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Date:      Wed, 14 Jun 2006 11:38:47 -0400
From:      Kris Kennaway <kris@obsecurity.org>
To:        John Baldwin <jhb@freebsd.org>
Cc:        freebsd-current@freebsd.org, Robert Watson <rwatson@freebsd.org>, John Birrell <jb@what-creek.com>, current@freebsd.org
Subject:   Re: DTrace for FreeBSD - fbt output
Message-ID:  <20060614153846.GA37259@xor.obsecurity.org>
In-Reply-To: <200606141021.45669.jhb@freebsd.org>
References:  <20060613021543.GA71283@what-creek.com> <20060614103614.Y34121@fledge.watson.org> <20060614095449.GA82424@what-creek.com> <200606141021.45669.jhb@freebsd.org>

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On Wed, Jun 14, 2006 at 10:21:44AM -0400, John Baldwin wrote:
> On Wednesday 14 June 2006 05:54, John Birrell wrote:
> > > > critical_enter                                           4364385478
> > > > critical_exit                                            4364385478
> >=20
> > The points I wanted to make about these calls are:
> >=20
> > 1. critical_enter is called so often that the debug printf that /could/
> >    be compiled into it should cause an increase in global warming if
> >    you tried. 8-)
>=20
> No, it's not a printf to the console it's a printf into the a circular
> buffer (see options KTR) and the system has been run with it enabled w/o
> burning the machine up. :)

For point of comparison, I added an implementation of spinlock
profiling in my kris-contention p4 branch which does a KTR every 100
mtx_lock_spin() operations, and this has less than 5% performance
impact.  KTR is very cheap since it is just a single atomic op to
increment the buffer index, and then a couple of pointer assignments.

Kris

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