Re: [Qemu-devel] [PATCH 00/15] Make migration work with hotplug

[Yoshiaki Tamura]

Alex Williamson wrote:
> On Thu, 2010-06-24 at 09:04 -0600, Alex Williamson wrote:
> > On Thu, 2010-06-24 at 15:02 +0900, Yoshiaki Tamura wrote:
> > > Hi Alex,
> > >
> > > Is there additional overhead to save rams introduce by this series?
> > > If so, how much?
> >
> > Yes, there is overhead, but it's typically quite small.  If I migrate a
> > 1G VM immediately after I boot to a login prompt (lots of zero pages), I
> > get an overhead of 0.000076%.  That's only 226 extra bytes over the
> > 297164995 bytes otherwise transferred.  If I build a kernel on the guest
> > and migrate during the compilation, the overhead is 0.000019%.  The
> > overhead is tiny largely due to patch 12/15, which avoids sending the
> > block name if we're working within the same block as sent previously.
> > If I disable this optimization, the overhead goes up to 0.93% after boot
> > and 0.26% during a kernel compile.

Thank your for the detailed numbers and analysis!
If the overhead is at this level, I think it's worth introducing to support 
migration with hot plug devices.

> > Note that an x86 VM does a separate qemu_ram_alloc for memory above 4G,
> > which means in bigger VMs we may end up needing to resend the ramblock
> > name once in a while as we bounce between above and below 4G.  Worst
> > case for this could match the 0.26% above, but in my testing during a
> > kernel compile, this seems to increase the overhead to 0.000026% on a 6G
> > VM.

If we run a program which bounce between the region intentionally, we should get 
a number close to 0.26%, but the overhead is still low enough, and it shouldn't 
be a big problem.

> > I don't see any reason why we couldn't allocate all the ram in a
> > single qemu_ram_alloc call, so I'll add another patch to make that
> > change (which will also shorten the name to "pc.ram" for even less
> > overhead ;).  Thanks,

Hmm.  I didn't know about the qemu_ram_alloc over 4G issue.
If there isn't any reason, how about submitting a patch to fix it besides this 
series?

> FWIW, with this change, my migration during kernel compile on the 6G VM
> seems to be running just at 0.000019%-0.000020%, so that eliminates the
> penalty for bigger memory VMs.  Thanks,

It make sense:-)

Thanks,

Yoshi

> Alex