Re: [PATCH] mm: memcontrol: asynchronous reclaim for memory.high

[Shakeel Butt]

On Wed, Feb 26, 2020 at 3:59 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2/26/20 12:25 PM, Shakeel Butt wrote:
> > On Wed, Feb 19, 2020 at 10:12 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >> We have received regression reports from users whose workloads moved
> >> into containers and subsequently encountered new latencies. For some
> >> users these were a nuisance, but for some it meant missing their SLA
> >> response times. We tracked those delays down to cgroup limits, which
> >> inject direct reclaim stalls into the workload where previously all
> >> reclaim was handled my kswapd.
> >>
> >> This patch adds asynchronous reclaim to the memory.high cgroup limit
> >> while keeping direct reclaim as a fallback. In our testing, this
> >> eliminated all direct reclaim from the affected workload.
> >>
> >> memory.high has a grace buffer of about 4% between when it becomes
> >> exceeded and when allocating threads get throttled. We can use the
> >> same buffer for the async reclaimer to operate in. If the worker
> >> cannot keep up and the grace buffer is exceeded, allocating threads
> >> will fall back to direct reclaim before getting throttled.
> >>
> >> For irq-context, there's already async memory.high enforcement. Re-use
> >> that work item for all allocating contexts, but switch it to the
> >> unbound workqueue so reclaim work doesn't compete with the workload.
> >> The work item is per cgroup, which means the workqueue infrastructure
> >> will create at maximum one worker thread per reclaiming cgroup.
> >>
> >> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> >> ---
> >>   mm/memcontrol.c | 60 +++++++++++++++++++++++++++++++++++++------------
> >>   mm/vmscan.c     | 10 +++++++--
> > This reminds me of the per-memcg kswapd proposal from LSFMM 2018
> > (https://lwn.net/Articles/753162/).
>
> Thanks for bringing this up.
>
> >
> > If I understand this correctly, the use-case is that the job instead
> > of direct reclaiming (potentially in latency sensitive tasks), prefers
> > a background non-latency sensitive task to do the reclaim. I am
> > wondering if we can use the memory.high notification along with a new
> > memcg interface (like memory.try_to_free_pages) to implement a user
> > space background reclaimer. That would resolve the cpu accounting
> > concerns as the user space background reclaimer can share the cpu cost
> > with the task.
>
> Actually I'm interested how you implement userspace reclaimer. Via a new
> syscall or a variant of existing syscall?
>

We have a per-memcg interface memory.try_to_free_pages on which user
space can echo two numbers i.e. number of bytes to reclaim and a byte
representing flags (I/O allowed or just reclaim zombies e.t.c).
However nowadays we are just using it for zombie cleanup.

Shakeel