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

[Vladimir Davydov]

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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