linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 03 Jun 2010 09:36:34 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	markgross@...gnar.org, 640e9920@...il.com,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Swetland <swetland@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Thu, 2010-06-03 at 00:10 -0700, Arve Hjønnevåg wrote:
> On Wed, Jun 2, 2010 at 10:40 PM, mark gross <640e9920@...il.com> wrote:
> > On Wed, Jun 02, 2010 at 09:54:15PM -0700, Brian Swetland wrote:
> >> On Wed, Jun 2, 2010 at 8:18 PM, mark gross <640e9920@...il.com> wrote:
> >> > On Wed, Jun 02, 2010 at 02:58:30PM -0700, Arve Hjønnevåg wrote:
> >> >>
> >> >> The list is not short. You have all the inactive and active
> >> >> constraints on the same list. If you change it to a two level list
> >> >> though, the list of unique values (which is the list you have to walk)
> >> >> may be short enough for a tree to be overkill.
> >> >
> >> > what have you seen in practice from the wake-lock stats?
> >> >
> >> > I'm having a hard time seeing where you could get more than just a
> >> > handfull.  However; one could go to a dual list (like the scheduler) and
> >> > move inactive nodes from an active to inactive list, or we could simply
> >> > remove them from the list uppon inactivity.  which would would well
> >> > after I change the api to have the client allocate the memory for the
> >> > nodes...  BUT, if your moving things in and out of a list a lot, I'm not
> >> > sure the break even point where changing the structure helps.
> >> >
> >> > We'll need to try it.
> >> >
> >> > I think we will almost never see more than 10 list elements.
> >> >
> >> > --mgross
> >> >
> >> >
> >>
> >> I see about 80 (based on the batteryinfo dump) on my Nexus One
> >> (QSD8250, Android Froyo):
> >
> > shucks.
> >
> > well I think for a pm_qos class that has boolean dynamic range we can
> > get away with not walking the list on every request update.  we can use
> > a counter, and the list will be for mostly for stats.
> >
> 
> Did you give any thought to my suggestion to only use one entry per
> unique value on the first level list and then use secondary lists of
> identical values. That way if you only have two constraints values the
> list you have to walk when updating a request will never have more
> than two entries regardless of how many total request you have.
> 
> A request update then becomes something like this:
>   if on primary list {
>     unlink from primary list
>     if secondary list is not empty
>       get next secondary entry and add in same spot on primary list
>   }
>   unlink from secondary list
>   find new spot on primary list
>   if already there
>     add to secondary list
>   else
>     add to primary list

This is just reinventing hash bucketed lists.  To get the benefits, all
we do is implement an N state constraint as backed by an N bucketed hash
list, which the kernel already has all the internal mechanics for.

James


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