linux-kernel - Re: [PATCH 0/8] mm: memcontrol: account socket memory in unified hierarchy

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Message-ID: <20151022184509.GM18351@esperanza>
Date:	Thu, 22 Oct 2015 21:45:10 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Tejun Heo <tj@...nel.org>,
	<netdev@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8] mm: memcontrol: account socket memory in unified
 hierarchy

Hi Johannes,

On Thu, Oct 22, 2015 at 12:21:28AM -0400, Johannes Weiner wrote:
...
> Patch #5 adds accounting and tracking of socket memory to the unified
> hierarchy memory controller, as described above. It uses the existing
> per-cpu charge caches and triggers high limit reclaim asynchroneously.
> 
> Patch #8 uses the vmpressure extension to equalize pressure between
> the pages tracked natively by the VM and socket buffer pages. As the
> pool is shared, it makes sense that while natively tracked pages are
> under duress the network transmit windows are also not increased.

First of all, I've no experience in networking, so I'm likely to be
mistaken. Nevertheless I beg to disagree that this patch set is a step
in the right direction. Here goes why.

I admit that your idea to get rid of explicit tcp window control knobs
and size it dynamically basing on memory pressure instead does sound
tempting, but I don't think it'd always work. The problem is that in
contrast to, say, dcache, we can't shrink tcp buffers AFAIU, we can only
stop growing them. Now suppose a system hasn't experienced memory
pressure for a while. If we don't have explicit tcp window limit, tcp
buffers on such a system might have eaten almost all available memory
(because of network load/problems). If a user workload that needs a
significant amount of memory is started suddenly then, the network code
will receive a notification and surely stop growing buffers, but all
those buffers accumulated won't disappear instantly. As a result, the
workload might be unable to find enough free memory and have no choice
but invoke OOM killer. This looks unexpected from the user POV.

That said, I think we do need per memcg tcp window control similar to
what we have system-wide. In other words, Glauber's work makes sense to
me. You might want to point me at my RFC patch where I proposed to
revert it (https://lkml.org/lkml/2014/9/12/401). Well, I've changed my
mind since then. Now I think I was mistaken, luckily I was stopped.
However, I may be mistaken again :-)

Thanks,
Vladimir
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