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Re: [Xen-devel] pthread_mutex_lock() and Xenstored

To: Peter Teoh <htmldeveloper@xxxxxxxxx>
Subject: Re: [Xen-devel] pthread_mutex_lock() and Xenstored
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Thu, 13 Sep 2007 16:21:40 +0100
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On Fri, Aug 31, 2007 at 01:51:18PM +0800, Peter Teoh wrote:
> pthread_mutex_lock() are not async-signal safe (ref:
> http://www.gelato.org/pdf/Illinois/gelato_IL2004_libatomic_boehm.pdf)
> but I still see that it is used extensively in xenstored implementation
> (eg, xs.c).

So what ?  The pthread_mutex_lock calls aren't used inside any signal
handlers, so the question of whether its async-signal safe is irrelevant

> Moreover, pthread_mutex_lock() suffered a higher performance penalty 
> than other synchronization option.   For a one-time effort like domain
> creation this is ok, but Xenstore is used repeatedly to access data, 
> and therefore performance could potentially be enhanced.

That presentation giving figures comparing pthread_mutex_lock with other
primitives is useless. It doesn't say what OS / version it was tested on,
or what those figures are measuring. Linux NPTL (2.6.x) threading has radically
different (better) pthread_mutex performance characteristics that the older
LinuxThreads impl (2.4.x) for example, and that's ignoring any other non-Linux
OS' impl of pthreads. The performance characteristics of a contended mutex lock,
vs an uncontended lock are also completely different & not considered in those

Finally there's no performance profile data to suggest that mutex locking
is even remotely relevant to xenstore performance. There is however data to
show that the huge amount of I/O xenstored does hurts performance.

By all means consider how to improve the performance of xenstored, but do
so based on *measured* facts about where the current problems are, rather
than bullet points in a presentation unrelated to Xen.

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