Date: Thu, 6 Nov 2008 06:06:24 -0800
From: Arjan van de Ven <arjan@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       mingo@elte.hu
Subject: Re: [PATCH] ftrace: add an fsync tracer
Message-ID: <20081106060624.58a0f967@infradead.org>
In-Reply-To: <1225976138.7803.4485.camel@twins>
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	<1225976138.7803.4485.camel@twins>
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On Thu, 06 Nov 2008 13:55:38 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, 2008-11-05 at 09:49 -0800, Arjan van de Ven wrote:
> > From 63c1b869d94eb31a98015af09fb24e22151f2f00 Mon Sep 17 00:00:00
> > 2001 From: Arjan van de Ven <arjan@linux.intel.com>
> > Date: Tue, 4 Nov 2008 21:08:11 -0800
> > Subject: [PATCH] ftrace: add an fsync tracer
> > 
> > fsync() (and its cousin, fdatasync()) are important chokepoints in
> > the kernel as they imply very expensive operations, both in terms
> > of filesystem operations (ext3 writes back its entire journal) as
> > well as the block layer (fsync() implies sending a cache flushing
> > barrier to the SATA/SCSI disk).
> > 
> > This tracer makes a log of which application calls fsync() on which
> > file, so that developers and others interested in finding these
> > choke points can locate them and fix them in the apps that call
> > this function.
> 
> Sorry, but I have to object to such single purpose tracers..
> 
> If we go this way we'll end up with a gazillion little tracers, non of
> which are really useful.

If we go this way we'll end up with a bunch of little tracers, all of
which will be useful in their area, and people can also make "super
tracers" out of the useful trace points.

> 
> Please work on getting something like a syscall tracer,

a syscall tracer will exactly not tell you which file(name) was being
fsync()'d which was the whole point.
LatencyTOP already KNOWS that fsync is the problem. What it doesn't
know is which file is being fsync()d.

fsync is a problem when used incorrectly, not just for ext3 but also
due to barriers. That's why it's important to be able to find who calls
it when it impacts interactive performance.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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