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

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