linux-kernel - Re: [PATCH 1/1] suspend: delete sys

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Message-ID: <20150508201310.5c4ccfaa@lxorguk.ukuu.org.uk>
Date:	Fri, 8 May 2015 20:13:10 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Len Brown <lenb@...nel.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 1/1] suspend: delete sys_sync()

> 2. worst case latency is obscene, there are examples of some
>     syncs which take over 3,000 ms to complete.

ATA is pretty open ended on this. I believe the vendors use 7 seconds
just for the cache flush as their limit because after 7 seconds some non
Linux OS's blow up. However if my suspend/resume crashes (as still I'm
sorry to say happens far too often) I don't want my last ten minutes of
work trashed.

> Unfortunately, sys_sync() can be a significant pain point,
> even for systems that run Android.

Android devices often have slow I/O devices coupled with a lot of memory
so yes that is true.

There are however some very important reasons for using sync() in a
suspend

- I can read data off the suspended machines disk volumes even though I
  can't write to them. People do this.

- Suspend requires the firmware, drivers and kernel all get it exactly
  right. On a lot of machines therefore suspend is still a buggy pile of
  crap. Sync is extremely valuable given that you can't be entirely
  sure your system will resume.

- Users habitually do stupid things like removing USB dongles from
  suspended boxes and thinking afterwards. Perception is that the device
  is off therefore you can unplug it.

So I think its inappropriate to change the default. Allow users to turn
it off by all means, and I imagine many phones would use that.

Some of this however is crappy suspend/resume handling. If the suspend
subsystem was doing its job then for the cases of timeout triggered
suspend it would have triggered most of the disk writes ten seconds
before it tried to suspend properly ;-)

Dirty page management is at the end of the day partly a power management
issue. We already reflect that in other places like laptop mode.

Alan
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