Re: I/O descriptor ring size bottleneck?

[Nivedita Singhvi <niv@us.ibm.com>]

Diwaker Gupta wrote:

> Hi everyone,
> 
> I'm doing some networking experiments over high BDP topologies. Right
> now the configuration is quite simple -- two Xen boxes connected via a
> dummynet router. The dummynet router is set to limit bandwidth to
> 500Mbps and simulate an RTT of 80ms.
> 
> I'm using the following sysctl values:
> net.ipv4.tcp_rmem = 4096        87380   4194304
> net.ipv4.tcp_wmem = 4096        65536   4194304

If you're trying to tune TCP traffic, then you might
want to increase the default TCP socket size (87380) above
as well, as simply increasing the core size won't
help there.

> Now if I run 50 netperf flows lasting 80 seconds (1000RTTs) from
> inside  a VM on one box talking to the netserver on the VM on the
> other box, I get a per flow throughput of around ~2.5Mbps (which
> sucks, but lets ignore the absolute value for the moment).
> 
> If I run the same test, but this time from inside dom0, I get a per
> flow throughput of around 6Mbps.

Could you get any further information on your test/data?
Which netperf test were you running, btw?

> I'm trying to understand the difference in performance. It seems to me
> that the I/O descriptor ring sizes are hard coded to 256 -- could that
> be a bottleneck here? If not, have people experience similar problems?

Someone on this list had posted that they would be getting
oprofile working soon - you might want to retry your testing
with that patch.

thanks,
Nivedita



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