linux-kernel - Re: [PATCH 2/3] Fix fsync livelock

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Message-ID: <20081005162847.7bf0ead1@infradead.org>
Date:	Sun, 5 Oct 2008 16:28:47 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: [PATCH 2/3] Fix fsync livelock

On Sun, 5 Oct 2008 19:18:22 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> On Sun, 5 Oct 2008, Arjan van de Ven wrote:
> 
> 
> The problem here is that you have two processes --- one is writing,
> the other is simultaneously syncing. The syncing process can't
> distinguish the pages that were created before fsync() was invoked
> (it has to wait on them) and the pages that were created while
> fsync() was running (it doesn't have to wait on them) --- so it waits
> on them all.

for pages it already passed that's not true.
but yes while you walk, you may find new ones. tough luck.
> 
> Or, how otherwise would you implement "Submit them all in one go,
> then wait"? The current code is:
> you grab page 0, see it is under writeback, wait on it
> you grab page 1, see it is under writeback, wait on it
> you grab page 2, see it is under writeback, wait on it
> you grab page 3, see it is under writeback, wait on it

it shouldn't be doing this. It should be "submit the lot"
"wait on the lot that got submitted".

keeping away new writers is just shifting the latency to an innocent
party (since the fsync() is just bloody expensive, the person doing it
should be paying the price, not the rest of the system), and a grave
mistake.

If the fsync() implementation isn't smart enough, sure, lets improve
it. But not by shifting latency around... lets make it more efficient
at submitting IO.
If we need to invent something like "chained IO" where if you wait on
the last of the chain, you wait on the entirely chain, so be it.


-- 
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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