Re: [RFC][PATCH 0/2] Swap token re-tuned

[Andrew Morton <akpm@osdl.org>]

On Mon, 02 Oct 2006 09:35:52 +0200
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Sun, 2006-10-01 at 15:56 -0700, Andrew Morton wrote:
> > On Sat, 30 Sep 2006 00:11:51 +0530
> > Ashwin Chaugule <ashwin.chaugule@celunite.com> wrote:
> 
> > > PATCH 2: 
> > > 
> > > Instead of using TIMEOUT as a parameter to transfer the token, I think a
> > > better solution is to hand it over to a process that proves its
> > > eligibilty. 
> > > 
> > > What my scheme does, is to find out how frequently a process is calling
> > > these functions. The processes that call these more frequently get a
> > > higher priority. 
> > > The idea is to guarantee that a high priority process gets the token.
> > > The priority of a process is determined by the number of consecutive
> > > calls to swap-in and no-page. I mean "consecutive" not from the
> > > scheduler point of view, but from the process point of view. In other
> > > words, if the task called these functions every time it was scheduled,
> > > it means it is not getting any further with its execution. 
> > > 
> > > This way, its a matter of simple comparison of task priorities, to
> > > decide whether to transfer the token or not. 
> > 
> > Does this introduce the possibility of starvation?  Where the
> > fast-allocating process hogs the system and everything else makes no
> > progress?
> 
> I tinkered with this a bit yesterday, and didn't get good results for:
> mem=64M ; make -j5
> 
> -vanilla: 2h32:55
> -swap-token: 2h41:48
> 
> various other attempts at tweaking the code only made it worse. (will
> have to rerun these test, but a ~3h test is well, a 3h test ;-)

I don't think that's a region of operation where we care a great deal. 
What was the average CPU utlisation?  Only a few percent.

It's just thrashing too much to bother optimising for.  Obviously we want
it to terminate in a sane period of time and we'd _like_ to improve it. 
But I think we'd accept a 10% slowdown in this region of operation if it
gave us a 10% speedup in the 25%-utilisation region.

IOW: does the patch help mem=96M;make -j5??

> Being frustrated with these results - I mean the idea made sense, so
> what is going on - I came up with this answer:
> 
> Tasks owning the swap token will retain their pages and will hence swap
> less, other (contending) tasks will get less pages and will fault more
> frequent. This prio mechanism will favour exactly those tasks not
> holding the token. Which makes for token bouncing.

OK.

(We need to do something with
ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.18/2.6.18-mm2/broken-out/mm-thrash-detect-process-thrashing-against-itself.patch,
btw.  Has been in -mm since March and I'm still waiting for some benchmarks
which would justify its inclusion..)

> The current mechanism seemingly assigns the token randomly (whomever
> asks while not held gets it - and the hold time is fixed), however this
> change in paging behaviour (holder less, contenders more) shifts the
> odds in favour of one of the contenders. Also the fixed holding time
> will make sure the token doesn't get released too soon and can make some
> progress.
> 
> So while I agree it would be nice to get rid of all magic variables
> (holding time in the current impl) this proposed solution hasn't
> convinced me (for one it introduces another).
> 
> (for the interrested, the various attempts I tried are available here:
>   http://programming.kicks-ass.net/kernel-patches/swap_token/ )

OK, thanks or looking into it.  I do think this is rich ground for
optimisation.