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Re: [Qemu-devel] [PATCH] Add a memory barrier to DMA functions

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] Add a memory barrier to DMA functions
Date: Tue, 22 May 2012 15:03:59 +0300

On Tue, May 22, 2012 at 09:41:41PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2012-05-22 at 14:14 +0300, Michael S. Tsirkin wrote:
> > > The baseline is the laptop without kvm talking to the server. The
> > > TCP_STREAM test results are:
> > 
> > It's not a good test. The thing most affecting throughput results is
> > how
> > much CPU does you guest get. So as a minumum you need to measure CPU
> > utilization on the host and divide by that. 
> The simple fact that we don't reach the baseline while in qemu seems to
> be a reasonably good indication that we tend to be CPU bound already so
> it's not -that- relevant. It would be if we were saturating the network.
> But yes, I can try to do more tests tomorrow, it would be nice if you
> could contribute a proper test protocol (or even test on some machines)
> since you seem to be familiar with those measurements (and I have a very
> limited access to x86 gear ... basically just my laptop).
> Cheers,
> Ben.

I have a deja vu. Amos sent perf results when you argued about
exactly the same issue in guest virtio. Delta was small but
measureable. At the moment I have no free time or free hardware
to redo the same work all over again. It's a well known fact that
actual memory barrier is slow on x86 CPUs. You can't see
it with network on your laptop? Write a microbenchmark.


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