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SubjectRe: [PATCH 3/3] Provide control over unmapped pages (v4)
On Fri, 28 Jan 2011 16:24:19 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Fri, Jan 28, 2011 at 3:48 PM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * MinChan Kim <minchan.kim@gmail.com> [2011-01-28 14:44:50]:
> >
> >> On Fri, Jan 28, 2011 at 11:56 AM, Balbir Singh
> >> <balbir@linux.vnet.ibm.com> wrote:
> >> > On Thu, Jan 27, 2011 at 4:42 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> >> > [snip]
> >> >
> >> >>> index 7b56473..2ac8549 100644
> >> >>> --- a/mm/page_alloc.c
> >> >>> +++ b/mm/page_alloc.c
> >> >>> @@ -1660,6 +1660,9 @@ zonelist_scan:
> >> >>>                        unsigned long mark;
> >> >>>                        int ret;
> >> >>>
> >> >>> +                       if (should_reclaim_unmapped_pages(zone))
> >> >>> +                               wakeup_kswapd(zone, order, classzone_idx);
> >> >>> +
> >> >>
> >> >> Do we really need the check in fastpath?
> >> >> There are lost of caller of alloc_pages.
> >> >> Many of them are not related to mapped pages.
> >> >> Could we move the check into add_to_page_cache_locked?
> >> >
> >> > The check is a simple check to see if the unmapped pages need
> >> > balancing, the reason I placed this check here is to allow other
> >> > allocations to benefit as well, if there are some unmapped pages to be
> >> > freed. add_to_page_cache_locked (check under a critical section) is
> >> > even worse, IMHO.
> >>
> >> It just moves the overhead from general into specific case(ie,
> >> allocates page for just page cache).
> >> Another cases(ie, allocates pages for other purpose except page cache,
> >> ex device drivers or fs allocation for internal using) aren't
> >> affected.
> >> So, It would be better.
> >>
> >> The goal in this patch is to remove only page cache page, isn't it?
> >> So I think we could the balance check in add_to_page_cache and trigger reclaim.
> >> If we do so, what's the problem?
> >>
> >
> > I see it as a tradeoff of when to check? add_to_page_cache or when we
> > are want more free memory (due to allocation). It is OK to wakeup
> > kswapd while allocating memory, somehow for this purpose (global page
> > cache), add_to_page_cache or add_to_page_cache_locked does not seem
> > the right place to hook into. I'd be open to comments/suggestions
> > though from others as well.

I don't like add hook here.
AND I don't want to run kswapd because 'kswapd' has been a sign as
there are memory shortage. (reusing code is ok.)

How about adding new daemon ? Recently, khugepaged, ksmd works for
managing memory. Adding one more daemon for special purpose is not
very bad, I think. Then, you can do
- wake up without hook
- throttle its work.
- balance the whole system rather than zone.
I think per-node balance is enough...








> >
> >> >
> >> >
> >> >>
> >> >>>                        mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> >> >>>                        if (zone_watermark_ok(zone, order, mark,
> >> >>>                                    classzone_idx, alloc_flags))
> >> >>> @@ -4167,8 +4170,12 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
> >> >>>
> >> >>>                zone->spanned_pages = size;
> >> >>>                zone->present_pages = realsize;
> >> >>> +#if defined(CONFIG_UNMAPPED_PAGE_CONTROL) || defined(CONFIG_NUMA)
> >> >>>                zone->min_unmapped_pages = (realsize*sysctl_min_unmapped_ratio)
> >> >>>                                                / 100;
> >> >>> +               zone->max_unmapped_pages = (realsize*sysctl_max_unmapped_ratio)
> >> >>> +                                               / 100;
> >> >>> +#endif
> >> >>>  #ifdef CONFIG_NUMA
> >> >>>                zone->node = nid;
> >> >>>                zone->min_slab_pages = (realsize * sysctl_min_slab_ratio) / 100;
> >> >>> @@ -5084,6 +5091,7 @@ int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
> >> >>>        return 0;
> >> >>>  }
> >> >>>
> >> >>> +#if defined(CONFIG_UNMAPPED_PAGE_CONTROL) || defined(CONFIG_NUMA)
> >> >>>  int sysctl_min_unmapped_ratio_sysctl_handler(ctl_table *table, int write,
> >> >>>        void __user *buffer, size_t *length, loff_t *ppos)
> >> >>>  {
> >> >>> @@ -5100,6 +5108,23 @@ int sysctl_min_unmapped_ratio_sysctl_handler(ctl_table *table, int write,
> >> >>>        return 0;
> >> >>>  }
> >> >>>
> >> >>> +int sysctl_max_unmapped_ratio_sysctl_handler(ctl_table *table, int write,
> >> >>> +       void __user *buffer, size_t *length, loff_t *ppos)
> >> >>> +{
> >> >>> +       struct zone *zone;
> >> >>> +       int rc;
> >> >>> +
> >> >>> +       rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
> >> >>> +       if (rc)
> >> >>> +               return rc;
> >> >>> +
> >> >>> +       for_each_zone(zone)
> >> >>> +               zone->max_unmapped_pages = (zone->present_pages *
> >> >>> +                               sysctl_max_unmapped_ratio) / 100;
> >> >>> +       return 0;
> >> >>> +}
> >> >>> +#endif
> >> >>> +
> >> >>>  #ifdef CONFIG_NUMA
> >> >>>  int sysctl_min_slab_ratio_sysctl_handler(ctl_table *table, int write,
> >> >>>        void __user *buffer, size_t *length, loff_t *ppos)
> >> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> >>> index 02cc82e..6377411 100644
> >> >>> --- a/mm/vmscan.c
> >> >>> +++ b/mm/vmscan.c
> >> >>> @@ -159,6 +159,29 @@ static DECLARE_RWSEM(shrinker_rwsem);
> >> >>>  #define scanning_global_lru(sc)        (1)
> >> >>>  #endif
> >> >>>
> >> >>> +#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
> >> >>> +static unsigned long reclaim_unmapped_pages(int priority, struct zone *zone,
> >> >>> +                                               struct scan_control *sc);
> >> >>> +static int unmapped_page_control __read_mostly;
> >> >>> +
> >> >>> +static int __init unmapped_page_control_parm(char *str)
> >> >>> +{
> >> >>> +       unmapped_page_control = 1;
> >> >>> +       /*
> >> >>> +        * XXX: Should we tweak swappiness here?
> >> >>> +        */
> >> >>> +       return 1;
> >> >>> +}
> >> >>> +__setup("unmapped_page_control", unmapped_page_control_parm);
> >> >>> +
> >> >>> +#else /* !CONFIG_UNMAPPED_PAGECACHE_CONTROL */
> >> >>> +static inline unsigned long reclaim_unmapped_pages(int priority,
> >> >>> +                               struct zone *zone, struct scan_control *sc)
> >> >>> +{
> >> >>> +       return 0;
> >> >>> +}
> >> >>> +#endif
> >> >>> +
> >> >>>  static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
> >> >>>                                                  struct scan_control *sc)
> >> >>>  {
> >> >>> @@ -2359,6 +2382,12 @@ loop_again:
> >> >>>                                shrink_active_list(SWAP_CLUSTER_MAX, zone,
> >> >>>                                                        &sc, priority, 0);
> >> >>>
> >> >>> +                       /*
> >> >>> +                        * We do unmapped page reclaim once here and once
> >> >>> +                        * below, so that we don't lose out
> >> >>> +                        */
> >> >>> +                       reclaim_unmapped_pages(priority, zone, &sc);
> >> >>> +
> >> >>>                        if (!zone_watermark_ok_safe(zone, order,
> >> >>>                                        high_wmark_pages(zone), 0, 0)) {
> >> >>>                                end_zone = i;
> >> >>> @@ -2396,6 +2425,11 @@ loop_again:
> >> >>>                                continue;
> >> >>>
> >> >>>                        sc.nr_scanned = 0;
> >> >>> +                       /*
> >> >>> +                        * Reclaim unmapped pages upfront, this should be
> >> >>> +                        * really cheap
> >> >>> +                        */
> >> >>> +                       reclaim_unmapped_pages(priority, zone, &sc);
> >> >>
> >> >> Why should we do by two phase?
> >> >> It's not a direct reclaim path. I mean it doesn't need to reclaim tighly
> >> >> If we can't reclaim enough, next allocation would wake up kswapd again
> >> >> and kswapd try it again.
> >> >>
> >> >
> >> > I am not sure I understand, the wakeup will occur only if the unmapped
> >> > pages are still above the max_unmapped_ratio. They are tunable control
> >> > points.
> >>
> >> I mean you try to reclaim twice in one path.
> >> one is when select highest zone to reclaim.
> >> one is when VM reclaim the zone.
> >>
> >> What's your intention?
> >>
> >
> > That is because some zones can be skipped, we need to ensure we go
> > through all zones, rather than selective zones (limited via search for
> > end_zone).
>
> If kswapd is wake up by unmapped memory of some zone, we have to
> include the zone while selective victim zones to prevent miss the
> zone.
> I think it would be better than reclaiming twice
>

That sounds checking all zones and loop again is enough.


BTW, it seems this doesn't work when some apps use huge shmem.
How to handle the issue ?

Thanks,
-Kame


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