linux-kernel - Re: [PATCH 1/1] Add check for dirty_writeback_interval in bdi_wakeup_thread

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Date:	Thu, 21 Apr 2011 00:47:57 +0530
From:	Raghavendra D Prabhu <rprabhu@...hang.net>
To:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
Cc:	linux-mm@...ck.org, Jens Axboe <jaxboe@...ionio.com>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Add check for dirty_writeback_interval in
 bdi_wakeup_thread_delayed

* On Mon, Apr 18, 2011 at 03:26:29PM +0300, Artem Bityutskiy <Artem.Bityutskiy@...ia.com> wrote:
>On Mon, 2011-04-18 at 14:46 +0530, Raghavendra D Prabhu wrote:
>> I have set it to 500 centisecs as that is the default value of
>> dirty_writeback_interval. I used this logic for following reason: the
>> purpose for which dirty_writeback_interval is set to 0 is to disable
>> periodic writeback
>> (http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/fs/fs-writeback.c#L818)
>> , whereas here (in bdi_wakeup_thread_delayed) it is being used for a
>> different purpose -- to delay the bdi wakeup in order to reduce context
>> switches for  dirty inode writeback.
>
>But why it wakes up the bdi thread? Exactly to make sure the periodic
>write-back happen.
I checked the callgraph of bdi_wakeup_thread_delayed and found out that
even though it may be called in the aftermath of wb_do_writeback(), it
is certainly called in the call-chain of sync. So effectively making
that function do nothing when dirty_writeback_interval is unset will
also make sync do nothing. On the other hand, not applying the original
change at all will make it run instantly (jiffies + 0, 0 being the
writeback interval in this case ) thus reversing the benefits of
d7dd01adc098eadc5d5fb07a7d2bf942d09b15df.

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