On Tuesday 05 October 2010, Krishna Kumar2 wrote:
> After testing various combinations of #txqs, #vhosts, #netperf
> sessions, I think the drop for 1 stream is due to TX and RX for
> a flow being processed on different cpus. I did two more tests:
> 1. Pin vhosts to same CPU:
> - BW drop is much lower for 1 stream case (- 5 to -8% range)
> - But performance is not so high for more sessions.
> 2. Changed vhost to be single threaded:
> - No degradation for 1 session, and improvement for upto
> 8, sometimes 16 streams (5-12%).
> - BW degrades after that, all the way till 128 netperf sessions.
> - But overall CPU utilization improves.
> Summary of the entire run (for 1-128 sessions):
> txq=4: BW: (-2.3) CPU: (-16.5) RCPU: (-5.3)
> txq=16: BW: (-1.9) CPU: (-24.9) RCPU: (-9.6)
>
> I don't see any reasons mentioned above. However, for higher
> number of netperf sessions, I see a big increase in retransmissions:
> _______________________________________
> #netperf ORG NEW
> BW (#retr) BW (#retr)
> _______________________________________
> 1 70244 (0) 64102 (0)
> 4 21421 (0) 36570 (416)
> 8 21746 (0) 38604 (148)
> 16 21783 (0) 40632 (464)
> 32 22677 (0) 37163 (1053)
> 64 23648 (4) 36449 (2197)
> 128 23251 (2) 31676 (3185)
> _______________________________________
This smells like it could be related to a problem that Ben Greear found
recently (see "macvlan: Enable qdisc backoff logic"). When the hardware
is busy, used to just drop the packet. With Ben's patch, we return -EAGAIN
to qemu (or vhost-net) to trigger a resend.
I suppose what we really should do is feed that condition back to the
guest network stack and implement the backoff in there.
Arnd
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