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    SubjectRe: [PATCH v2 2/2] vmscan: shrink_slab() require number of lru_pages, not page order
    On Thu, 8 Jul 2010 09:04:18 -0500 (CDT)
    Christoph Lameter <cl@linux-foundation.org> wrote:

    > On Thu, 8 Jul 2010, KOSAKI Motohiro wrote:
    >
    > > Fix simple argument error. Usually 'order' is very small value than
    > > lru_pages. then it can makes unnecessary icache dropping.
    >
    > AFAICT this is not argument error but someone changed the naming of the
    > parameter.

    It's been there since day zero:

    : commit 2a16e3f4b0c408b9e50297d2ec27e295d490267a
    : Author: Christoph Lameter <clameter@engr.sgi.com>
    : AuthorDate: Wed Feb 1 03:05:35 2006 -0800
    : Commit: Linus Torvalds <torvalds@g5.osdl.org>
    : CommitDate: Wed Feb 1 08:53:16 2006 -0800
    :
    : [PATCH] Reclaim slab during zone reclaim
    :
    : If large amounts of zone memory are used by empty slabs then zone_reclaim
    : becomes uneffective. This patch shakes the slab a bit.
    :
    : The problem with this patch is that the slab reclaim is not containable to a
    : zone. Thus slab reclaim may affect the whole system and be extremely slow.
    : This also means that we cannot determine how many pages were freed in this
    : zone. Thus we need to go off node for at least one allocation.
    :
    : The functionality is disabled by default.
    :
    : We could modify the shrinkers to take a zone parameter but that would be quite
    : invasive. Better ideas are welcome.
    :
    : Signed-off-by: Christoph Lameter <clameter@sgi.com>
    : Signed-off-by: Andrew Morton <akpm@osdl.org>
    : Signed-off-by: Linus Torvalds <torvalds@osdl.org>
    :
    : diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
    : index 4bca2a3..a46c10f 100644
    : --- a/Documentation/sysctl/vm.txt
    : +++ b/Documentation/sysctl/vm.txt
    : @@ -137,6 +137,7 @@ This is value ORed together of
    : 1 = Zone reclaim on
    : 2 = Zone reclaim writes dirty pages out
    : 4 = Zone reclaim swaps pages
    : +8 = Also do a global slab reclaim pass
    :
    : zone_reclaim_mode is set during bootup to 1 if it is determined that pages
    : from remote zones will cause a measurable performance reduction. The
    : @@ -160,6 +161,11 @@ Allowing regular swap effectively restricts allocations to the local
    : node unless explicitly overridden by memory policies or cpuset
    : configurations.
    :
    : +It may be advisable to allow slab reclaim if the system makes heavy
    : +use of files and builds up large slab caches. However, the slab
    : +shrink operation is global, may take a long time and free slabs
    : +in all nodes of the system.
    : +
    : ================================================================
    :
    : zone_reclaim_interval:
    : diff --git a/mm/vmscan.c b/mm/vmscan.c
    : index 9e2ef36..aa4b80d 100644
    : --- a/mm/vmscan.c
    : +++ b/mm/vmscan.c
    : @@ -1596,6 +1596,7 @@ int zone_reclaim_mode __read_mostly;
    : #define RECLAIM_ZONE (1<<0) /* Run shrink_cache on the zone */
    : #define RECLAIM_WRITE (1<<1) /* Writeout pages during reclaim */
    : #define RECLAIM_SWAP (1<<2) /* Swap pages out during reclaim */
    : +#define RECLAIM_SLAB (1<<3) /* Do a global slab shrink if the zone is out of memory */
    :
    : /*
    : * Mininum time between zone reclaim scans
    : @@ -1666,6 +1667,19 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    :
    : } while (sc.nr_reclaimed < nr_pages && sc.priority > 0);
    :
    : + if (sc.nr_reclaimed < nr_pages && (zone_reclaim_mode & RECLAIM_SLAB)) {
    : + /*
    : + * shrink_slab does not currently allow us to determine
    : + * how many pages were freed in the zone. So we just
    : + * shake the slab and then go offnode for a single allocation.
    : + *
    : + * shrink_slab will free memory on all zones and may take
    : + * a long time.
    : + */
    : + shrink_slab(sc.nr_scanned, gfp_mask, order);
    : + sc.nr_reclaimed = 1; /* Avoid getting the off node timeout */
    : + }
    : +
    : p->reclaim_state = NULL;
    : current->flags &= ~PF_MEMALLOC;

    > The "lru_pages" parameter is really a division factor affecting
    > the number of pages scanned. This patch increases this division factor
    > significantly and therefore reduces the number of items scanned during
    > zone_reclaim.
    >

    And for that reason I won't apply the patch. I'd be crazy to do so.
    It tosses away four years testing, replacing it with something which
    could have a large effect on reclaim behaviour, with no indication
    whether that effect is good or bad.



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