Re: [Results] [RFC PATCH v4 00/40] mm: Memory Power Management

[Arjan van de Ven]

On 9/26/2013 6:42 AM, Srivatsa S. Bhat wrote:
> On 09/26/2013 08:29 AM, Andrew Morton wrote:
> > On Thu, 26 Sep 2013 03:50:16 +0200 Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> >
> > > On Wed, Sep 25, 2013 at 06:21:29PM -0700, Andrew Morton wrote:
> > > > On Wed, 25 Sep 2013 18:15:21 -0700 Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > > On 9/25/2013 4:47 PM, Andi Kleen wrote:
> > > > > > > Also, the changelogs don't appear to discuss one obvious downside: the
> > > > > > > latency incurred in bringing a bank out of one of the low-power states
> > > > > > > and back into full operation.  Please do discuss and quantify that to
> > > > > > > the best of your knowledge.
> > > > > >
> > > > > > On Sandy Bridge the memry wakeup overhead is really small. It's on by default
> > > > > > in most setups today.
> > > > >
> > > > > btw note that those kind of memory power savings are content-preserving,
> > > > > so likely a whole chunk of these patches is not actually needed on SNB
> > > > > (or anything else Intel sells or sold)
> > > >
> > > > (head spinning a bit).  Could you please expand on this rather a lot?
> > >
> > > As far as I understand there is a range of aggressiveness. You could
> > > just group memory a bit better (assuming you can sufficiently predict
> > > the future or have some interface to let someone tell you about it).
> > >
> > > Or you can actually move memory around later to get as low footprint
> > > as possible.
> > >
> > > This patchkit seems to do both, with the later parts being on the
> > > aggressive side (move things around)
> > >
> > > If you had non content preserving memory saving you would
> > > need to be aggressive as you couldn't afford any mistakes.
> > >
> > > If you had very slow wakeup you also couldn't afford mistakes,
> > > as those could cost a lot of time.
> > >
> > > On SandyBridge is not slow and it's preserving, so some mistakes are ok.
> > >
> > > But being aggressive (so move things around) may still help you saving
> > > more power -- i guess only benchmarks can tell. It's a trade off between
> > > potential gain and potential worse case performance regression.
> > > It may also depend on the workload.
> > >
> > > At least right now the numbers seem to be positive.
> >
> > OK.  But why are "a whole chunk of these patches not actually needed on SNB
> > (or anything else Intel sells or sold)"?  What's the difference between
> > Intel products and whatever-it-is-this-patchset-was-designed-for?
>
> Arjan, are you referring to the fact that Intel/SNB systems can exploit
> memory self-refresh only when the entire system goes idle? Is that why this
> patchset won't turn out to be that useful on those platforms?

no we can use other things (CKE and co) all the time.

just that we found that statistical grouping gave 95%+ of the benefit,
without the cost of being aggressive on going to a 100.00% grouping
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