Re: [PATCH RFC 0/3] mm: Reduce IO by improving algorithm of memcg pagecache pages eviction

[Kirill Tkhai]

Hi, Shakeel,

On 09.01.2019 20:37, Shakeel Butt wrote:
> Hi Kirill,
> 
> On Wed, Jan 9, 2019 at 4:20 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>
>> On nodes without memory overcommit, it's common a situation,
>> when memcg exceeds its limit and pages from pagecache are
>> shrinked on reclaim, while node has a lot of free memory.
>> Further access to the pages requires real device IO, while
>> IO causes time delays, worse powerusage, worse throughput
>> for other users of the device, etc.
>>
>> Cleancache is not a good solution for this problem, since
>> it implies copying of page on every cleancache_put_page()
>> and cleancache_get_page(). Also, it requires introduction
>> of internal per-cleancache_ops data structures to manage
>> cached pages and their inodes relationships, which again
>> introduces overhead.
>>
>> This patchset introduces another solution. It introduces
>> a new scheme for evicting memcg pages:
>>
>>   1)__remove_mapping() uncharges unmapped page memcg
>>     and leaves page in pagecache on memcg reclaim;
>>
>>   2)putback_lru_page() places page into root_mem_cgroup
>>     list, since its memcg is NULL. Page may be evicted
>>     on global reclaim (and this will be easily, as
>>     page is not mapped, so shrinker will shrink it
>>     with 100% probability of success);
>>
>>   3)pagecache_get_page() charges page into memcg of
>>     a task, which takes it first.
>>
> 
> From what I understand from the proposal, on memcg reclaim, the file
> pages are uncharged but kept in the memory and if they are accessed
> again (either through mmap or syscall), they will be charged again but
> to the requesting memcg. Also it is assumed that the global reclaim of
> such uncharged file pages is very fast and deterministic. Is that
> right?

Yes, this was my assumption. But Michal, Josef and Johannes pointed a diving
into reclaim in general is not fast. So, maybe we need some more creativity
here to minimize the effect of this diving..

Thanks,
Kirill