From: "Brown, Len" <len.brown@intel.com>
To: Austin S Hemmelgarn <ahferroin7@gmail.com>, Pavel Machek <pavel@ucw.cz>,
        Len Brown <lenb@kernel.org>
CC: "rjw@rjwysocki.net" <rjw@rjwysocki.net>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional
Thread-Topic: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional
Thread-Index: AQHQvqV85I7cASIWTkignhs+Qu1MSZ3ciyKAgAB77QD//5ZZkA==
Date: Wed, 15 Jul 2015 14:58:22 +0000
Message-ID: <1A7043D5F58CCB44A599DFD55ED4C948468A7978@fmsmsx115.amr.corp.intel.com>
References: <1436927091-32520-1-git-send-email-lenb@kernel.org>
 <505338dbbcdce9416ac05f67288b10442fbda179.1436926113.git.len.brown@intel.com>
 <20150715064320.GA23713@amd> <55A668FE.80409@gmail.com>
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> -----Original Message-----
> From: Austin S Hemmelgarn [mailto:ahferroin7@gmail.com]
> Sent: Wednesday, July 15, 2015 10:07 AM
> To: Pavel Machek; Len Brown
> Cc: rjw@rjwysocki.net; linux-pm@vger.kernel.org; linux-
> kernel@vger.kernel.org; Brown, Len
> Subject: Re: [PATCH 1/1] suspend: make sync() on suspend-to-RAM optional
> 
> On 2015-07-15 02:43, Pavel Machek wrote:
> > On Tue 2015-07-14 22:24:51, Len Brown wrote:
> >> From: Len Brown <len.brown@intel.com>
> >>
> >> The Linux kernel suspend path has traditionally invoked sys_sync().
> >>
> >> But sys_sync() can be expensive, and some systems do not want
> >> to pay the cost of sys_sync() on every suspend.
> >
> > Have you measured how expensive it can be, and why it is expensive?

> How expensive it is can vary widely, but it pretty much boils down to
> how much dirty data still needs written out, and how slow the storage it
> needs written to is.  There's not really much that can be done in the
> kernel to change this, and most userspace suspend systems call sync
> themselves during the suspend sequence.

Right.
And now, user-space gets is no longer forced to incur that
delay on every suspend if they do not want it.

Yes, have measured this under many conditions.
The bottom line is that sys_sync() is rarely as fast as 1ms,
and is sometimes as slow as hundreds of ms.

>> Why do you need CONFIG parameter?

So that an OS that doesn't want to change their user-space,
can build a kernel that does what they want by default.

Originally I had the config parameter remove this code entirely,
which would achieve the same goal.
But Rafael prefers the sysfs attribute always exist
and the config simply set the default.

thanks,
-Len

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