** Description changed:
[Impact]
- Performance degradation for read/write workloads in bcache devices,
occasional system stalls
+ 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
-
- [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
+ [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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https://bugs.launchpad.net/bugs/1840043
Title:
bcache: Performance degradation when querying priority_stats
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