On Mon, 5 Mar 2001, Matt Dillon wrote:
> :throughput. For example, on the PIII-850 (116MHz FSB and SDRAM, its
> :overclocked) here on my desk with 256KB L2 cache:
> :
> :dd if=/dev/zero of=/dev/null bs=512k count=4000
> :4000+0 records in
> :4000+0 records out
> :2097152000 bytes transferred in 8.229456 secs (254834825 bytes/sec)
> :
> :dd if=/dev/zero of=/dev/null bs=128k count=16000
> :16000+0 records in
> :16000+0 records out
> :2097152000 bytes transferred in 1.204001 secs (1741819224 bytes/sec)
> :
> :Now THAT is a significant difference. :-)
>
> Interesting. I get very different results with the 1.3 GHz P4. The
> best I seem to get is 1.4 GBytes/sec. I'm not sure what the L2 cache
> is on the box, but it's definitely a consumer model.
>
> dd if=/dev/zero of=/dev/null bs=512k count=4000
> 2097152000 bytes transferred in 2.363903 secs (887156520 bytes/sec)
>
> dd if=/dev/zero of=/dev/null bs=128k count=16000
> 2097152000 bytes transferred in 1.471046 secs (1425619621 bytes/sec)
>
> If I use lower block sizes the syscall overhead blows up the
> performance (it gets lower rather then higher). So I figure I don't
> have as much L2 as on your system.
IIRC, Intel is using a very different caching method on the P4 from
what we are used to on just about every other x86 processor we've
seen. Well, I can't remember if the data cache has changed much, but
the instruction cache has. I doubt the difference in instruction
cache behaviour would make a difference here though. Hmm.
I wonder if it makes any difference that I'm using -march=pentium
-mcpu=pentium for my CFLAGS? Actually, the kernel I tested on might
even be using -march/-mcpu=pentiumpro, since I only recently changed
it to =pentium to allow me to do buildworlds for another Pentium-class
machine. I did wonder the same thing a while back and did the same
test with and without the optimizations, and with pentiumpro opts the
big block size transfer rate went _down_ a little bit, which was odd.
I didn't compare with L2-cache-friendly blocks, though.
-- Chris Dillon - [EMAIL PROTECTED] - [EMAIL PROTECTED]
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