Re: [Xen-devel] Xen cluster n/w performance (again!)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> > Hey everyone,
> > 
> > A few weeks back there was some discussion on Xen's I/O performance in
> > clusters on the list. I did some experiments today myself using iperf
> > (not ttcp):
> > 
> > o Xen dom0 talking to another machine in the cluster running native
> > linux: b/w around 904Mbps, thats nice :)
> > 
> > o Xen VM (running on the same machine as the dom0 in the previous
> > experiment) talking to another machine running native linux (again,
> > same as in previous experiment) only achieves 128 Mbps
> > 
> > I read on the list that you folks at Cambridge got upto 800+ Mbps
> > across VMs? Did you guys do any special optimizations or set any
> > special parameters? I read something about socket buffer size?

One thing you might want to try is to change a line in the file
linux-2.6.9-xenU/drivers/xen/netfront/netfront.c.
From:
#define RX_MIN_TARGET 8
To:
#define RX_MIN_TARGET NETIF_RX_RING_SIZE

One possibility is that dynamic buffer sizing is dropping some packets
and causing TCP to crap itself.

If this improves things then I'll have to be much more careful about
shrinking the buffers, and/or add a config option to disable the
resizing completely. 

 -- Keir


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