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    SubjectRe: [PATCH -mm 07/25] second chance replacement for anonymous pages
    On Fri, 06 Jun 2008 16:28:45 -0400
    Rik van Riel <riel@redhat.com> wrote:

    > From: Rik van Riel <riel@redhat.com>
    >
    > We avoid evicting and scanning anonymous pages for the most part, but
    > under some workloads we can end up with most of memory filled with
    > anonymous pages. At that point, we suddenly need to clear the referenced
    > bits on all of memory, which can take ages on very large memory systems.
    >
    > We can reduce the maximum number of pages that need to be scanned by
    > not taking the referenced state into account when deactivating an
    > anonymous page. After all, every anonymous page starts out referenced,
    > so why check?
    >
    > If an anonymous page gets referenced again before it reaches the end
    > of the inactive list, we move it back to the active list.
    >
    > To keep the maximum amount of necessary work reasonable, we scale the
    > active to inactive ratio with the size of memory, using the formula
    > active:inactive ratio = sqrt(memory in GB * 10).

    Should be scaled by PAGE_SIZE?

    > Kswapd CPU use now seems to scale by the amount of pageout bandwidth,
    > instead of by the amount of memory present in the system.
    >
    > Signed-off-by: Rik van Riel <riel@redhat.com>
    > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    >
    > ---
    > include/linux/mm_inline.h | 12 ++++++++++++
    > include/linux/mmzone.h | 5 +++++
    > mm/page_alloc.c | 40 ++++++++++++++++++++++++++++++++++++++++
    > mm/vmscan.c | 38 +++++++++++++++++++++++++++++++-------
    > mm/vmstat.c | 6 ++++--
    > 5 files changed, 92 insertions(+), 9 deletions(-)
    >
    > Index: linux-2.6.26-rc2-mm1/include/linux/mm_inline.h
    > ===================================================================
    > --- linux-2.6.26-rc2-mm1.orig/include/linux/mm_inline.h 2008-05-23 14:21:34.000000000 -0400
    > +++ linux-2.6.26-rc2-mm1/include/linux/mm_inline.h 2008-05-28 12:09:06.000000000 -0400
    > @@ -97,4 +97,16 @@ del_page_from_lru(struct zone *zone, str
    > __dec_zone_state(zone, NR_INACTIVE_ANON + l);
    > }
    >
    > +static inline int inactive_anon_low(struct zone *zone)
    > +{
    > + unsigned long active, inactive;
    > +
    > + active = zone_page_state(zone, NR_ACTIVE_ANON);
    > + inactive = zone_page_state(zone, NR_INACTIVE_ANON);
    > +
    > + if (inactive * zone->inactive_ratio < active)
    > + return 1;
    > +
    > + return 0;
    > +}

    inactive_anon_low: "number of inactive anonymous pages which are in lowmem"?

    Nope.

    Needs a comment. And maybe a better name, like inactive_anon_is_low.
    Although making the return type a bool kind-of does that.

    > #endif
    > Index: linux-2.6.26-rc2-mm1/include/linux/mmzone.h
    > ===================================================================
    > --- linux-2.6.26-rc2-mm1.orig/include/linux/mmzone.h 2008-05-23 14:21:34.000000000 -0400
    > +++ linux-2.6.26-rc2-mm1/include/linux/mmzone.h 2008-05-28 12:09:06.000000000 -0400
    > @@ -311,6 +311,11 @@ struct zone {
    > */
    > int prev_priority;
    >
    > + /*
    > + * The ratio of active to inactive pages.
    > + */
    > + unsigned int inactive_ratio;

    That comment needs a lot of help please. For a start, it's plain wrong
    - inactive_ratio would need to be a float to be able to record that ratio.

    The comment should describe the units too.

    Now poor-old-reviewer has to go off and work out what this thing is.

    >
    > ZONE_PADDING(_pad2_)
    > /* Rarely used or read-mostly fields */
    > Index: linux-2.6.26-rc2-mm1/mm/page_alloc.c
    > ===================================================================
    > --- linux-2.6.26-rc2-mm1.orig/mm/page_alloc.c 2008-05-23 14:21:34.000000000 -0400
    > +++ linux-2.6.26-rc2-mm1/mm/page_alloc.c 2008-05-28 12:09:06.000000000 -0400
    > @@ -4269,6 +4269,45 @@ void setup_per_zone_pages_min(void)
    > calculate_totalreserve_pages();
    > }
    >
    > +/**
    > + * setup_per_zone_inactive_ratio - called when min_free_kbytes changes.
    > + *
    > + * The inactive anon list should be small enough that the VM never has to
    > + * do too much work, but large enough that each inactive page has a chance
    > + * to be referenced again before it is swapped out.
    > + *
    > + * The inactive_anon ratio is the ratio of active to inactive anonymous

    target ratio? Desired ratio?

    > + * pages. Ie. a ratio of 3 means 3:1 or 25% of the anonymous pages are
    > + * on the inactive list.
    > + *
    > + * total return max
    > + * memory value inactive anon

    This function doesn't "return" a "value".

    > + * -------------------------------------
    > + * 10MB 1 5MB
    > + * 100MB 1 50MB
    > + * 1GB 3 250MB
    > + * 10GB 10 0.9GB
    > + * 100GB 31 3GB
    > + * 1TB 101 10GB
    > + * 10TB 320 32GB
    > + */
    > +void setup_per_zone_inactive_ratio(void)
    > +{
    > + struct zone *zone;
    > +
    > + for_each_zone(zone) {
    > + unsigned int gb, ratio;
    > +
    > + /* Zone size in gigabytes */
    > + gb = zone->present_pages >> (30 - PAGE_SHIFT);
    > + ratio = int_sqrt(10 * gb);
    > + if (!ratio)
    > + ratio = 1;
    > +
    > + zone->inactive_ratio = ratio;
    > + }
    > +}

    OK, so inactive_ratio is an integer 1 .. N which determines our target
    number of inactive pages according to the formula

    nr_inactive = nr_active / inactive_ratio

    yes?

    Can nr_inactive get larger than this? I assume so. I guess that
    doesn't matter much. Except the problems which you're trying to sovle
    here can reoccur. What would I need to do to trigger that?

    > /*
    > * Initialise min_free_kbytes.
    > *
    > @@ -4306,6 +4345,7 @@ static int __init init_per_zone_pages_mi
    > min_free_kbytes = 65536;
    > setup_per_zone_pages_min();
    > setup_per_zone_lowmem_reserve();
    > + setup_per_zone_inactive_ratio();
    > return 0;
    > }
    > module_init(init_per_zone_pages_min)
    > Index: linux-2.6.26-rc2-mm1/mm/vmscan.c
    > ===================================================================
    > --- linux-2.6.26-rc2-mm1.orig/mm/vmscan.c 2008-05-23 14:21:34.000000000 -0400
    > +++ linux-2.6.26-rc2-mm1/mm/vmscan.c 2008-05-28 12:11:38.000000000 -0400
    > @@ -114,7 +114,7 @@ struct scan_control {
    > /*
    > * From 0 .. 100. Higher means more swappy.
    > */
    > -int vm_swappiness = 60;
    > +int vm_swappiness = 20;

    <goes back to check the changelog>

    Whoa. Where'd this come from?

    > long vm_total_pages; /* The total number of pages which the VM controls */
    >
    > static LIST_HEAD(shrinker_list);
    > @@ -1008,7 +1008,7 @@ static inline int zone_is_near_oom(struc
    > static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
    > struct scan_control *sc, int priority, int file)
    > {
    > - unsigned long pgmoved;
    > + unsigned long pgmoved = 0;
    > int pgdeactivate = 0;
    > unsigned long pgscanned;
    > LIST_HEAD(l_hold); /* The pages which were snipped off */
    > @@ -1036,17 +1036,32 @@ static void shrink_active_list(unsigned
    > __mod_zone_page_state(zone, NR_ACTIVE_ANON, -pgmoved);
    > spin_unlock_irq(&zone->lru_lock);
    >
    > + pgmoved = 0;

    didn't we just do that?

    > while (!list_empty(&l_hold)) {
    > cond_resched();
    > page = lru_to_page(&l_hold);
    > list_del(&page->lru);
    > - if (page_referenced(page, 0, sc->mem_cgroup))
    > - list_add(&page->lru, &l_active);
    > - else
    > + if (page_referenced(page, 0, sc->mem_cgroup)) {
    > + if (file) {
    > + /* Referenced file pages stay active. */
    > + list_add(&page->lru, &l_active);
    > + } else {
    > + /* Anonymous pages always get deactivated. */

    hm. That's going to make the machine swap like hell. I guess I don't
    understand all this yet.

    > + list_add(&page->lru, &l_inactive);
    > + pgmoved++;
    > + }
    > + } else
    > list_add(&page->lru, &l_inactive);
    > }
    >
    > /*
    > + * Count the referenced anon pages as rotated, to balance pageout
    > + * scan pressure between file and anonymous pages in get_sacn_ratio.

    tpyo




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