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Date: Mon, 19 Nov 2007 19:57:06 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: Ivan Voras <ivoras@freebsd.org>
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Cc: freebsd-stable@freebsd.org
Subject: Re: 2 x quad-core system is slower that 2 x dual core on FreeBSD
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Ivan Voras wrote:
> Kris Kennaway wrote:
> 
>> My guess is that you're hitting contention in the TCP send path, but I
>> missed the start of this conversation so I don't know what problems you
>> are seeing.
> 
> Here it is:
> http://lists.freebsd.org/pipermail/freebsd-stable/2007-November/038371.html
> 
> there's some mutex profiling there.

OK, but that is on 6.x and only (I guess) on the 8 core machine.  What 
is needed is at least a comparison of the two machines under identical 
conditions, and preferably on 7.0.

It looks like the most serious issue might be due to lockmgr contention, 
but on 6.x lockmgr usage is not profiled directly (it only shows up via 
secondary mutexes).  Comparing two 7.0 traces should help to shed more 
light on the subject.

> Offtopic: How to you read output from debug.mutex.prof.stats? Is
> cnt_lock the number of times a lock has been attempted to be acquired
> but it wasn't available?

It's explained in the MUTEX_PROFILING(9) manpage (LOCK_PROFILING(9) on 7.0)

              cnt_hold  The number of times the lock was held and another
                        thread attempted to acquire the lock.

              cnt_lock  The number of times the lock was already held 
when this
                        point was reached.

Kris