Le mardi 30 novembre 2010 à 15:42 +0000, Ben Hutchings a écrit :

On Tue, 2010-11-30 at 22:55 +0900, Simon Horman wrote:

quoted

The only other parameter that seemed to have significant effect was to
increase the mtu.  In the case of MTU=9000, GRO seemed to have a negative
impact on throughput, though a significant positive effect on CPU
utilisation.

[...]

Increasing MTU also increases the interval between packets on a TCP flow
using maximum segment size so that it is more likely to exceed the
difference in delay.

GRO really is operational _if_ we receive in same NAPI run several
packets for the same flow.

As soon as we exit NAPI mode, GRO packets are flushed.

Big MTU --> bigger delays between packets, so big chance that GRO cannot
trigger at all, since NAPI runs for one packet only.

One possibility with big MTU is to tweak "ethtool -c eth0" params
rx-usecs: 20
rx-frames: 5
rx-usecs-irq: 0
rx-frames-irq: 5
so that "rx-usecs" is bigger than the delay between two MTU full sized
packets.

Gigabit speed means 1 nano second per bit, and MTU=9000 means 72 us
delay between packets.

So try :

ethtool -C eth0 rx-usecs 100

to get chance that several packets are delivered at once by NIC.

Unfortunately, this also add some latency, so it helps bulk transferts,
and slowdown interactive traffic