Re: [PATCH 1/1] Add check for dirty_writeback_interval inbdi_wakeup_thread_delayed

[Raghavendra D Prabhu]

* On Mon, Apr 18, 2011 at 03:26:29PM +0300, Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx> 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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