Re: [RFC][PATCH] new timeofday core subsystem (v. A2)

[Nigel Cunningham]

Hi.

On Wed, 2005-02-02 at 10:32, john stultz wrote:
> On Wed, 2005-02-02 at 10:14 +1100, Nigel Cunningham wrote:
> > Hi John and Tim.
> > 
> > On Wed, 2005-02-02 at 09:48, john stultz wrote:
> > > > I didn't scan for all uses of read_persistent_clock, but
> > > > in my experience get_cmos_time() has a latency of up to
> > > > 1 second on x86 because it synchronizes with the rollover
> > > > of the RTC seconds.
> > > 
> > > I believe you're right. Although we don't call read_persistent_clock()
> > > very frequently, nor do we call it in ways we don't already call
> > > get_cmos_time(). So I'm not sure exactly what the concern is.
> > 
> > Tim and I talked about this at the recent CELF conference. I have a
> > concern in that suspend-to-disk calls the suspend methods and then
> > (after the atomic copy) the resume methods. Since the copy usually takes
> > < 1s, and the suspend and resume methods both make two calls to
> > get_coms_time, that's an average of 1.5s per suspend call and 1.5s per
> > resume call - but if the copy does take next to no time (as normal),
> > it's really 1.5s + 2s = 3.5s average just for getting the time. I
> > believe Tim has similar issues in code he is working on. It's a concern
> > if your battery is running out and you're trying to hibernate!
> 
> Well, counting the atomic copy in the "3.5s average just for getting the
> time" doesn't quite seem fair, but I think I understand. Its

You're right. Maybe we could say 3s, accounting .5s of that 3.5s average
delay as being for the atomic copy.

> interesting, I wasn't aware of the suspend/copy/resume process that
> occurs for suspend-to-disk. The thing I don't quite get is why are the
> resume methods called before we really suspend to disk?

We call the suspend and resume methods because the suspend is supposed
to achieve atomicity, and the resume is necessary for us to be able to
write the image. (Remember that these calls are invoked as part of the
drivers_suspend and drivers_resume code). Until recently the
sysdev_suspend and resume methods weren't called and things did still
work, but that was an omission and we did then run into time issues.

> > > I've only lightly tested the suspend code, but on my system I didn't see
> > > very much drift appear. Regardless, it should be better then what the
> > > current suspend/resume code does, which doesn't keep any sub-second
> > > resolution across suspend.
> > 
> > My question is, "Is there a way we can get sub-second resolution without
> > waiting for the start of a new second four times in a row?" I'm sure
> > there must be.
> 
> Well, I'm not sure what else we could use for the persistent clock, but
> I'd be happy to change the read/set_persistent_clock function to use it.

Is it possible to still use the persistent clock, but do the math for
the portions of seconds?

By the way, Tim, I hope I didn't misunderstand anything, and that these
_are_ the same issues you had!

Regards,

Nigel

> thanks
> -john
-- 
Nigel Cunningham
Software Engineer, Canberra, Australia
http://www.cyclades.com

Ph: +61 (2) 6292 8028      Mob: +61 (417) 100 574

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