Re: [RFC v2 PATCH 0/5] Promotion of Unmapped Page Cache Folios.

Gregory Price
2024-12-21T14:49:04Z

On Sat, Dec 21, 2024 at 01:18:04PM +0800, Huang, Ying wrote:
> Gregory Price <gourry@gourry.net> writes:
>
> >
> > Single-reader DRAM: ~16.0-16.4s
> > Single-reader CXL (after demotion): ~16.8-17s
>
> The difference is trivial. This makes me thought that why we need this
> patchset?
>

That's 3-6% performance in this contrived case.

We're working to testing a real workload we know suffers from this
problem as it is long-running. Should be early in the new year hopefully.

> > Next we turned promotion on with only a single reader running.
> >
> > Before promotions:
> > Node 0 MemFree: 636478112 kB
> > Node 0 FilePages: 59009156 kB
> > Node 1 MemFree: 250336004 kB
> > Node 1 FilePages: 14979628 kB
>
> Why are there some many file pages on node 1 even if there're a lot of
> free pages on node 0? You moved some file pages from node 0 to node 1?
>

This was explicit and explained in the test notes:

First we ran with promotion disabled to show consistent overhead as
a result of forcing a file out to CXL memory. We first ran a single
reader to see uncontended performance, launched many readers to force
demotions, then dropped back to a single reader to observe.

The goal here was to simply demonstrate functionality and stability.

> > After promotions:
> > Node 0 MemFree: 632267268 kB
> > Node 0 FilePages: 72204968 kB
> > Node 1 MemFree: 262567056 kB
> > Node 1 FilePages: 2918768 kB
> >
> > Single-reader (after_promotion): ~16.5s

This represents a 2.5-6% speedup depending on the spread.

> >
> > numa_migrate_prep: 93 - time(3969867917) count(42576860)
> > migrate_misplaced_folio_prepare: 491 - time(3433174319) count(6985523)
> > migrate_misplaced_folio: 1635 - time(11426529980) count(6985523)
> >
> > Thoughts on a good throttling heuristic would be appreciated here.
>
> We do have a throttle mechanism already, for example, you can used
>
> $ echo 100 > /proc/sys/kernel/numa_balancing_promote_rate_limit_MBps
>
> to rate limit the promotion throughput under 100 MB/s for each DRAM
> node.
>

Can easily piggyback on that, just wasn't sure if overloading it was
an acceptable idea. Although since that promotion rate limit is also
per-task (as far as I know, will need to read into it a bit more) this
is probably fine.

~Gregory