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

[Alex Williamson]

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.

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.  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,

Alex