linux-kernel - Re: [PATCH 0/6] x86: reduce paravirtualized spinlock overhead

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Message-ID: <20150517053036.GB16607@gmail.com>
Date:	Sun, 17 May 2015 07:30:36 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Juergen Gross <jgross@...e.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
	tglx@...utronix.de, mingo@...hat.com,
	xen-devel@...ts.xensource.com, konrad.wilk@...cle.com,
	david.vrabel@...rix.com, boris.ostrovsky@...cle.com,
	chrisw@...s-sol.org, akataria@...are.com, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org, gleb@...nel.org,
	pbonzini@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH 0/6] x86: reduce paravirtualized spinlock overhead


* Juergen Gross <jgross@...e.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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