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. > > 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,
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, Alex