Re: [PATCH] memcg: introduce per-memcg reclaim interface

[Michal Hocko]

On Mon 21-09-20 10:50:14, Shakeel Butt wrote:
> On Mon, Sep 21, 2020 at 9:30 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Wed 09-09-20 14:57:52, Shakeel Butt wrote:
> > > Introduce an memcg interface to trigger memory reclaim on a memory cgroup.
> > >
> > > Use cases:
> > > ----------
> > >
> > > 1) Per-memcg uswapd:
> > >
> > > Usually applications consists of combination of latency sensitive and
> > > latency tolerant tasks. For example, tasks serving user requests vs
> > > tasks doing data backup for a database application. At the moment the
> > > kernel does not differentiate between such tasks when the application
> > > hits the memcg limits. So, potentially a latency sensitive user facing
> > > task can get stuck in high reclaim and be throttled by the kernel.
> > >
> > > Similarly there are cases of single process applications having two set
> > > of thread pools where threads from one pool have high scheduling
> > > priority and low latency requirement. One concrete example from our
> > > production is the VMM which have high priority low latency thread pool
> > > for the VCPUs while separate thread pool for stats reporting, I/O
> > > emulation, health checks and other managerial operations. The kernel
> > > memory reclaim does not differentiate between VCPU thread or a
> > > non-latency sensitive thread and a VCPU thread can get stuck in high
> > > reclaim.
> >
> > As those are presumably in the same cgroup what does prevent them to get
> > stuck behind shared resources with taken during the reclaim performed by
> > somebody else? I mean, memory reclaim might drop memory used by the high
> > priority task. Or they might simply stumble over same locks.
> >
> 
> Yes there are a lot of challenges in providing isolation between
> latency sensitive and latency tolerant jobs/threads. This proposal
> aims to solve one specific challenge memcg limit reclaim.

I am fully aware that a complete isolation is hard to achieve. I am just
trying evaluate how is this specific usecase worth a new interface that
we will have to maintain for ever. Especially when I suspect that the
interface will likely only paper over immediate problems rather than
offer a long term maintainable solution for it.

> > I am also more interested in actual numbers here. The high limit reclaim
> > is normally swift and should be mostly unnoticeable. If the reclaim gets
> > more expensive then it can get really noticeable for sure. But for the
> > later the same can happen with the external pro-activee reclaimer as
> 
> I think you meant 'uswapd' here instead of pro-active reclaimer.
>
> > well, right? So there is no real "guarantee". Do you have any numbers
> > from your workloads where you can demonstrate that the external reclaim
> > has saved you this amount of effective cpu time of the sensitive
> > workload? (Essentially measure how much time it has to consume in the
> > high limit reclaim)
> >
> 
> What we actually use in our production is the 'proactive reclaim'
> which I have explained in the original message but I will add a couple
> more sentences below.
> 
> For the uswapd use-case, let me point to the previous discussions and
> feature requests by others [1, 2]. One of the limiting factors of
> these previous proposals was the lack of CPU accounting of the
> background reclaimer which the current proposal solves by enabling the
> user space solution.
> 
> [1] https://lwn.net/Articles/753162/
> [2] http://lkml.kernel.org/r/20200219181219.54356-1-hannes@xxxxxxxxxxx

I remember those. My understanding was that the only problem is to
properly account for CPU on behalf of the reclaimed cgroup and that has
been work in progress for that.

Outsourcing all that to userspace surely sounds like an attractive
option but it comes with usual user API price. More on that later.

> Let me add one more point. Even if the high limit reclaim is swift, it
> can still take 100s of usecs. Most of our jobs are anon-only and we
> use zswap. Compressing a page can take a couple usec, so 100s of usecs
> in limit reclaim is normal. For latency sensitive jobs, this amount of
> hiccups do matters.

Understood. But isn't this an implementation detail of zswap? Can it
offload some of the heavy lifting to a different context and reduce the
general overhead?

> For the proactive reclaim, based on the refault medium, we define
> tolerable refault rate of the applications. Then we proactively
> reclaim memory from the applications and monitor the refault rates.
> Based on the refault rates, the memory overcommit manager controls the
> aggressiveness of the proactive reclaim.
> 
> This is exactly what we do in the production. Please let me know if
> you want to know why we do proactive reclaim in the first place.

This information is definitely useful and having it in the changelog
would be useful. IIUC the only reason why you cannot use high limit
to control this pro-active reclaim is the potential throttling due to
expensive reclaim, correct?

> > To the feature itself, I am not yet convinced we want to have a feature
> > like that. It surely sounds easy to use and attractive for a better user
> > space control. It is also much well defined than drop_caches/force_empty
> > because it is not all or nothing. But it also sounds like something too
> > easy to use incorrectly (remember drop_caches). I am also a bit worried
> > about corner cases wich would be easier to hit - e.g. fill up the swap
> > limit and turn anonymous memory into unreclaimable and who knows what
> > else.
> 
> The corner cases you are worried about are already possible with the
> existing interfaces. We can already do all such things with
> memory.high interface but with some limitations. This new interface
> resolves that limitation as explained in the original email.

You are right that misconfigured limits can result in problems. But such
a configuration should be quite easy to spot which is not the case for
targetted reclaim calls which do not leave any footprints behind.
Existing interfaces are trying to not expose internal implementation
details as much as well. You are proposing a very targeted interface to
fine control the memory reclaim. There is a risk that userspace will
start depending on a specific reclaim implementation/behavior and future
changes would be prone to regressions in workloads relying on that. So
effectively, any user space memory reclaimer would need to be tuned to a
specific implementation of the memory reclaim. My past experience tells
me that this is not a great thing for maintainability of neither kernel
nor the userspace part.

All that being said, we really should consider whether the proposed
interface is trying to work around existing limitations in the reclaim
or the interface. If this is the former then I do not think we should be
adding it. If the later then we should discuss on how to improve our
existing interfaces (or their implementations) to be better usable and
allow your usecase to work better.

What is your take on that Johannes?
-- 
Michal Hocko
SUSE Labs