Re: CFQ read performance regression

[Miklos Szeredi <mszeredi@suse.cz>]

Corrado,

On Tue, 2010-04-20 at 22:50 +0200, Corrado Zoccolo wrote:
> can you give more information about the setup?
> How much memory do you have, what is the disk configuration (is this a
> hw raid?) and so on.

8G of memory 8-way Xeon CPU, fiber channel attached storage array (HP
HSV200).  I don't know the configuration of the array.

> > low_latency is set to zero in all tests.
> >
> > The layout difference doesn't explain why setting the scheduler to
> > "noop" consistently speeds up read throughput in 8-thread tiobench to
> > almost twice.  This fact alone pretty clearly indicates that the I/O
> > scheduler is the culprit.
> From the attached btt output, I see that a lot of time is spent
> waiting to allocate new request structures.
> > S2G               0.022460311   6.581680621  23.144763751          15
> Since noop doesn't attach fancy data to each request, it can save
> those allocations, thus resulting in no sleeps.
> The delays in allocation, though, may not be completely imputable to
> the I/O scheduler, and working in constrained memory conditions will
> negatively affect it.

I verified with the simple dd test that this happens even if there's no
memory pressure from the cache by dd-ing only 5G of files, after
clearing the cache.  This way ~2G of memory are completely free
throughout the test. 

> > I've also tested with plain "dd" instead of tiobench where the
> > filesystem layout stayed exactly the same between tests.  Still the
> > speed difference is there.
> Does dropping caches before the read test change the situation?

In all my tests I drop caches before running it.

Please let me know if you need more information.

Thanks,
Miklos