Re: latency doubled on tg3 device from 2.6.11 to 2.6.12

Eric Dumazet
2005-09-01T21:46:50Z

Rick Warner a écrit :
> Hello,
> We have been testing latency and bandwidth using our proprietary MPI link
> checker tool (http://www.microway.com/mpilinkchecker.html) and have found
> that the latency increased from ~25ms to ~45ms going from 2.6.11 to 2.6.12.
> 2.6.13 has the same result. We also tried the latest bcm5700 from broadcom
> (8.2.18) and got the same ~45ms latencies. This was tried on several
> different opteron and em64t motherboards.
>
> We see 20-25ms latencies with the e1000 driver (with some module options) on
> all 3 kernel versions. For those interested, the e1000 options used are:
>
> InterruptThrottleRate=0 RxIntDelay=0 TxIntDelay=0 RxAbsIntDelay=0
> TxAbsIntDelay=0
>
> Digging through source, it seems that a new locking mechanism for tg3 was put
> in place in 2.6.12. Is this the likely cause? What can we do to restore our
> lower latency?
>
>

Could you please define latency ?

tg3 driver was recently updated to use coalescing.

So when the nic receives one frame, it may delay up to XXXX us ( XXXX < 1024)
the interrupt.

But 25 ms is far more than 1024 us, so I dont think this coalescing can
explain your problem.

The HZ change from 1000 to 250 could be the root of the problem ?

Using a simple ping between 2 machines with tg3, I get less than 1ms time.

Eric