[Xen-users] tracking down the Xen+NFS slowness problem

["Mark Stier" <markstier@xxxxxxxxx>]

Hello,

I recently had some problems with Xen timer and tcp congestion
algorithms, especially tcp_vegas which crashes machines because it
seems to measure round trip times to be zero -- and then divides by
them. So I thought I play around with congestion algorithms a bit in
order to if they have an effect on NFS throughput.

In the end, I detected a totally different correlation:

I have mounted an NFS share within domU. The share is exported from
dom0 on the same physical machine. I have a routed/masq setup. Writing
to the NFS share happens at approx 11 MB/s (much less than phsically
possible), reading happens at 1.3 MB/s. dom0 and domU are not running
*anything* else. The CPU usage is at approx 0% inside dom0 and domU.
Now what I found out: the read speed goes up dramatically when I start
bzip2 in dom0!

Best regards,
Mark

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