Re: iSCSI latency issue

[Vladislav Bolkhovitin]

Shachar f, on 11/25/2009 07:57 PM wrote:
> I'm running open-iscsi with scst on Broadcom 10Gig network and facing
> write latency issues.
> When using netperf over an idle network the latency for a single block
> round trip transfer is 30 usec and with open-iscsi it is 90-100 usec.
>
> I see that Nagle (TCP_NODELAY) is disabled when openning socket on the
> initiator side and I'm not sure about the target side.
> Vlad, Can you elaborate on this?

TCP_NODELAY is always enabled in iSCSI-SCST. You can at any time have
latency statistics on the target side by enabling
CONFIG_SCST_MEASURE_LATENCY (see README). Better also enable
CONFIG_PREEMPT_NONE to not count CPU scheduler latency.

> Are others in the mailing list aware to possible environment changes
> that effext latency?
>
> more info -
> I'm running this test with Centos5.3 machines with almost latest open-iscsi.
>
> Thanks,
> Shachar

[Shachar f]

I'm running open-iscsi with scst on Broadcom 10Gig network and facing write latency issues.

When using netperf over an idle network the latency for a single block round trip transfer is 30 usec and with open-iscsi it is 90-100 usec.

 

I see that Nagle (TCP_NODELAY) is disabled when openning socket on the initiator side and I'm not sure about the target side. 

Vlad, Can you elaborate on this?

 

[Bart Van Assche]

Please make sure that interrupt coalescing has been disabled -- see
also ethtool -c.

Bart.

[Pasi Kärkkäinen]

Did you ever figure out the problem for the additional latency?

-- Pasi