SMP Strangeness

[Simon Fowler]

On Fri, Jun 28, 2002 at 11:50:51AM +1000, Brett Worth wrote:
> I've just run some very rudimentary performance testing on a server in our
> office and see some poor SMP results.  Can anyone suggest why?
> 
> Kernel: 2.4.18
> Disk: Ultra Wide SCSI on AIC7XXX
> CPUs: 2 x 700Mhz PIII
> MB: Intel L440GX+
> RAM: 4 x 64Mb = 256M
> 
> I'm just doing kernel compiles and timing them.
> 
> time (make -jN dep; make -jN clean; make -jN ; make -jN module; ; make -jN install ; make -jN modules_install)
> 
> N=1	264.79s real   251.83s user    20.74s system
> N=2	304            502             46
> N=3     402            660             53
> 
> Is this a memory bandwidth problem?  The machine is otherwise idle.  I also
> ran the same command (j1) once before starting incase the cache made any
> difference.
> 
> BTW:
> # hdparm -T -t /dev/sda
> 
> /dev/sda:
>  Timing buffer-cache reads:   128 MB in  0.79 seconds =162.03 MB/sec
>  Timing buffered disk reads:  64 MB in  4.12 seconds = 15.53 MB/sec

This last one is rather slow - my 7200rpm IDE disk gets 22-23MB/s
off the platters, and I'd expect a good scsi disk to be a good bit
faster . . . 
> 
> 
What filesystem are you using with this? ext3 is currently not at
all well optimised for SMP, so if you're using that you may be
seeing the results . . . Try mounting with ext2 instead, and see if
that fixes things.

Other than that, I can't think of anything right now, but you
definitely shouldn't be seeing that kind of slowdown.

Simon

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