linux-kernel - Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback

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Message-Id: <1252434746.7035.7.camel@laptop>
Date:	Tue, 08 Sep 2009 20:32:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Artem Bityutskiy <dedekind1@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	david@...morbit.com, hch@...radead.org, akpm@...ux-foundation.org,
	jack@...e.cz, Theodore Ts'o <tytso@....edu>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

On Tue, 2009-09-08 at 19:55 +0200, Peter Zijlstra wrote:
> 
> I think I'm somewhat confused here though..
> 
> There's kernel threads doing writeout, and there's apps getting stuck in
> balance_dirty_pages().
> 
> If we want all writeout to be done by kernel threads (bdi/pd-flush like
> things) then we still need to manage the actual apps and delay them.
> 
> As things stand now, we kick pdflush into action when dirty levels are
> above the background level, and start writing out from the app task when
> we hit the full dirty level.
> 
> Moving all writeout to a kernel thread sounds good from writing linear
> stuff pov, but what do we make apps wait on then?

OK, so like said in the previous email, we could have these app tasks
simply sleep on a waitqueue which gets periodic wakeups from
__bdi_writeback_inc() every time the dirty threshold drops.

The woken tasks would then check their bdi dirty limit (its task
dependent) against the current values and either go back to sleep or
back to work.

The only problem would be the mass wakeups when lots of tasks are
blocked on dirty, but I'm guessing there's no way around that anyway,
and its better to have a limited number of writers than have everybody
write something, which would result in massive write fragmentation.

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