Re: [RFC Patch] Thrashing notification

["Daniel Spång" <daniel.spang@gmail.com>]

On 11/5/07, Marcelo Tosatti <marcelo@kvack.org> wrote:
> Hooking into try_to_free_pages() makes the scheme suspectible to
> specifics such as:
>
> - can the task writeout pages?
> - is the allocation a higher order one?
> - in what zones is it operating on?
>
> Remember that notifications are sent to applications which can allocate
> globally... It is not very useful to send notifications for a userspace
> which has a large percentage of its memory in highmem if the system is
> having a lowmem zone shortage (granted that the notify-on-swap heuristic
> has that problem, but you can then argue that swap affects system
> performance globally, and it generally does in desktop systems).

On a swapless system, the alternative is often to get killed by the oom killer.

> Other than that tuning "priority" from try_to_free_pages() is rather
> difficult for users/admins.

Yes, that parameter might need some tuning, but my initial tests show
that is pretty robust if you keep out of the ends of the interval.

> My previous patches had the zone limitation, but the following way of
> asking "are we low on memory?" gets rid of it:
>
> +static unsigned int mem_notify_poll(struct file *file, poll_table *wait)
> +{
> +       unsigned int val = 0;
> +       struct zone *zone;
> +       int tpages_low, tpages_free, tpages_reserve;
> +
> +       tpages_low = tpages_free = tpages_reserve = 0;
> +
> +       poll_wait(file, &mem_wait, wait);
> +
> +       for_each_zone(zone) {
> +               if (!populated_zone(zone))
> +                       continue;
> +               tpages_low += zone->pages_low;
> +               tpages_free += zone_page_state(zone, NR_FREE_PAGES);
> +               /* always use the reserve of the highest allocation type */
> +               tpages_reserve += zone->lowmem_reserve[MAX_NR_ZONES-1];
> +       }
> +
> +       if (mem_notify_status || (tpages_free <= tpages_low + tpages_reserve))
> +               val = POLLIN;
> +
> +       return val;
> +}

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