Re: Why throughput increases as MTU size is increased

From: Stephen Satchell (list@fluent2.pyramid.net)
Date: Wed Apr 30 2003 - 10:30:51 EST

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In reply to: Shesha@asu.edu: "Why throughput increases as MTU size is increased"
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At 07:10 AM 4/30/03 -0700, Shesha@asu.edu wrote:
> When I measure the performance of iSCSI on XScale with MTU size = 1500
> bytes,
>a throughput of 32 Mbps was observed. As the MTU size was increased, the
>throughput also increased.
>1500 -> 32 Mbps
>3000 -> 56
>4500 -> 80
>6000 -> 100
>7500 -> 108
>9000 -> 108

The non-linear increase as you change MTU is the dead give-away: the
slow-start algorithm is working just fine. The reason you didn't get
double the throughput as you doubled the MTU is that the slow-start
algorithm ramps based on packet count, so as you increase the MTU the
packet rate slope remains reasonably constant. If you have the source
code, you can turn off slow-start using setsocketopt(), and see what happens.

Have you considered trying a longer file?

-- X -> unknown; Spurt -> drip of water under pressure Expert -> X-Spurt -> Unknown drip under pressure.

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