Re: [RFC 0/4] Introduce unbalance proactive reclaim

From: Huan Yang
Date: Thu Nov 09 2023 - 22:49:07 EST


在 2023/11/9 21:46, Michal Hocko 写道:
On Thu 09-11-23 21:07:29, Huan Yang wrote:
[...]
Of course, as you suggested, madvise can also achieve this, but
implementing it in the agent may be more complex.(In terms of
achieving the same goal, using memcg to group all the processes of an
APP and perform proactive reclamation is simpler than using madvise
and scanning multiple processes of an application using an agent?)
It might be more involved but the primary question is whether it is
usable for the specific use case. Madvise interface is not LRU aware but
you are not really talking about that to be a requirement? So it would
really help if you go deeper into details on how is the interface
actually supposed to be used in your case.
In mobile field, we usually configure zram to compress anonymous page.
We can approximate to expand memory usage with limited hardware memory
by using zram.

With proper strategies, an 8GB RAM phone can approximate the usage of a 12GB
phone
(or more).

In our strategy, we group memcg by application. When the agent detects that
an
application has entered the background, then frozen, and has not been used
for a long time,
the agent will slowly issue commands to reclaim the anonymous page of that
application.

With this interface, `echo memory anon > memory.reclaim`
This doesn't really answer my questions above.
Also make sure to exaplain why you cannot use other existing interfaces.
For example, why you simply don't decrease the limit of the frozen
cgroup and rely on the normal reclaim process to evict the most cold
This is a question of reclamation tendency, and simply decreasing the limit
of the frozen cgroup cannot achieve this.
Why?

Can I ask how to limit the reclamation to only anonymous pages using the limit?

memory? What are you basing your anon vs. file proportion decision on?
When zram is configured and anonymous pages are reclaimed proactively, the
refault
probability of anonymous pages is low when an application is frozen and not
reopened.
Also, the cost of refaulting from zram is relatively low.

However, file pages usually have shared properties, so even if an
application is frozen,
other processes may still access the file pages. If a limit is set and the
reclamation encounters
file pages, it will cause a certain amount of refault I/O, which is costly
for mobile devices.
Two points here (and the reason why I am repeatedly asking for some
data) 1) are you really seeing shared and actively used page cache pages
When we call the current proactive reclamation interface to actively reclaim memory,
the debug program can usually observe that file pages are partially reclaimed.

However, when we start other APPs for testing(the current reclaimed APP is in the background),
the trace shows that there is a lot of block I/O for the background application.
being reclaimed? 2) Is the refault IO really a problem. What kind of
storage those phone have that this is more significant than potentially
GB of compressed anonymous memory which would need CPU to refaulted
Phone typically use UFS.
back. I mean do you have any actual numbers to show that the default
reclaim strategy would lead to a less utilized or less performant
system?
Also, When the application enters the foreground, the startup speed may be slower. Also trace
show that here are a lot of block I/O. (usually 1000+ IO count and 200+ms IO Time)
We usually observe very little block I/O caused by zram refault.(read: 1698.39MB/s, write: 995.109MB/s),
usually, it is faster than random disk reads.(read: 48.1907MB/s write: 49.1654MB/s). This test by
zram-perf and I change a little to test UFS.

Therefore, if the proactive reclamation encounters many file pages, the application may become
slow when it is opened.

--
Thanks,
Huan Yang