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

Dave Chinner
2021-04-14T04:50:20Z

On Tue, Apr 13, 2021 at 09:40:12PM -0600, Yu Zhao wrote:
> On Tue, Apr 13, 2021 at 5:14 PM Dave Chinner <david@fromorbit.com> wrote:
> > 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...
>
> I appreciate the profile. But there is no batching in
> __remove_mapping() -- it locks the mapping for each page, and
> therefore the lock contention penalizes the mainline and this patchset
> equally. It looks worse on your system because the four kswapd threads
> from different nodes were working on the same file.

I think you misunderstand exactly what I mean by "batching" here.
I'm not talking about doing multiple pieces of work under a single
lock. What I mean is that the overall amount of work done in a
single reclaim scan (i.e a "reclaim batch") is packaged differently.

We already batch up page reclaim via building a page list and then
passing it to shrink_page_list() to process the batch of pages in a
single pass. Each page in this page list batch then calls
remove_mapping() to pull the page form the LRU, we have a run of
contention between the foreground read() thread and the background
kswapd.

If the size or nature of the pages in the batch passed to
shrink_page_list() changes, then the amount of time a reclaim batch
is going to put pressure on the mapping tree lock will also change.
That's the "change in batching behaviour" I'm referring to here. I
haven't read through the patchset to determine if you change the
shrink_page_list() algorithm, but it likely changes what is passed
to be reclaimed and that in turn changes the locking patterns that
fall out of shrink_page_list...

> And kswapd is only one of two paths that could affect the performance.
> The kernel context of the test process is where the improvement mainly
> comes from.
>
> I also suspect you were testing a file much larger than your memory
> size. If so, sorry to tell you that a file only a few times larger,
> e.g. twice, would be worse.
>
> Here is my take:
>
> Claim
> -----
> This patchset is a "better" algorithm. (Technically it's not an
> algorithm, it's a feedback loop.)
>
> Theoretical basis
> -----------------
> An open-loop control (the mainline) can only be better if the margin
> of error in its prediction of the future events is less than that from
> the trial-and-error of a closed-loop control (this patchset). For
> simple machines, it surely can. For page reclaim, AFAIK, it can't.
>
> A typical example: when randomly accessing a (not infinitely) large
> file via buffered io long enough, we're bound to hit the same blocks
> multiple times. Should we activate the pages containing those blocks,
> i.e., to move them to the active lru list? No.
>
> RCA
> ---
> For the fio/io_uring benchmark, the "No" is the key.
>
> The mainline activates pages accessed multiple times. This is done in
> the buffered io access path by mark_page_accessed(), and it takes the
> lru lock, which is contended under memory pressure. This contention
> slows down both the access path and kswapd. But kswapd is not the
> problem here because we are measuring the io_uring process, not kswap.
>
> For this patchset, there are no activations since the refault rates of
> pages accessed multiple times are similar to those accessed only once
> -- activations will only be done to pages from tiers with higher
> refault rates.
>
> If you wish to debunk
> ---------------------

Nope, it's your job to convince us that it works, not the other way
around. It's up to you to prove that your assertions are correct,
not for us to prove they are false.

> git fetch https://linux-mm.googlesource.com/page-reclaim refs/changes/73/1173/1
>
> CONFIG_LRU_GEN=y
> CONFIG_LRU_GEN_ENABLED=y
>
> Run your benchmarks
>
> Profiles (200G mem + 400G file)
> -------------------------------
> A quick test from Jens' fio/io_uring:
>
> -rc7
> 13.30% io_uring xas_load
> 13.22% io_uring _copy_to_iter
> 12.30% io_uring __add_to_page_cache_locked
> 7.43% io_uring clear_page_erms
> 4.18% io_uring filemap_get_read_batch
> 3.54% io_uring get_page_from_freelist
> 2.98% io_uring ***native_queued_spin_lock_slowpath***
> 1.61% io_uring page_cache_ra_unbounded
> 1.16% io_uring xas_start
> 1.08% io_uring filemap_read
> 1.07% io_uring ***__activate_page***
>
> lru lock: 2.98% (lru addition + activation)
> activation: 1.07%
>
> -rc7 + this patchset
> 14.44% io_uring xas_load
> 14.14% io_uring _copy_to_iter
> 11.15% io_uring __add_to_page_cache_locked
> 6.56% io_uring clear_page_erms
> 4.44% io_uring filemap_get_read_batch
> 2.14% io_uring get_page_from_freelist
> 1.32% io_uring page_cache_ra_unbounded
> 1.20% io_uring psi_group_change
> 1.18% io_uring filemap_read
> 1.09% io_uring ****native_queued_spin_lock_slowpath****
> 1.08% io_uring do_mpage_readpage
>
> lru lock: 1.09% (lru addition only)

All this tells us is that there was *less contention on the mapping
tree lock*. It does not tell us why there was less contention.

You've handily omitted the kswapd profile, which is really the one
of interest to the discussion here - how did the memory reclaim CPU
usage profile also change at the same time?

> And I plan to reach out to other communities, e.g., PostgreSQL, to
> benchmark the patchset. I heard they have been complaining about the
> buffered io performance under memory pressure. Any other benchmarks
> you'd suggest?
>
> BTW, you might find another surprise in how less frequently slab
> shrinkers are called under memory pressure, because this patchset is a
> lot better at finding pages to reclaim and therefore doesn't overkill
> slabs.

That's actually very likely to be a Bad Thing and cause unexpected
perofrmance and OOM based regressions. When the machine finally runs
out of page cache it can easily reclaim, it's going to get stuck
with long tail latencies reclaiming huge slab caches as they've had
no substantial ongoing pressure put on them to keep them in balance
with the overall memory pressure the system is under...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com