On Thu, 18 Oct 2007, Matthew Faulkner wrote:
> Hey all
>
> I'm using netperf to perform TCP throughput tests via the localhost
> interface. This is being done on a SMP machine. I'm forcing the
> netperf server and client to run on the same core. However, for any
> packet sizes below 523 the throughput is much lower compared to the
> throughput when the packet sizes are greater than 524.
>
> Recv Send Send Utilization Service Demand
> Socket Socket Message Elapsed Send Recv Send Recv
> Size Size Size Time Throughput local remote local remote
> bytes bytes bytes secs. MBytes /s % S % S us/KB us/KB
> 65536 65536 523 30.01 81.49 50.00 50.00 11.984 11.984
> 65536 65536 524 30.01 460.61 49.99 49.99 2.120 2.120
>
> The chances are i'm being stupid and there is an obvious reason for
> this, but when i put the server and client on different cores i don't
> see this effect.
>
> Any help explaining this will be greatly appreciated.
>
> Machine details:
>
> Linux 2.6.22-2-amd64 #1 SMP Thu Aug 30 23:43:59 UTC 2007 x86_64 GNU/Linux
>
> sched_affinity is used by netperf internally to set the core affinity.
I don't know if it's relevant, but note that 524 bytes + 52 bytes
of IP(20)/TCP(20)/TimeStamp(12) overhead gives a 576 byte packet,
which is the specified size that all IP routers must handle (and
the smallest value possible during PMTU discovery I believe). A
message size of 523 bytes would be 1 less than that. Could this
possibly have to do with ABC (possibly try disabling it if set)?
-Bill
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