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

Dave Chinner
2021-04-13T23:14:49Z

On Tue, Apr 13, 2021 at 10:13:24AM -0600, Jens Axboe wrote:
> On 4/13/21 1:51 AM, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > Hello,
> >
> >
> > Very interesting work, thank you for sharing this :)
> >
> > On Tue, 13 Apr 2021 00:56:17 -0600 Yu Zhao <yuzhao@google.com> wrote:
> >
> >> What's new in v2
> >> ================
> >> Special thanks to Jens Axboe for reporting a regression in buffered
> >> I/O and helping test the fix.
> >
> > Is the discussion open? If so, could you please give me a link?
>
> I wasn't on the initial post (or any of the lists it was posted to), but
> it's on the google page reclaim list. Not sure if that is public or not.
>
> tldr is that I was pretty excited about this work, as buffered IO tends
> to suck (a lot) for high throughput applications. My test case was
> pretty simple:
>
> Randomly read a fast device, using 4k buffered IO, and watch what
> happens when the page cache gets filled up. For this particular test,
> we'll initially be doing 2.1GB/sec of IO, and then drop to 1.5-1.6GB/sec
> with kswapd using a lot of CPU trying to keep up. That's mainline
> behavior.

I see this exact same behaviour here, too, but I RCA'd it to
contention between the inode and memory reclaim for the mapping
structure that indexes the page cache. Basically the mapping tree
lock is the contention point here - you can either be adding pages
to the mapping during IO, or memory reclaim can be removing pages
from the mapping, but we can't do both at once.

So we end up with kswapd spinning on the mapping tree lock like so
when doing 1.6GB/s in 4kB buffered IO:

- 20.06% 0.00% [kernel] [k] kswapd ▒
- 20.06% kswapd ▒
- 20.05% balance_pgdat ▒
- 20.03% shrink_node ▒
- 19.92% shrink_lruvec ▒
- 19.91% shrink_inactive_list ▒
- 19.22% shrink_page_list ▒
- 17.51% __remove_mapping ▒
- 14.16% _raw_spin_lock_irqsave ▒
- 14.14% do_raw_spin_lock ▒
__pv_queued_spin_lock_slowpath ▒
- 1.56% __delete_from_page_cache ▒
0.63% xas_store ▒
- 0.78% _raw_spin_unlock_irqrestore ▒
- 0.69% do_raw_spin_unlock ▒
__raw_callee_save___pv_queued_spin_unlock ▒
- 0.82% free_unref_page_list ▒
- 0.72% free_unref_page_commit ▒
0.57% free_pcppages_bulk ▒

And these are the processes consuming CPU:

5171 root 20 0 1442496 5696 1284 R 99.7 0.0 1:07.78 fio
1150 root 20 0 0 0 0 S 47.4 0.0 0:22.70 kswapd1
1146 root 20 0 0 0 0 S 44.0 0.0 0:21.85 kswapd0
1152 root 20 0 0 0 0 S 39.7 0.0 0:18.28 kswapd3
1151 root 20 0 0 0 0 S 15.2 0.0 0:12.14 kswapd2

i.e. when memory reclaim kicks in, the read process has 20% less
time with exclusive access to the mapping tree to insert new pages.
Hence buffered read performance goes down quite substantially when
memory reclaim kicks in, and this really has nothing to do with the
memory reclaim LRU scanning algorithm.

I can actually get this machine to pin those 5 processes to 100% CPU
under certain conditions. Each process is spinning all that extra
time on the mapping tree lock, and performance degrades further.
Changing the LRU reclaim algorithm won't fix this - the workload is
solidly bound by the exclusive nature of the mapping tree lock and
the number of tasks trying to obtain it exclusively...

> The initial posting of this patchset did no better, in fact it did a bit
> worse. Performance dropped to the same levels and kswapd was using as
> much CPU as before, but on top of that we also got excessive swapping.
> Not at a high rate, but 5-10MB/sec continually.
>
> I had some back and forths with Yu Zhao and tested a few new revisions,
> and the current series does much better in this regard. Performance
> still dips a bit when page cache fills, but not nearly as much, and
> kswapd is using less CPU than before.

Profiles would be interesting, because it sounds to me like reclaim
*might* be batching page cache removal better (e.g. fewer, larger
batches) and so spending less time contending on the mapping tree
lock...

IOWs, I suspect this result might actually be a result of less lock
contention due to a change in batch processing characteristics of
the new algorithm rather than it being a "better" algorithm...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com