Re: swsusp performance problems in 2.6.15-rc3-mm1

[Andy Isaacson <adi@hexapodia.org>]

On Tue, Dec 06, 2005 at 12:05:04AM +0100, Rafael J. Wysocki wrote:
> On Monday, 5 December 2005 09:19, Andy Isaacson wrote:
> > On recent kernels such as 2.6.14-rc2-mm1, a swsusp of my laptop (1.25
> > GB, P4M 1.4 GHz) was a pretty fast process; freeing memory took about 3
> > seconds or less,
> 
> It took much more time on my box, but I won't discuss with your
> experience. ;-)

I may be misremembering, it didn't seem important at the time.  Less
than 10 seconds, anyways.

> > Certainly there's a tradeoff to be made, and I'm glad to lose the slow
> > re-paging after resume, but I'm hoping that some kind of improvement can
> > be made in the suspend/resume time.
> 
> Yes, there is a tradeoff.  Till now, we have used the simplistic approach
> based on freeing as much memory as possible before suspend.  Now, we
> are freeing only as much memory as necessary, which is on the other
> end of the scale, so to speak.  There are a whole lot of possibilities in
> between, and there's a question which one is the best.  Frankly, I'm afraid
> the answer is very system-dependent.

If you wanted to compute "What's the absolute perfect 99.9999th
percentile amount to free", yes, that would be impossibly
system-dependent.

But some kind of rule of thumb should get good results in most cases,
and it should be easy enough to add a knob for people who have
particular requirements.

> If you want a quick solution, you can get back to the previous behavior by
> commenting out the definition of FAST_FREE in kernel/power/power.h.

That's boring. :)  The current behavior isn't bad enough to force me
back.

> Alternatively, you can increase the value of PAGES_FOR_IO, defined
> in include/linux/suspend.h.  To see any effect, you'll probably have to
> increase it by tens of thousands, but please note the box may be unable
> to suspend if it's too great (if you try this anyway, please let me know what
> number seems to be the best to you).
> 
> Also, I can create a patch to improve this a bit, if you promise to help
> test/debug it. ;-)

I'll play with this a bit tonight but I'd love to see a patch that makes
it a tunable.  Rebooting my laptop is sloooow and annoying (due to slow
startup scripts and losing all my state), but trying various suspend
settings sounds like a fun experiment.

-andy