[PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
Yu Zhao
yuzhao at google.com
Mon Mar 21 21:39:21 PDT 2022
On Mon, Mar 21, 2022 at 7:01 AM Aneesh Kumar K.V
<aneesh.kumar at linux.ibm.com> wrote:
>
> Yu Zhao <yuzhao at google.com> writes:
>
> > To avoid confusion, the terms "promotion" and "demotion" will be
> > applied to the multi-gen LRU, as a new convention; the terms
> > "activation" and "deactivation" will be applied to the active/inactive
> > LRU, as usual.
> >
> > The aging produces young generations. Given an lruvec, it increments
> > max_seq when max_seq-min_seq+1 approaches MIN_NR_GENS. The aging
> > promotes hot pages to the youngest generation when it finds them
> > accessed through page tables; the demotion of cold pages happens
> > consequently when it increments max_seq. The aging has the complexity
> > O(nr_hot_pages), since it is only interested in hot pages. Promotion
> > in the aging path does not require any LRU list operations, only the
> > updates of the gen counter and lrugen->nr_pages[]; demotion, unless as
> > the result of the increment of max_seq, requires LRU list operations,
> > e.g., lru_deactivate_fn().
> >
> > The eviction consumes old generations. Given an lruvec, it increments
> > min_seq when the lists indexed by min_seq%MAX_NR_GENS become empty. A
> > feedback loop modeled after the PID controller monitors refaults over
> > anon and file types and decides which type to evict when both types
> > are available from the same generation.
> >
> > Each generation is divided into multiple tiers. Tiers represent
> > different ranges of numbers of accesses through file descriptors. A
> > page accessed N times through file descriptors is in tier
> > order_base_2(N). Tiers do not have dedicated lrugen->lists[], only
> > bits in folio->flags. In contrast to moving across generations, which
> > requires the LRU lock, moving across tiers only involves operations on
> > folio->flags. The feedback loop also monitors refaults over all tiers
> > and decides when to protect pages in which tiers (N>1), using the
> > first tier (N=0,1) as a baseline. The first tier contains single-use
> > unmapped clean pages, which are most likely the best choices. The
> > eviction moves a page to the next generation, i.e., min_seq+1, if the
> > feedback loop decides so. This approach has the following advantages:
> > 1. It removes the cost of activation in the buffered access path by
> > inferring whether pages accessed multiple times through file
> > descriptors are statistically hot and thus worth protecting in the
> > eviction path.
> > 2. It takes pages accessed through page tables into account and avoids
> > overprotecting pages accessed multiple times through file
> > descriptors. (Pages accessed through page tables are in the first
> > tier, since N=0.)
> > 3. More tiers provide better protection for pages accessed more than
> > twice through file descriptors, when under heavy buffered I/O
> > workloads.
> >
> > Server benchmark results:
> > Single workload:
> > fio (buffered I/O): +[47, 49]%
> > IOPS BW
> > 5.17-rc2: 2242k 8759MiB/s
> > patch1-5: 3321k 12.7GiB/s
> >
> > Single workload:
> > memcached (anon): +[101, 105]%
> > Ops/sec KB/sec
> > 5.17-rc2: 476771.79 18544.31
> > patch1-5: 972526.07 37826.95
> >
> > Configurations:
> > CPU: two Xeon 6154
> > Mem: total 256G
> >
> > Node 1 was only used as a ram disk to reduce the variance in the
> > results.
> >
> > patch drivers/block/brd.c <<EOF
> > 99,100c99,100
> > < gfp_flags = GFP_NOIO | __GFP_ZERO | __GFP_HIGHMEM;
> > < page = alloc_page(gfp_flags);
> > ---
> > > gfp_flags = GFP_NOIO | __GFP_ZERO | __GFP_HIGHMEM | __GFP_THISNODE;
> > > page = alloc_pages_node(1, gfp_flags, 0);
> > EOF
> >
> > cat >>/etc/systemd/system.conf <<EOF
> > CPUAffinity=numa
> > NUMAPolicy=bind
> > NUMAMask=0
> > EOF
> >
> > cat >>/etc/memcached.conf <<EOF
> > -m 184320
> > -s /var/run/memcached/memcached.sock
> > -a 0766
> > -t 36
> > -B binary
> > EOF
> >
> > cat fio.sh
> > modprobe brd rd_nr=1 rd_size=113246208
> > mkfs.ext4 /dev/ram0
> > mount -t ext4 /dev/ram0 /mnt
> >
> > mkdir /sys/fs/cgroup/user.slice/test
> > echo 38654705664 >/sys/fs/cgroup/user.slice/test/memory.max
> > echo $$ >/sys/fs/cgroup/user.slice/test/cgroup.procs
> > fio -name=mglru --numjobs=72 --directory=/mnt --size=1408m \
> > --buffered=1 --ioengine=io_uring --iodepth=128 \
> > --iodepth_batch_submit=32 --iodepth_batch_complete=32 \
> > --rw=randread --random_distribution=random --norandommap \
> > --time_based --ramp_time=10m --runtime=5m --group_reporting
> >
> > cat memcached.sh
> > modprobe brd rd_nr=1 rd_size=113246208
> > swapoff -a
> > mkswap /dev/ram0
> > swapon /dev/ram0
> >
> > memtier_benchmark -S /var/run/memcached/memcached.sock \
> > -P memcache_binary -n allkeys --key-minimum=1 \
> > --key-maximum=65000000 --key-pattern=P:P -c 1 -t 36 \
> > --ratio 1:0 --pipeline 8 -d 2000
> >
> > memtier_benchmark -S /var/run/memcached/memcached.sock \
> > -P memcache_binary -n allkeys --key-minimum=1 \
> > --key-maximum=65000000 --key-pattern=R:R -c 1 -t 36 \
> > --ratio 0:1 --pipeline 8 --randomize --distinct-client-seed
> >
> > Client benchmark results:
> > kswapd profiles:
> > 5.17-rc2
> > 38.05% page_vma_mapped_walk
> > 20.86% lzo1x_1_do_compress (real work)
> > 6.16% do_raw_spin_lock
> > 4.61% _raw_spin_unlock_irq
> > 2.20% vma_interval_tree_iter_next
> > 2.19% vma_interval_tree_subtree_search
> > 2.15% page_referenced_one
> > 1.93% anon_vma_interval_tree_iter_first
> > 1.65% ptep_clear_flush
> > 1.00% __zram_bvec_write
> >
> > patch1-5
> > 39.73% lzo1x_1_do_compress (real work)
> > 14.96% page_vma_mapped_walk
> > 6.97% _raw_spin_unlock_irq
> > 3.07% do_raw_spin_lock
> > 2.53% anon_vma_interval_tree_iter_first
> > 2.04% ptep_clear_flush
> > 1.82% __zram_bvec_write
> > 1.76% __anon_vma_interval_tree_subtree_search
> > 1.57% memmove
> > 1.45% free_unref_page_list
> >
> > Configurations:
> > CPU: single Snapdragon 7c
> > Mem: total 4G
> >
> > Chrome OS MemoryPressure [1]
> >
> > [1] https://chromium.googlesource.com/chromiumos/platform/tast-tests/
> >
>
> In shrink_active_list we do preferential treatment of VM_EXEC pages.
> Do we do similar thing with MGLRU? if not why is that not needed?
No, because MGLRU has a different set of assumptions than the
active/inactive LRU does [1]. It provides mmapped pages with equal
opportunities, and the tradeoff was discussed here [2].
Note that even with this preferential treatment of executable pages,
plus other heuristics added since then, executable pages are still
underprotected for at least desktop workloads [3]. And I can confirm
the problem reported is genuine -- we recently accidentally removed
our private patch that works around the problem for the last 12 years,
and observed immediate consequences on a small portion of devices not
using MGLRU [4].
[1] https://lore.kernel.org/linux-mm/20220309021230.721028-15-yuzhao@google.com/
[2] https://lore.kernel.org/linux-mm/20220208081902.3550911-5-yuzhao@google.com/
[3] https://lore.kernel.org/linux-mm/2dc51fc8-f14e-17ed-a8c6-0ec70423bf54@valdikss.org.ru/
[4] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3429559
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