Re: [Xen-devel] [RFC] use event channel to improve suspend speed

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Thu, May 10, 2007 at 03:13:10PM -0700, Brendan Cully wrote:
> The posted patch was a fairly conservative approach (backward
> compatible, equivalent to existing semantics). I've done some
> more experimental work that reduces the time for the final round to
> about 5ms. Here are the stats for 100 checkpoints:
> 
> avg: 5.62 ms, min: 3.96, max: 13.70, median: 4.86
> 
> It turns out the biggest remaining delay is (surprise!) xenstored. To
> get the above numbers I unwired xenstored from VIRQ_DOM_EXC and let
> xc_save bind to it.
> 
> Obviously this isn't a practical approach. I'd love to hear any ideas
> about the right way to avoid the xenstore penalty though. My current
> thought is that it might be possible to arrange to register a dynamic
> virq from xc_save into xen for a target domain, and then have xen fire
> it on suspend instead of DOM_EXC (iff it's installed, otherwise use
> the normal path).

It would be interesting to know what aspect of the xenstore interaction
is responsible for the slowdown. In particular, whether it is a fundamental
architectural constraint, or whether it is merely due to the poor performance
of the current impl. We already know from previous tests that XenD impl of
transactions absolutely kills performance of various XenD operations due to 
the vast amount of unneccessary I/O it does. 

If fixing the XenstoreD transaction code were to help suspend performance 
too, it might be a better option than re-writing all code which touches
xenstore. A quick test of putting /var/lib/xenstored on a ramdisk would
be a way of testing whether its the I/O which is hurting suspend time.

Dan
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