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.
-- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
