Re: [PATCH V5 06/10] Per-memcg background reclaim.

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Apr 19, 2011 at 6:38 AM, Ying Han <yinghan@google.com> wrote:
>
>
> On Sun, Apr 17, 2011 at 8:51 PM, Minchan Kim <minchan.kim@gmail.com> wrote:
>>
>> On Sat, Apr 16, 2011 at 8:23 AM, Ying Han <yinghan@google.com> wrote:
>> > +
>> > +               sc->nr_scanned = 0;
>> > +               shrink_zone(priority, zone, sc);
>> > +               total_scanned += sc->nr_scanned;
>> > +
>> > +               /*
>> > +                * If we've done a decent amount of scanning and
>> > +                * the reclaim ratio is low, start doing writepage
>> > +                * even in laptop mode
>> > +                */
>> > +               if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
>> > +                   total_scanned > sc->nr_reclaimed + sc->nr_reclaimed
>> > / 2) {
>> > +                       sc->may_writepage = 1;
>>
>> I don't want to add more random write any more although we don't have
>> a trouble of real memory shortage.
>
>
>>
>> Do you have any reason to reclaim memory urgently as writing dirty pages?
>> Maybe if we wait a little bit of time, flusher would write out the page.
>
> We would like to reduce the writing dirty pages from page reclaim,
> especially from direct reclaim. AFAIK, the try_to_free_mem_cgroup_pages()
> still need to write dirty pages when there is a need. removing this from the
> per-memcg kswap will only add more pressure to the per-memcg direct reclaim,
> which seems to be worse. (stack overflow as one example which we would like
> to get rid of)
>

Stack overflow would be another topic.

Normal situation :

The softlimit memory pressure of memcg isn't real memory shortage and
if we have gap between hardlimit and softlimit, periodic writeback of
flusher would write it out before reaching the hardlimit. In the end,
direct reclaim don't need to write it out.

Exceptional situation :

Of course, it doesn't work well in congestion of bdi, sudden big
memory consumption in memcg in wrong [hard/soft]limit(small gap)
configuration of administrator.

I think we have to design it by normal situation.
The point is that softlimit isn't real memory shortage so that we are
not urgent.

How about adding new function which checks global memory pressure and
if we have a trouble by global memory pressure, we can change
may_write with 1 dynamically in memcg_kswapd?

-- 
Kind regards,
Minchan Kim

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