Re: [page-reclaim] Re: [PATCH v2 00/16] Multigenerational LRU Framework

[Shakeel Butt]

On Wed, Apr 14, 2021 at 6:52 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> On Wed, 2021-04-14 at 16:27 +0800, Huang, Ying wrote:
> > Yu Zhao <yuzhao@xxxxxxxxxx> writes:
> >
> > > On Wed, Apr 14, 2021 at 12:15 AM Huang, Ying <ying.huang@xxxxxxxxx>
> > > wrote:
> > > >
> > > NUMA Optimization
> > > -----------------
> > > Support NUMA policies and per-node RSS counters.
> > >
> > > We only can move forward one step at a time. Fair?
> >
> > You don't need to implement that now definitely.  But we can discuss
> > the
> > possible solution now.
>
> That was my intention, too. I want to make sure we don't
> end up "painting ourselves into a corner" by moving in some
> direction we have no way to get out of.
>
> The patch set looks promising, but we need some plan to
> avoid the worst case behaviors that forced us into rmap
> based scanning initially.
>
> > Note that it's possible that only some processes are bound to some
> > NUMA
> > nodes, while other processes aren't bound.
>
> For workloads like PostgresQL or Oracle, it is common
> to have maybe 70% of memory in a large shared memory
> segment, spread between all the NUMA nodes, and mapped
> into hundreds, if not thousands, of processes in the
> system.
>
> Now imagine we have an 8 node system, and memory
> pressure in the DMA32 zone of node 0.
>
> How will the current VM behave?
>
> Wha
> t will the virtual scanning need to do?
>
> If we can come up with a solution to make virtual
> scanning scale for that kind of workload, great.
>
> If not ... if it turns out most of the benefits of
> the multigeneratinal LRU framework come from sorting
> the pages into multiple LRUs, and from being able
> to easily reclaim unmapped pages before having to
> scan mapped ones, could it be an idea to implement
> that first, independently from virtual scanning?
>
> I am all for improving
> our page reclaim system, I
> just want to make sure we don't revisit the old traps
> that forced us where we are today :)
>

One potential idea is to take the hybrid 'of rmap and virtual
scanning' approach. If the number of pages that are targeted to be
scanned is below some threshold, do rmap otherwise virtual scanning. I
think we can experimentally find good value for that threshold.