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["Guilherme G. Piccoli" <1840043@xxxxxxxxxxxxxxxxxx>]

** Changed in: linux
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1840043

Title:
  bcache: Performance degradation when querying priority_stats

Status in Linux:
  Fix Released
Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Bionic:
  Fix Released
Status in linux source package in Disco:
  Fix Released
Status in linux source package in Eoan:
  Fix Released

Bug description:
  [Impact]
  Querying bcache's priority_stats attribute in sysfs causes severe performance degradation for read/write workloads and occasional system stalls

  [Test Case]
  Note: As the sorting step has the most noticeable performance impact, the test case below pins a workload and the sysfs query to the same CPU. CPU contention issues still occur without any pinning, this just removes the scheduling factor of landing in different CPUs and affecting different tasks.

  1) Start a read/write workload on the bcache device with e.g. fio or dd, pinned to a certain CPU:
  # taskset 0x10 dd if=/dev/zero of=/dev/bcache0 bs=4k status=progress

  2) Start a sysfs query loop for the priority_stats attribute pinned to the same CPU:
  # for i in {1..100000}; do taskset 0x10 cat /sys/fs/bcache/*/cache0/priority_stats > /dev/null; done

  3) Monitor the read/write workload for any performance impact

  [Fix]
  To fix CPU contention and performance impact, a cond_resched() call is introduced in the priority_stats sort comparison.

  [Regression Potential]
  Regression potential is low, as the change is confined to the priority_stats sysfs query. In cases where frequent queries to bcache priority_stats take place (e.g. node_exporter), the impact should be more noticeable as those could now take a bit longer to complete. A regression due to this patch would most likely show up as a performance degradation in bcache-focused workloads.

  --

  [Description]
  In the latest bcache drivers, there's a sysfs attribute that calculates bucket priority statistics in /sys/fs/bcache/*/cache0/priority_stats. Querying this file has a big performance impact on tasks that run in the same CPU, and also affects read/write performance of the bcache device itself.

  This is due to the way the driver calculates the stats: the bcache
  buckets are locked and iterated through, collecting information about
  each individual bucket. An array of nbucket elements is constructed
  and sorted afterwards, which can cause very high CPU contention in
  cases of larger bcache setups.

  From our tests, the sorting step of the priority_stats query causes
  the most expressive performance reduction, as it can hinder tasks that
  are not even doing any bcache IO. If a task is "unlucky" to be
  scheduled in the same CPU as the sysfs query, its performance will be
  harshly reduced as both compete for CPU time. We've had users report
  systems stalls of up to ~6s due to this, as a result from monitoring
  tools that query the priority_stats periodically (e.g. Prometheus Node
  Exporter from [0]). These system stalls have triggered several other
  issues such as ceph-mon re-elections, problems in percona-cluster and
  general network stalls, so the impact is not isolated to bcache IO
  workloads.

  An example benchmark can be seen in [1], where the read performance on
  a bcache device suffered quite heavily (going from ~40k IOPS to ~4k
  IOPS due to priority_stats). Other comparison charts are found under
  [2].

  [0] https://github.com/prometheus/node_exporter
  [1] https://people.canonical.com/~halves/priority_stats/read/4k-iops-2Dsmooth.png
  [2] https://people.canonical.com/~halves/priority_stats/

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