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

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Date:	Fri, 25 Sep 2009 09:33:50 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

On Thu, Sep 24, 2009 at 11:38:16PM +0800, Peter Zijlstra wrote:
> On Thu, 2009-09-24 at 16:33 +0800, Wu Fengguang wrote:
> 
> > Yeah, FIFO queuing should be good enough.
> > 
> > I'd like to propose one more data structure for evaluation :)
> > 
> > - bdi->throttle_lock
> > - bdi->throttle_list    pages to sync for each waiting task, taken from sync_writeback_pages()
> > - bdi->throttle_pages   (counted down) pages to sync for the head task, shall be atomic_t
> > 
> > In balance_dirty_pages(), it would do
> > 
> >         nr_to_sync = sync_writeback_pages()
> >         if (list_empty(bdi->throttle_list))  # I'm the only task
> >                 bdi->throttle_pages = nr_to_sync
> >         append nr_to_sync to bdi->throttle_list
> >         kick off background writeback
> >         wait
> >         remove itself from bdi->throttle_list and wait list
> >         set bdi->throttle_pages for new head task (or LONG_MAX)
> > 
> > In __bdi_writeout_inc(), it would do
> > 
> >         if (--bdi->throttle_pages <= 0)
> >                 check and wake up head task
> > 
> > In wb_writeback(), it would do
> > 
> >         if (args->for_background && exiting)
> >                 wake up all throttled tasks

> > To prevent wake up too many tasks at the same time, it can relax the
> > background threshold a bit, so that __bdi_writeout_inc() become the
> > only wake up point in normal cases.
> > 
> >         if (args->for_background && !list_empty(bdi->throttle_list) &&
> >                 over background_thresh - background_thresh / 32)
> >                 keep write pages;

I realized this last change is not necessary, because we already
have a big enough buffer area:

(dirty_thresh + background_thresh)/2  ==> background_thresh

> Right, something like that ought to work well, or at least sounds like
> worth a try ;-)

Thanks :)
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