Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

From: Yang Shi
Date: Thu Jan 03 2019 - 12:35:14 EST

On 1/3/19 2:12 AM, Michal Hocko wrote:
On Thu 03-01-19 04:05:30, Yang Shi wrote:
Currently, force empty reclaims memory synchronously when writing to
memory.force_empty. It may take some time to return and the afterwards
operations are blocked by it. Although it can be interrupted by signal,
it still seems suboptimal.
Why it is suboptimal? We are doing that operation on behalf of the
process requesting it. What should anybody else pay for it? In other
words why should we hide the overhead?

Please see the below explanation.

Now css offline is handled by worker, and the typical usecase of force
empty is before memcg offline. So, handling force empty in css offline
sounds reasonable.
Hmm, so I guess you are talking about
echo 1 > $MEMCG/force_empty
rmdir $MEMCG

and you are complaining that the operation takes too long. Right? Why do
you care actually?

We have some usecases which create and remove memcgs very frequently, and the tasks in the memcg may just access the files which are unlikely accessed by anyone else. So, we prefer force_empty the memcg before rmdir'ing it to reclaim the page cache so that they don't get accumulated to incur unnecessary memory pressure. Since the memory pressure may incur direct reclaim to harm some latency sensitive applications.

And, the create/remove might be run in a script sequentially (there might be a lot scripts or applications are run in parallel to do this), i.e.
mkdir cg1
do something
echo 0 > cg1/memory.force_empty
rmdir cg1

mkdir cg2

The creation of the afterwards memcg might be blocked by the force_empty for long time if there are a lot page caches, so the overall throughput of the system may get hurt.
And, it is not that urgent to reclaim the page cache right away and it is not that important who pays the cost, we just need a mechanism to reclaim the pages soon in a short while. The overhead could be smoothed by background workqueue.

And, the patch still keeps the old behavior, just in case someone else still depends on it.