linux-kernel - Re: [PATCH] ftrace: add an fsync tracer

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Date:	Thu, 06 Nov 2008 15:50:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: [PATCH] ftrace: add an fsync tracer

On Thu, 2008-11-06 at 06:31 -0800, Arjan van de Ven wrote:
> On Thu, 06 Nov 2008 15:19:01 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > a syscall tracer will exactly not tell you which file(name) was
> > > being fsync()'d which was the whole point.
> > 
> > It will tell you the process and the fd, and when you have those two
> > its a simple step to find the actual file.
> 
> actually process+fd is absolutely useless; the typical useage is
> 
> fd = open(file)
> write(fd, <> )
> fsync(fd);
> close(fd);
> 
> by the time userland gets the data the fd is closed. And heck, even the
> program may have exited.
> Really, the fd number is only useful for the program itself, not for
> any outside part, and especially, later in time.

The syscall tracer will also have told you about that open.

Anyway, do_fsync() doesn't catch all sync actions (although I suspect it
catches most). We still have the mythical sync_file_range() that Andrew
still wants a real program to use.

And then there are things like sync and umount that do syncs too. But I
suspect you might not be interested in those.

> > 
> > > 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.
> > 
> > Which suggests you want a tracer that gives more information about who
> > generates barriers, not specifically fsync().
> 
> that would be a fine second tracer. because the filesystem part of it
> is also expensive, and you can diss ext3 all you want, it is reality
> for 99% of the people...

Lets hope btrfs will fix that quickly. fsync() causing latencies for
anyone else besides tasks interested in that file is utterly
unacceptable.

> (and I suspect that at the barrier level it'll be really hard to get to
> a filename)

I suspect you might be right, but that's not a reason not to try ;-)
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