Date: Tue, 25 Feb 1997 18:30:56 +1100 From: Bruce Evans <bde@zeta.org.au> To: asami@vader.cs.berkeley.edu, jin@george.lbl.gov, mark@quickweb.com Cc: freebsd-hardware@freebsd.org, kuku@gilberto.physik.rwth-aachen.de, robsch@robkaos.ruhr.de Subject: Re: Memory speed of P6-200 (256k) Message-ID: <199702250730.SAA00656@godzilla.zeta.org.au>
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>Do not waste time to play this game. The "dd" is O.S. dependent code.
No, dd is very machine-independent. It just loops calling read() and
write() with the specified block size. However, the implementation
of /dev/zero is very machine-dependent. FreeBSD happens to have an
implementation that copies memory in a straightforward way, so the speed
reported by dd is closely related to the memory write bandwidth. The
read bandwidth doesn't matter much because most reads are from the
cache.
>It does not give you what is real memory speed on your system. The result
>from dd is really depended on the O.S. you are running. If you run 2.2 or
>higher, you will get much better performance than 2.1.x.
There isn't much difference unless you have a P5 and the P5-optimized
copyout routine is not disabled.
>440FX does have worse memory speed than Triton-{I,II}; even though P6 has
>much better CPU speed, but the PCI controller (440FX) is worse.
That would explain why a P6 is more than twice as slow as a P5 for large
writes. It is inherently about twice as slow because P6's read cache
lines before writing to them. Read-before-write is useful if the memory
is read after writing to it, but the dd benchmark never does that.
Bruce
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