Date: Sun, 17 May 2015 07:30:36 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Juergen Gross <jgross@suse.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com,
        xen-devel@lists.xensource.com, konrad.wilk@oracle.com,
        david.vrabel@citrix.com, boris.ostrovsky@oracle.com,
        chrisw@sous-sol.org, akataria@vmware.com, rusty@rustcorp.com.au,
        virtualization@lists.linux-foundation.org, gleb@kernel.org,
        pbonzini@redhat.com, kvm@vger.kernel.org
Subject: Re: [PATCH 0/6] x86: reduce paravirtualized spinlock overhead
Message-ID: <20150517053036.GB16607@gmail.com>
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* Juergen Gross <jgross@suse.com> wrote:

> On 05/05/2015 07:21 PM, Jeremy Fitzhardinge wrote:
> >On 05/03/2015 10:55 PM, Juergen Gross wrote:
> >>I did a small measurement of the pure locking functions on bare metal
> >>without and with my patches.
> >>
> >>spin_lock() for the first time (lock and code not in cache) dropped from
> >>about 600 to 500 cycles.
> >>
> >>spin_unlock() for first time dropped from 145 to 87 cycles.
> >>
> >>spin_lock() in a loop dropped from 48 to 45 cycles.
> >>
> >>spin_unlock() in the same loop dropped from 24 to 22 cycles.
> >
> >Did you isolate icache hot/cold from dcache hot/cold? It seems to me the
> >main difference will be whether the branch predictor is warmed up rather
> >than if the lock itself is in dcache, but its much more likely that the
> >lock code is icache if the code is lock intensive, making the cold case
> >moot. But that's pure speculation.
> >
> >Could you see any differences in workloads beyond microbenchmarks?
> >
> >Not that its my call at all, but I think we'd need to see some concrete
> >improvements in real workloads before adding the complexity of more pvops.
> 
> I did another test on a larger machine:
> 
> 25 kernel builds (time make -j 32) on a 32 core machine. Before each
> build "make clean" was called, the first result after boot was omitted
> to avoid disk cache warmup effects.
> 
> System time without my patches: 861.5664 +/- 3.3665 s
>                with my patches: 852.2269 +/- 3.6629 s

So how does the profile look like in the guest, before/after the PV 
spinlock patches? I'm a bit surprised to see so much spinlock 
overhead.

Thanks,

	Ingo
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