[libvirt] qemu-kvm spending time in do_info_migrate() during virDomainSave(); 50ms polling too fast?

[Matthias Bolte]

2010/5/3 Charles Duffy <charles at dyfis.net>:
> ...or more particularly, in ram_bytes_remaining() called by do_info_migrate();
> oddly, this is much more pronounced when running with <emulator> pointing at a
> shim prepending -no-kvm-irqchip to the invoked command line.
>
> This VM was intended to be paused for the save event (if my software was doing
> its job correctly), so we shouldn't be spending time running the guest CPU and
> writing updates for already-once-written blocks.
>
> I'm seeing much more CPU time spent inside qemu-kvm than in the exec'd lzop
> process compressing and writing the data stream; on attaching gdb and taking
> some stack traces to sample where execution time was spent, it appeared that we
> were spending our time responding to requests from the monitor.
>
> The question then -- is the 50ms poll in qemuDomainWaitForMigrationComplete
> (called from qemudDomainSave) perhaps too frequent?
>
> Thanks!

What's the libvirt version?

Maybe the "Poll for migration end every 50ms instead of 50us" [1]
commit (part of 0.8.1) fixes your problem, if you're currently using
libvirt < 0.8.1.

[1] http://libvirt.org/git/?p=libvirt.git;a=commit;h=e2c059485cf062bf1f9066237039b6e398a2fe33

Matthias