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

[Josef Bacik]

On Wed, Jan 09, 2019 at 07:08:09PM +0300, Kirill Tkhai wrote:
> Hi, Josef,
> 
> On 09.01.2019 18:49, Josef Bacik wrote:
> > On Wed, Jan 09, 2019 at 03:20:18PM +0300, Kirill Tkhai 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.
> >>
> >> Below is small test, which shows profit of the patchset.
> >>
> >> Create memcg with limit 20M (exact value does not matter much):
> >>   $ mkdir /sys/fs/cgroup/memory/ct
> >>   $ echo 20M > /sys/fs/cgroup/memory/ct/memory.limit_in_bytes
> >>   $ echo $$ > /sys/fs/cgroup/memory/ct/tasks
> >>
> >> Then twice read 1GB file:
> >>   $ time cat file_1gb > /dev/null
> >>
> >> Before (2 iterations):
> >>   1)0.01user 0.82system 0:11.16elapsed 7%CPU
> >>   2)0.01user 0.91system 0:11.16elapsed 8%CPU
> >>
> >> After (2 iterations):
> >>   1)0.01user 0.57system 0:11.31elapsed 5%CPU
> >>   2)0.00user 0.28system 0:00.28elapsed 100%CPU
> >>
> >> With the patch set applied, we have file pages are cached
> >> during the second read, so the result is 39 times faster.
> >>
> >> This may be useful for slow disks, NFS, nodes without
> >> overcommit by memory, in case of two memcg access the same
> >> files, etc.
> >>
> > 
> > This isn't going to work for us (Facebook).  The whole reason the hard limit
> > exists is to keep different groups from messing up other groups.  Page cache
> > reclaim is not free, most of our pain and most of the reason we use cgroups
> > is to limit the effect of flooding the machine with pagecache from different
> > groups.
> 
> I understand the problem.
> 
> > Memory leaks happen few and far between, but chef doing a yum
> > update in the system container happens regularly.  If you talk about suddenly
> > orphaning these pages to the root container it still creates pressure on the
> > main workload, pressure that results in it having to take time from what it's
> > doing and free up memory instead.
> 
> Could you please to clarify additional pressure, which introduces the patchset?
> The number of actions, which are needed to evict a pagecache page, remain almost
> the same: we just delay __delete_from_page_cache() to global reclaim. Global
> reclaim should not introduce much pressure, since it's the iteration on a single
> memcg (we should not dive into hell of children memcg, since root memcg reclaim
> should be successful and free enough pages, should't we?).

If we go into global reclaim at all.  If we're unable to allocate a page as the
most important cgroup we start shrinking ourselves first right?  And then
eventually end up in global reclaim, right?  So it may be easily enough
reclaimed, but we're going to waste a lot of time getting there in the meantime,
which means latency that's hard to pin down.

And secondly this allows hard limited cgroups to essentially leak pagecache into
the whole system, creating waaaaaaay more memory pressure than what I think you
intend.  Your logic is that we'll exceed our limit, evict some pagecache to the
root cgroup, and we avoid a OOM and everything is ok.  However what will really
happen is some user is going to do dd if=/dev/zero of=file and we'll just
happily keep shoving these pages off into the root cg and suddenly we have 100gb
of useless pagecache that we have to reclaim.  Yeah we just have to delete it
from the root, but thats only once we get to that part, before that there's a
bunch of latency inducing work that has to be done to get to deleting the pages.

> 
> Also, what is about implementing this as static key option? What about linking
> orphaned pagecache pages into separate list, which is easy-to-iterate?

Yeah if we have a way to short-circuit the normal reclaim path and just go to
evicting these easily evicted pages then that would make it more palatable.  But
I'd like to see testing to verify that this faster way really is faster and
doesn't induce latency on other protected workloads.  We put hard limits on
groups we don't care about, we want those things to die in a fire.  The excess
IO from re-reading those pages is mitigated with io.latency, and eventually
io.weight for proportional control, so really isn't an argument for keeping
pages around.  Thanks,

Josef