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SubjectRe: Spinlock performance on Athlon MP (2.4)
   From: Alan Cox <alan@lxorguk.ukuu.org.uk>
Date: 30 Jul 2003 23:59:00 +0100

On Mer, 2003-07-30 at 22:50, Scott L. Burson wrote:
> First, and probably the reason you haven't heard more complaints about the
> problem, its severity is evidently dependent on the size of main memory. At
> 512MB it doesn't seem to be much of a problem (right, Mathieu?). At 2.5GB,
> which is what I have, it can be quite serious. For instance, if I start two
> `find' processes at the roots of different filesystems, the system can spend
> (according to `top') 95% - 98% of its time in the kernel. It even gets
> worse than that, but `top' stops updating -- in fact, the system can seem
> completely frozen, but it does recover eventually. Stopping or killing one
> of the `find' processes brings it back fairly quickly, though it can take a
> while to accomplish that.
Thats the well understood DMA bounce buffers problem.

It's definitely not the bounce buffers problem. I installed the patch and
it doesn't help (well, maybe it helps a little; it's hard to tell).

However, I have pretty strong evidence that it's not the spinlock handoff
time either. I wrote a small benchmark that starts two threads that do
nothing but hand two spinlocks back and forth. The Athlon runs it an order
of magnitude _faster_ than the P4 (5ns vs. 50ns, roughly, per handoff).

I'm fairly certain that lock contention is involved somehow, though.
Lockmeter reports that lock waiting is consuming about 35% of the CPU cycles
when the problem is happening. This isn't the 90% - 95% number I expected
-- the latter being the percentage of time spent in the kernel, as reported
by `top' -- but it's high enough to wonder about, and it may be artificially
low. Lockmeter has a spinlock that protects its data structures, and the
profile says 36% of the time is being spent in the routine that acquires
that lock. This suggests that lockmeter isn't counting that time.

One oddity pointed up by lockmeter is that `pagemap_lru_lock' is held by
`shrink_cache' some 85% of the time. This seems way too high, and I am
looking into it.

-- Scott
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