From: Yu Zhao <firstname.lastname@example.org> To: Dave Chinner <email@example.com> Cc: Jens Axboe <firstname.lastname@example.org>, SeongJae Park <email@example.com>, Linux-MM <firstname.lastname@example.org>, Andi Kleen <email@example.com>, Andrew Morton <firstname.lastname@example.org>, Benjamin Manes <email@example.com>, Dave Hansen <firstname.lastname@example.org>, Hillf Danton <email@example.com>, Johannes Weiner <firstname.lastname@example.org>, Jonathan Corbet <email@example.com>, Joonsoo Kim <firstname.lastname@example.org>, Matthew Wilcox <email@example.com>, Mel Gorman <firstname.lastname@example.org>, Miaohe Lin <email@example.com>, Michael Larabel <firstname.lastname@example.org>, Michal Hocko <email@example.com>, Michel Lespinasse <firstname.lastname@example.org>, Rik van Riel <email@example.com>, Roman Gushchin <firstname.lastname@example.org>, Rong Chen <email@example.com>, SeongJae Park <firstname.lastname@example.org>, Tim Chen <email@example.com>, Vlastimil Babka <firstname.lastname@example.org>, Yang Shi <email@example.com>, Ying Huang <firstname.lastname@example.org>, Zi Yan <email@example.com>, linux-kernel <firstname.lastname@example.org>, email@example.com, Kernel Page Reclaim v2 <firstname.lastname@example.org> Subject: Re: [PATCH v2 00/16] Multigenerational LRU Framework Date: Tue, 13 Apr 2021 21:40:12 -0600 [thread overview] Message-ID: <CAOUHufa7RCK6gcYSeLv98w3_NY-TUpUNkDS0p_W4u5_ZfSXTsg@mail.gmail.com> (raw) In-Reply-To: <20210413231436.GF63242@dread.disaster.area> On Tue, Apr 13, 2021 at 5:14 PM Dave Chinner <email@example.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 <firstname.lastname@example.org> > > > > > > Hello, > > > > > > > > > Very interesting work, thank you for sharing this :) > > > > > > On Tue, 13 Apr 2021 00:56:17 -0600 Yu Zhao <email@example.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. 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 --------------------- 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) 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. Thanks.
next prev parent reply other threads:[~2021-04-14 3:40 UTC|newest] Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-04-13 6:56 Yu Zhao 2021-04-13 6:56 ` [PATCH v2 01/16] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG_MEMCG Yu Zhao 2021-04-13 6:56 ` [PATCH v2 02/16] include/linux/nodemask.h: define next_memory_node() if !CONFIG_NUMA Yu Zhao 2021-04-13 6:56 ` [PATCH v2 03/16] include/linux/huge_mm.h: define is_huge_zero_pmd() if !CONFIG_TRANSPARENT_HUGEPAGE Yu Zhao 2021-04-13 6:56 ` [PATCH v2 04/16] include/linux/cgroup.h: export cgroup_mutex Yu Zhao 2021-04-13 6:56 ` [PATCH v2 05/16] mm/swap.c: export activate_page() Yu Zhao 2021-04-13 6:56 ` [PATCH v2 06/16] mm, x86: support the access bit on non-leaf PMD entries Yu Zhao 2021-04-13 6:56 ` [PATCH v2 07/16] mm/vmscan.c: refactor shrink_node() Yu Zhao 2021-04-13 6:56 ` [PATCH v2 08/16] mm: multigenerational lru: groundwork Yu Zhao 2021-04-13 6:56 ` [PATCH v2 09/16] mm: multigenerational lru: activation Yu Zhao 2021-04-13 6:56 ` [PATCH v2 10/16] mm: multigenerational lru: mm_struct list Yu Zhao 2021-04-14 14:36 ` Matthew Wilcox 2021-04-13 6:56 ` [PATCH v2 11/16] mm: multigenerational lru: aging Yu Zhao 2021-04-13 6:56 ` [PATCH v2 12/16] mm: multigenerational lru: eviction Yu Zhao 2021-04-13 6:56 ` [PATCH v2 13/16] mm: multigenerational lru: page reclaim Yu Zhao 2021-04-13 6:56 ` [PATCH v2 14/16] mm: multigenerational lru: user interface Yu Zhao 2021-04-13 6:56 ` [PATCH v2 15/16] mm: multigenerational lru: Kconfig Yu Zhao 2021-04-13 6:56 ` [PATCH v2 16/16] mm: multigenerational lru: documentation Yu Zhao 2021-04-13 7:51 ` [PATCH v2 00/16] Multigenerational LRU Framework SeongJae Park 2021-04-13 16:13 ` Jens Axboe 2021-04-13 16:42 ` SeongJae Park 2021-04-13 23:14 ` Dave Chinner 2021-04-14 2:29 ` Rik van Riel [not found] ` <CAOUHufafMcaG8sOS=1YMy2P_6p0R1FzP16bCwpUau7g1-PybBQ@mail.gmail.com> 2021-04-14 6:15 ` Huang, Ying 2021-04-14 7:58 ` Yu Zhao 2021-04-14 8:27 ` Huang, Ying 2021-04-14 13:51 ` Rik van Riel 2021-04-14 15:56 ` Andi Kleen 2021-04-14 15:58 ` [page-reclaim] " Shakeel Butt 2021-04-14 18:45 ` Yu Zhao 2021-04-14 15:51 ` Andi Kleen 2021-04-14 15:58 ` Rik van Riel 2021-04-14 19:14 ` Yu Zhao 2021-04-14 19:41 ` Rik van Riel 2021-04-14 20:08 ` Yu Zhao 2021-04-14 19:04 ` Yu Zhao 2021-04-15 3:00 ` Andi Kleen 2021-04-15 7:13 ` Yu Zhao 2021-04-15 8:19 ` Huang, Ying 2021-04-15 9:57 ` Michel Lespinasse 2021-04-24 2:33 ` Yu Zhao 2021-04-24 3:30 ` Andi Kleen 2021-04-24 4:16 ` Yu Zhao 2021-04-14 3:40 ` Yu Zhao [this message] 2021-04-14 4:50 ` Dave Chinner 2021-04-14 7:16 ` Yu Zhao 2021-04-14 10:00 ` Yu Zhao 2021-04-15 1:36 ` Dave Chinner 2021-04-24 21:21 ` Yu Zhao 2021-04-14 14:43 ` Jens Axboe 2021-04-14 19:42 ` Yu Zhao 2021-04-15 1:21 ` Dave Chinner 2021-04-14 17:43 ` Johannes Weiner 2021-04-27 10:35 ` Yu Zhao 2021-04-29 23:46 ` Konstantin Kharlamov 2021-04-30 6:37 ` Konstantin Kharlamov 2021-04-30 19:31 ` Yu Zhao
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