Re: latency doubled on tg3 device from 2.6.11 to 2.6.12

[Eric Dumazet]

Rick Warner a écrit :
> On Thursday 01 September 2005 05:46 pm, Eric Dumazet wrote:
>
> > 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
>
>
> Our mpi link checker tool does a 1 way transfer of a 0 byte message (+ header) 
> and times it to each system (in addition to other tests).  All systems in a 
> cluster are showing the same high latency.  The numbers I gave were supposed 
> to be us, I used the wrong unit by accident.
>
> Low latency is often essential to clustered applications.  While a delay of up 
> to 1024 us doesn't affect regular communications too much, it may severely 
> affect mpi jobs.
>
> Thanks.

OK

tg3 driver set a rx-usecs of 20us per default.
This is certainly the root of your problem.

Try to lower it on mpi links ?

ethtool -C eth0 rx-usecs 0

Eric
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