Re: Low TCP throughput due to vmpressure with swap enabled

From: Shakeel Butt <shakeelb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ivan Babrou <ivan@cloudflare.com>, Linux MM <linux-mm@kvack.org>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <songmuchun@bytedance.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Eric Dumazet <edumazet@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	cgroups@vger.kernel.org, kernel-team <kernel-team@cloudflare.com>
Subject: Re: Low TCP throughput due to vmpressure with swap enabled
Date: Mon, 5 Dec 2022 19:28:14 +0000	[thread overview]
Message-ID: <20221205192814.diiwtktsrgxzccw2@google.com> (raw)
In-Reply-To: <Y4T43Tc54vlKjTN0@cmpxchg.org>

On Mon, Nov 28, 2022 at 01:07:25PM -0500, Johannes Weiner wrote:
> 
[...]
> > With the patch applied I'm capped at ~120MB/s, which is a symptom of a
> > clamped window.
> > 
> > I can't find any sockets with memcg->socket_pressure = 1, but at the
> > same time I only see the following rcv_ssthresh assigned to sockets:
> 
> Hm, I don't see how socket accounting would alter the network behavior
> other than through socket_pressure=1.
> 

I think what is happening is that the tcp stack is calling
tcp_under_memory_pressure() and making decisions without going through
the memcg charge codepath which set or reset memcg->socket_pressure.
Most probably the socket is clamped due to memcg->socket_pressure and
then the kernel never tried to grow its buffers because
memcg->socket_pressure is still set and thus never tried the memcg
charge codepath which would have reset memcg->socket_pressure. (Maybe)
That is my guess but network experts CCed can correct me.

Anyways, I don't think the pressure mechanism which relies on successful
charging will work. I am brainstorming towards memory.high based network
throttling. Basically use penalty_jiffies (or something similar) to set
memcg->socket_pressure. However I want this to be opt-in as we do have
applications which prefer to be killed than be throttled. So, still
working on the fine details how this can be done without introducing a
rigid API.