Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Yu Zhao <yuzhao@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? 

	if (page_referenced(page, 0, sc->target_mem_cgroup,
			    &vm_flags)) {
		/*
		 * Identify referenced, file-backed active pages and
		 * give them one more trip around the active list. So
		 * that executable code get better chances to stay in
		 * memory under moderate memory pressure.  Anon pages
		 * are not likely to be evicted by use-once streaming
		 * IO, plus JVM can create lots of anon VM_EXEC pages,
		 * so we ignore them here.
		 */
		if ((vm_flags & VM_EXEC) && page_is_file_lru(page)) {
			nr_rotated += thp_nr_pages(page);
			list_add(&page->lru, &l_active);
			continue;
		}
	}