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    SubjectRe: [RFC][PATCH 7/7] drain batch list during long operations
    On Tue, May 07, 2013 at 02:20:03PM -0700, Dave Hansen wrote:
    >
    > From: Dave Hansen <dave.hansen@linux.intel.com>
    >
    > This was a suggestion from Mel:
    >
    > http://lkml.kernel.org/r/20120914085634.GM11157@csn.ul.ie
    >
    > Any pages we collect on 'batch_for_mapping_removal' will have
    > their lock_page() held during the duration of their stay on the
    > list. If some other user is trying to get at them during this
    > time, they might end up having to wait for a while, especially if
    > we go off and do pageout() on some other page.
    >
    > This ensures that we drain the batch if we are about to perform a
    > writeout.
    >
    > I added some statistics to the __remove_mapping_batch() code to
    > track how large the lists are that we pass in to it. With this
    > patch, the average list length drops about 10% (from about 4.1 to
    > 3.8). The workload here was a make -j4 kernel compile on a VM
    > with 200MB of RAM.
    >
    > I've still got the statistics patch around if anyone is
    > interested.
    >
    > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    > ---
    >
    > linux.git-davehans/kernel/sched/fair.c | 2 ++
    > linux.git-davehans/mm/vmscan.c | 10 ++++++++++
    > 2 files changed, 12 insertions(+)
    >
    > diff -puN kernel/sched/fair.c~drain-batch-list-during-long-operations kernel/sched/fair.c
    > --- linux.git/kernel/sched/fair.c~drain-batch-list-during-long-operations 2013-05-07 13:48:15.267113941 -0700
    > +++ linux.git-davehans/kernel/sched/fair.c 2013-05-07 13:48:15.275114295 -0700
    > @@ -5211,6 +5211,8 @@ more_balance:
    > if (sd->balance_interval < sd->max_interval)
    > sd->balance_interval *= 2;
    > }
    > + //if (printk_ratelimit())
    > + // printk("sd->balance_interval: %d\n", sd->balance_interval);
    >
    > goto out;
    >

    heh

    > diff -puN mm/vmscan.c~drain-batch-list-during-long-operations mm/vmscan.c
    > --- linux.git/mm/vmscan.c~drain-batch-list-during-long-operations 2013-05-07 13:48:15.268113985 -0700
    > +++ linux.git-davehans/mm/vmscan.c 2013-05-07 13:48:15.272114163 -0700
    > @@ -936,6 +936,16 @@ static unsigned long shrink_page_list(st
    > if (!sc->may_writepage)
    > goto keep_locked;
    >
    > + /*
    > + * We hold a bunch of page locks on the batch.
    > + * pageout() can take a while, so drain the
    > + * batch before we perform pageout.
    > + */
    > + nr_reclaimed +=
    > + __remove_mapping_batch(&batch_for_mapping_removal,
    > + &ret_pages,
    > + &free_pages);
    > +

    There is also a wait_on_page_writeback() above that would affect
    memcg's but this alone alleviates a lot of my concerns about lock hold
    times.

    --
    Mel Gorman
    SUSE Labs


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