Subject: Re: Early SPECWeb99 results on 2.5.33 with TSO on e1000

From: [email-masked] (Eric W. Biederman)
Date: 11 Sep 2002 09:31:53 -0600

"David S. Miller" <[email-masked]> writes:

>    From: [email-masked] (Eric W. Biederman)
>    Date: 11 Sep 2002 09:06:36 -0600
> 
>    "Martin J. Bligh" <[email-masked]> 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