Re: [Qemu-devel] [RFC PATCH RDMA support v1: 10/13] introduce new command migrate_check_for_zero

[Paolo Bonzini]

Il 11/04/2013 15:24, Michael R. Hines ha scritto:
> That's very accurate. Zero page scanning *after* the bulk phase
> is not very helpful in general.
> 
> Are we proposing to skip is_dup_page() after the bulk phase
> has finished?

No, I'm saying that is_dup_page() should not be a problem.  I'm saying
it should only loop a lot during the bulk phase.  The only effect I can
imagine after the bulk phase is one cache miss.

Perhaps the stress-test you're using does not reproduce realistic
conditions with respect to zero pages.  Peter Lieven benchmarked real
guests, both Linux and Windows, and confirmed the theory that I
mentioned upthread.  Almost all non-zero pages are detected within the
first few words, and almost all zero pages come from the bulk phase.

Considering that one cache miss, RDMA is indeed different here.  TCP
would have this cache miss later anyway, RDMA does not.  Let's say 300
cycles/miss; at 2.5 GHz that is 300/2500 microseconds, i.e 0.12
microseconds per page.  This would say that we can run is_dup_page on 30
GB worth of nonzero pages every second or more.  Ok, the estimate is
quite generous in many ways, but is_dup_page() is only a bottleneck if
it can do less than 5 GB/s.

Paolo