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SubjectRE: Problem with e100 driver and latency on different packet sizes
> I've attached a small program to measure latency of 
> round-trip time on UDP. If I send 85-byte packets between
> two of my machines, I get 170us round-trip latency. If I
> send 86-byte packets, I get 1329us latency.
> This seems quite odd. If I test on the eepro100 driver, I
> get expected linear increase in round-trip time as the packet
> size increases, and it never gets close to 1300us.

This sounds like a side-effect of the "CPU Cycle Saver" feature to
bundle Rx packets per one interrupt. See
Documentation/networking/e100.txt. I haven't tried your setup, but I
would guess that you can play around with the BundleSmallFr module
parameter, or better yet, if you want the lowest latencies, turn off CPU
Saver (ucode=0).

CPU Saver trades latency for reduced interrupts, resulting in CPU
savings, hence the name.

Hope this helps.

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