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    SubjectRe: [rfc] lru_add_drain_all() vs isolation
    On Thu, 10 Sep 2009 10:15:07 +0900 (JST)
    KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

    > > On Thu, 10 Sep 2009 08:58:20 +0900 (JST)
    > > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
    > >
    > > > > On Wed, Sep 9, 2009 at 1:27 PM, KOSAKI Motohiro
    > > > > <kosaki.motohiro@jp.fujitsu.com> wrote:
    > > > > >> The usefulness of a scheme like this requires:
    > > > > >>
    > > > > >> 1. There are cpus that continually execute user space code
    > > > > >>    without system interaction.
    > > > > >>
    > > > > >> 2. There are repeated VM activities that require page isolation /
    > > > > >>    migration.
    > > > > >>
    > > > > >> The first page isolation activity will then clear the lru caches of the
    > > > > >> processes doing number crunching in user space (and therefore the first
    > > > > >> isolation will still interrupt). The second and following isolation will
    > > > > >> then no longer interrupt the processes.
    > > > > >>
    > > > > >> 2. is rare. So the question is if the additional code in the LRU handling
    > > > > >> can be justified. If lru handling is not time sensitive then yes.
    > > > > >
    > > > > > Christoph, I'd like to discuss a bit related (and almost unrelated) thing.
    > > > > > I think page migration don't need lru_add_drain_all() as synchronous, because
    > > > > > page migration have 10 times retry.
    > > > > >
    > > > > > Then asynchronous lru_add_drain_all() cause
    > > > > >
    > > > > >  - if system isn't under heavy pressure, retry succussfull.
    > > > > >  - if system is under heavy pressure or RT-thread work busy busy loop, retry failure.
    > > > > >
    > > > > > I don't think this is problematic bahavior. Also, mlock can use asynchrounous lru drain.
    > > > >
    > > > > I think, more exactly, we don't have to drain lru pages for mlocking.
    > > > > Mlocked pages will go into unevictable lru due to
    > > > > try_to_unmap when shrink of lru happens.
    > > >
    > > > Right.
    > > >
    > > > > How about removing draining in case of mlock?
    > > >
    > > > Umm, I don't like this. because perfectly no drain often make strange test result.
    > > > I mean /proc/meminfo::Mlock might be displayed unexpected value. it is not leak. it's only lazy cull.
    > > > but many tester and administrator wiill think it's bug... ;)
    > >
    > > I agree. I have no objection to your approach. :)
    > >
    > > > Practically, lru_add_drain_all() is nearly zero cost. because mlock's page fault is very
    > > > costly operation. it hide drain cost. now, we only want to treat corner case issue.
    > > > I don't hope dramatic change.
    > >
    > > Another problem is as follow.
    > >
    > > Although some CPUs don't have any thing to do, we do it.
    > > HPC guys don't want to consume CPU cycle as Christoph pointed out.
    > > I liked Peter's idea with regard to this.
    > > My approach can solve it, too.
    > > But I agree it would be dramatic change.
    >
    > Is Perter's + mine approach bad?

    It's good to me! :)

    > It mean,
    >
    > - RT-thread binding cpu is not grabbing the page
    > -> mlock successful by Peter's improvement
    > - RT-thread binding cpu is grabbing the page
    > -> mlock successful by mine approach
    > the page is culled later.
    >
    >
    >
    >


    --
    Kind regards,
    Minchan Kim
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