Re: [RFC PATCH] mm: memcontrol: memory+swap accounting for cgroup-v2

From: Tejun Heo
Date: Thu Dec 21 2017 - 08:37:37 EST

Hello, Shakeel.

On Wed, Dec 20, 2017 at 05:15:41PM -0800, Shakeel Butt wrote:
> Let's say we have a job that allocates 100 MiB memory and suppose 80
> MiB is anon and 20 MiB is non-anon (file & kmem).
> [With memsw] Scheduler sets the memsw limit of the job to 100 MiB and
> memory to max. Now suppose the job tries to allocates memory more than
> 100 MiB, it will hit the memsw limit and will try to reclaim non-anon
> memory. The memcg OOM behavior will only depend on the reclaim of
> non-anon memory and will be independent of the underlying swap device.

Sure, the direct reclaim on memsw limit won't reclaim anon pages, but
think about how the state at that point would have formed. You're
claiming that memsw makes memory allocation and balancing behavior an
invariant against the performance of the swap device that the machine
has. It's simply not possible.

On top of that, what's the point?

1. As I wrote earlier, given the current OOM killer implementation,
whether OOM kicks in or not is not even that relevant in
determining the health of the workload. There are frequent failure
modes where OOM killer fails to kick in while the workload isn't
making any meaningful forward progress.

2. On hitting memsw limit, the OOM decision is dependent on the
performance of the file backing devices. Why is that necessarily
better than being dependent on swap or both, which would increase
the reclaim efficiency anyway? You can't avoid being affected by
the underlying hardware one way or the other.

3. The only thing memsw does is that memsw direct reclaim will only
consider file backed pages, which I think is more of an accident
(in an attemp to avoid lower swap setting meaning higher actual
memory usage) than the intended outcome. This is obviously
suboptimal and an implementation detail. I don't think it's
something we want to expose to userland as a feature.