Re: [PATCH v7 00/12] Multigenerational LRU Framework

[Yu Zhao]

On Sat, Feb 12, 2022 at 05:12:19AM +0900, Alexey Avramov wrote:
> Aggressive swapping even with vm.swappiness=1 with MGLRU
> ========================================================
> 
> Reading a large mmapped file leads to a super agressive swapping.
> Reducing vm.swappiness even to 1 does not have effect.

Mind explaining why you think it's "super agressive"? I assume you
expected a different behavior that would perform better. If so,
please spell it out.

> Demo: https://www.youtube.com/watch?v=J81kwJeuW58
> 
> Linux 5.17-rc3, Multigenerational LRU v7, 
> vm.swappiness=1, MemTotal: 11.5 GiB.
> 
> $ cache-bench -r 35000 -m1 -b1 -p1 -f test20000
> Reading mmapped file (file size: 20000 MiB)
> cache-bench v0.2.0: https://github.com/hakavlad/cache-bench

Writing your own benchmark is a good exercise but fio is the standard
benchmark in this case. Please use it with --ioengine=mmap.

> Swapping started with MemAvailable=71%.
> At the end 33 GiB was swapped out when MemAvailable=60%.
> 
> Is it OK?

MemAvailable is an estimate (free + page cache), and it doesn't imply
any reclaim preferences. In the worst case scenario, e.g., out of swap
space, MemAvailable *may* be reclaimed.

Here is my benchmark result with file mmap + *high* swap usage. Ram
disk was used to reduce the variance in the result (and SSD wear out
if you care). More details on additional configurations here:
https://lore.kernel.org/linux-mm/20220208081902.3550911-6-yuzhao@xxxxxxxxxx/

  Mixed workloads:
    fio (buffered I/O): +13%
                IOPS         BW
      5.17-rc3: 275k         1075MiB/s
            v7: 313k         1222MiB/s

    memcached (anon): +12%
                Ops/sec      KB/sec
      5.17-rc3: 511282.72    19861.04
            v7: 572408.80    22235.49

  cat mmap.sh
  systemctl restart memcached
  swapoff -a
  umount /mnt
  rmmod brd
  
  modprobe brd rd_nr=2 rd_size=56623104
  
  mkswap /dev/ram0
  swapon /dev/ram0
  
  mkfs.ext4 /dev/ram1
  mount -t ext4 /dev/ram1 /mnt
  
  memtier_benchmark -S /var/run/memcached/memcached.sock \
  -P memcache_binary -n allkeys --key-minimum=1 \
  --key-maximum=50000000 --key-pattern=P:P -c 1 \
  -t 36 --ratio 1:0 --pipeline 8 -d 2000
  
  sysctl vm.overcommit_memory=1
  
  fio -name=mglru --numjobs=36 --directory=/mnt --size=1408m \
  --buffered=1 --ioengine=mmap --iodepth=128 --iodepth_batch_submit=32 \
  --iodepth_batch_complete=32 --rw=randread --random_distribution=random \
  --norandommap --time_based --ramp_time=10m --runtime=990m \
  --group_reporting &
  pid=$!
  
  sleep 200
  
  memcached.sock -P memcache_binary -n allkeys --key-minimum=1 \
  --key-maximum=50000000 --key-pattern=R:R -c 1 -t 36 --ratio 0:1 \
  --pipeline 8 --randomize --distinct-client-seed
  
  kill -INT $pid
  wait