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SubjectRe: Intel P6 vs P7 system call performance
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Mike Hayward <hayward@loup.net> wrote:

> I have been benchmarking Pentium 4 boxes against my Pentium III laptop
> with the exact same kernel and executables as well as custom compiled
> kernels. The Pentium III has a much lower clock rate and I have
> noticed that system call performance (and hence io performance) is up
> to an order of magnitude higher on my Pentium III laptop. 1k block IO
> reads/writes are anemic on the Pentium 4, for example, so I'm trying
> to figure out why and thought someone might have an idea.
>
> Notice below that the System Call overhead is much higher on the
> Pentium 4 even though the cpu runs more than twice the speed and the
> system has DDRAM, a 400 Mhz FSB, etc. I even get pretty remarkable
> syscall/io performance on my Pentium III laptop vs. an otherwise idle
> dual Xeon.
>
> See how the performance is nearly opposite of what one would expect:
...
> M-Pentium III 850Mhz Sys Call Rate 433741.8
> Pentium 4 2Ghz Sys Call Rate 233637.8
> Xeon x 2 2.4Ghz Sys Call Rate 207684.2
...[other benchmark deleted]...
> Any ideas? Not sure I want to upgrade to the P7 architecture if this
> is right, since for me system calls are probably more important than
> raw cpu computational power.

You're assuming that ALL operations in a P4 are linearly faster than
a P-III. This is definitely not the case.

A P4 has a much longer pipeline (for a many cases, considerably
longer than the diagrams imply) than the P-III, and in particular
it has a much longer latency in handling mode transitions.

The results you got don't surprise me whatsoever. In fact the raw
system call transition instructions are likely 5x slower on the
P4.

--
Erich Stefan Boleyn <erich@uruk.org> http://www.uruk.org/
"Reality is truly stranger than fiction; Probably why fiction is so popular"
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