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Re: [Xen-devel] suspend evetchn creation failure

On Tue, Mar 11, 2014 at 4:21 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
On Mon, 2014-03-03 at 12:24 -0500, Prateek Sharma wrote:
> Hi all,
>       During xm save, I xc_save throws up this error :
> "failed to get the suspend evtchn port".
>       From my understanding, the port is supposed to be stored by
> xenstore in /local/domain/, but I can't see the port being created using
> xenstore-ls either.

I think this event channel is created by the guest and written to
xenstore as part of support for the fast event channel based save
mechanism used by e.g. remus. In its absence save/suspend is triggered
via the traditional method of the toolstack writing commands to the
"control/shutdown" node.

IIRC the fast event channel based save stuff is not in mainline kernels,
so the tools message is correct but harmless.

CCing Shriram (Remus maintainer) in case I've got all the above wrong...

Ian is right. Mainline kernels don't have suspend event channel.
Unfortunately, not having suspend event channel results in a pretty big performance hit,
as each suspend call takes about 7-10ms and a resume takes 2-4ms. You are looking
at approx 10% loss of execution time just to suspend/resume the VM (assuming 
a 100ms checkpoint interval).

However, OpenSUSE kernels have suspend event channel support. That said I have
had issues with 3.7+ versions (IIRC), where the kernel starts crashing during 
recovery (which is basically a full resume).

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