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

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Message-Id: <201006032305.58082.rjw@sisk.pl>
Date:	Thu, 3 Jun 2010 23:05:58 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	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 Thursday 03 June 2010, James Bottomley wrote:
> 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.

Agreed.

Rafael
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