Re: Buffered I/O slowness

[Jesse Barnes]

On Friday, October 29, 2004 5:30 pm, Andrew Morton wrote:
> Jesse Barnes <jbarnes@xxxxxxxxxxxx> wrote:
> > > I'm not sure that we know what's going on yet.  I certainly don't.  The
> > > above numbers look good, so what's the problem???
> >
> > The numbers are ~1/3 of what the machine is capable of with direct I/O.
>
> Are there CPU cycles to spare?  If you have just one CPU copying 1GB/sec
> out of pagecache, maybe it is pegged?

Hm, I thought I had more CPU to spare, but when I set the readahead to a large 
value, I'm taking ~100% of the CPU time on the CPU doing the read.  ~98% of 
that is system time.  When I run 8 copies (this is an 8 CPU system), I get 
~4GB/s and all the CPUs are near fully busy.  I guess things aren't as bad as 
I initially thought.

Thanks,
Jesse
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