock+generic semaphore thing probably makes that "lucky" 
behavior be exponentially less likely, because now to hit the lucky case, 
rather than the hot path having just *one* access to the interesting cache 
line, it has basically something like 4 accesses (spinlock, count test, 
count decrement, spinunlock), in addition to various serializing 
instructions, so I suspect it quite often gets serialized simply because 
even the "fast path" is actually about ten times as long!

As a result, a slow "fast path" means that the thing gets saturated much 
more easily, and that in turn means that the "fast path" turns into a 
"slow path" more easily, which is how you end up in the scheduler rather 
than just taking the fast path.

This is why sleeping locks are more expensive in general: they have a 
*huge* cost from when they get contended. Hundreds of times higher than a 
spinlock. And the faster they are, the longer it takes for them to get 
contended under load. So slowing them down in the fast path is a double 
whammy, in that it shows their bad behaviour much earlier.

And the generic semaphores really are slower than the old optimized ones 
in that fast path. By a *big* amount.

Which is why I'm 100% convinced it's not even worth saving the old code. 
It needs to use mutexes, or spinlocks. I bet it has *nothing* to do with 
"slow path" other than the fact that it gets to that slow path much more 
these days.

			Linus
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