linux-kernel - RE: [PATCH] bdi: use deferable timer for sync

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Date:	Fri, 08 Oct 2010 13:09:46 +0300
From:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
To:	"ext Wu, Xia" <xia.wu@...el.com>
Cc:	Christoph Hellwig <hch@....de>,
	Yong Wang <yong.y.wang@...ux.intel.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	"Wu, Fengguang" <fengguang.wu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH] bdi: use deferable timer for sync_supers task

On Fri, 2010-10-08 at 12:04 +0200, ext Wu, Xia wrote:
> On Fri, Oct 08, 2010 at 04:35:14PM +0800, Yong Wang wrote:
> > > sync_supers task currently wakes up periodically for superblock
> > > writeback. This hurts power on battery driven devices. This patch
> > > turns this housekeeping timer into a deferable timer so that it
> > > does not fire when system is really idle.
> 
> > How long can the timer be defereed?  We can't simply stop writing
> > out data for a long time.  I think the current timer value should be
> > the upper bound, but allowing to fire earlier to run during the
> > same wakeup cycle as others is fine.
> 
> If the system is in sleep state, this timer can be deferred to the next wake-up interrupt.
> If the system is busy, this timer will fire at the scheduled time.

However, when the next wake-up interrupt happens is not defined. It can
happen 1ms after, or 1 minute after, or 1 hour after. What Christoph
says is that there should be some guarantee that sb writeout starts,
say, within 5 to 10 seconds interval. Deferrable timers do not guarantee
this. But take a look at the range hrtimers - they do exactly this.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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