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List:       kvm
Subject:    RE: Virtio network performance problem
From:       "Adrian Schmitz" <aschmitz () lehighgas ! com>
Date:       2008-12-11 17:03:33
Message-ID: 3D753CDC7D3A054CADC4DC2DF4C9F665020278A3 () LGC-MX01 ! corp ! lehighgas ! local
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> I think it is an unsync tsc problem.
> First, make sure you pin all of the process threads. There is thread
> per
> vcpu + io thread +more non relevant.
> You can do it by adding the taskset before the cmdline.
> Second, you said that you use smp guest. So windows also sees unsync
> tsc.
> So, either test with UP guest or learn how to pin windows receiving
> ISR,
> DPC and the user app.
> 
> Well, testing on Intel or newer AMD is another option.
> I tested it again now on Intel with UP guest and there is no such a
> problem.
> Hope to test it next week on AMD SMP guest.
> 
> Regards,
> Dor

I made sure to pin all 5 threads from my guest kvm process to two cores
on a single socket, and also tried pinning them all to a single core,
but neither made a difference. I then powered down all the cores (using
the /sys entries) until only two cores on a single socket were left,
which made no improvement. I then powered all cores down except one (and
verified that only the one showed up in /proc/cpuinfo). Naturally the
guest slowed a bit, but the erratic and negative pings from the guest to
the host br0 were still there. If I boot the guest with smp 1 on the kvm
command line, the ping times are fine. It's only when I start the guest
with 2 VCPUs that the ping times are erratic and some pings are
negative.

Just to recap, this is on a brand new Dell Poweredge R805 with dual
quad-core Opteron 2350s, with KVM-79 built against the CentOS-packaged
2.6.18 kernel. The guest in question is Windows Server 2003 x64 SP2 with
2 VCPUs.

I couldn't think of anything else to try, so I loaded up another server,
a new Dell Poweredge 2950 with dual quad-core Xeon E5430s. I installed
and configured kvm-79 following the exact same process I used to
configure it on the AMD host. When I boot the _same_ guest (it's in
iscsi) on the PE2950/Intel host, the ping times are perfect.

I'm interested to know why this is (before I purchase any more AMD
servers) but my priority right now is trying to figure out what I'm
doing wrong that is causing virtio networking to be extremely slow
(usually around 55Mb/s) on every host I've built it on. I'm sure I'm
doing something wrong and I just can't place it. I'll start another
thread for that, though, because it seems to be a different problem. In
the meantime I'll just keep this guest on my Intel servers, I guess.
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