"David S. Miller" <davem@xxxxxxxxxx> writes:
> From: ebiederm@xxxxxxxxxxxx (Eric W. Biederman)
> Date: 11 Sep 2002 09:06:36 -0600
>
> "Martin J. Bligh" <mbligh@xxxxxxxxxxx> writes:
>
> > We can push about 420MB/s of IO out of this thing (out of that
> > theoretical 800Mb/s).
>
> Sounds about average for a P3. I have pushed the full 800MiB/s out of
> a P3 processor to memory but it was a very optimized loop.
>
> You pushed that over the PCI bus of your P3? Just to RAM
> doesn't count, lots of cpu's can do that.
>
> That's what makes his number interesting.
I agree. Getting 420MB/s to the pci bus is nice, especially with a P3.
The 800MB/s to memory was just the test I happened to conduct about 2 years
ago when I was still messing with slow P3 systems. It was a proof of
concept test to see if we could plug in an I/O card into a memory
slot.
On a current P4 system with the E7500 chipset this kind of thing is
easy. I have gotten roughly 450MB/s to a single myrinet card. And there
is enough theoretical bandwidth to do 4 times that. I haven't had a
chance to get it working in practice. When I attempted to run to gige
cards simultaneously I had some weird problem (probably interrupt
related) where adding additional pci cards did not deliver any extra
performance.
On a P3 to get writes from the cpu to hit 800MB/s you use the special
cpu instructions that bypass the cache.
My point was that I have tested the P3 bus in question and I achieved
a real world 800MB/s over it. So I expect that on the system in
question unless another bottleneck is hit, it should be possible to
achieve a real world 800MB/s of I/O. There are enough pci busses
to support that kind of traffic.
Unless the memory controller is carefully placed on the system though
doing 400+MB/s could easily eat up most of the available memory
bandwidth and reduce the system to doing some very slow cache line fills.
Eric
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