linux-kernel - Re: [RFC v3 PATCH 0/5] mm: memcontrol: do memory reclaim when offlining

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Date:   Wed, 9 Jan 2019 14:32:47 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mhocko@...e.com, shakeelb@...gle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v3 PATCH 0/5] mm: memcontrol: do memory reclaim when
 offlining

On Thu, Jan 10, 2019 at 03:14:40AM +0800, Yang Shi wrote:
> 
> 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.

We have kswapd for exactly this purpose. Can you lay out more details
on why that is not good enough, especially in conjunction with tuning
the watermark_scale_factor etc.?

We've been pretty adamant that users shouldn't use drop_caches for
performance for example, and that the need to do this usually is
indicative of a problem or suboptimal tuning in the VM subsystem.

How is this different?