High load (25% system) on 6450, 1+1+1=3 ;-)

Matt_Domsch@Dell.com Matt_Domsch at Dell.com
Tue Mar 19 12:36:01 CST 2002


> > > I am running stock 2.4.17 on a 6450. I have recently 
> upgraded to 4 CPUs,

How many did you have before?

>              total       used       free     shared    buffers     cached
> Mem:       3996696    3890456     106240          0    75036    3300428
> -/+ buffers/cache:     514992    3481704
> Swap:       530104      32792     497312

You've got nearly 4GB RAM.  With the stock 2.4.17 kernel, you're going to
see a lot of bounce buffer usage, which could easily account for the
increased CPU utilization.

Essentially, memory between 1GB and 4GB that is used for I/O buffers, is
getting "bounced", that is, copied into memory between 0-1GB, then the I/O
happens, then that low memory copied back to the high memory.  See, two
extra copies for every I/O.  Not ideal, especially when the controller can
DMA to/from high memory directly, there shouldn't be the need for this
bouncing.

There's work being done at Red Hat and elsewhere to decrease the need for
bounce buffers, so your performance would increase.  The
announced-but-not-yet-released Red Hat Linux Advanced Server product has
significant improvements in this area, and of course there's the whole block
I/O rewrite (bio) in the 2.5.x kernel series to look forward to.

There's also SMP scalability improvements (Ingo Molnar's O(1) scheduler)
happening in the kernel, so if you've recently gone from 1 to 4 CPUs, you'll
probably see performance improvements with work too.

Thanks,
Matt

--
Matt Domsch
Sr. Software Engineer
Dell Linux Solutions www.dell.com/linux
Linux on Dell mailing lists @ http://lists.us.dell.com
#1 US Linux Server provider for 2001!




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