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On a system that executes multiple cgrouped jobs and independent workloads, we don't just care about the health of the overall system, but also that of individual jobs, so that we can ensure individual job health, fairness between jobs, or prioritize some jobs over others. This patch implements pressure stall tracking for cgroups. In kernels with CONFIG_PSI=y, cgroup2 groups will have cpu.pressure, memory.pressure, and io.pressure files that track aggregate pressure stall times for only the tasks inside the cgroup. Link: http://lkml.kernel.org/r/20180828172258.3185-10-hannes@cmpxchg.org Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Tejun Heo <tj@kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Daniel Drake <drake@endlessm.com> Tested-by: Suren Baghdasaryan <surenb@google.com> Cc: Christopher Lameter <cl@linux.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Johannes Weiner <jweiner@fb.com> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Enderborg <peter.enderborg@sony.com> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Shakeel Butt <shakeelb@google.com> Cc: Vinayak Menon <vinmenon@codeaurora.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
74 lines
2.9 KiB
Plaintext
74 lines
2.9 KiB
Plaintext
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PSI - Pressure Stall Information
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================================
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:Date: April, 2018
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:Author: Johannes Weiner <hannes@cmpxchg.org>
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When CPU, memory or IO devices are contended, workloads experience
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latency spikes, throughput losses, and run the risk of OOM kills.
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Without an accurate measure of such contention, users are forced to
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either play it safe and under-utilize their hardware resources, or
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roll the dice and frequently suffer the disruptions resulting from
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excessive overcommit.
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The psi feature identifies and quantifies the disruptions caused by
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such resource crunches and the time impact it has on complex workloads
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or even entire systems.
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Having an accurate measure of productivity losses caused by resource
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scarcity aids users in sizing workloads to hardware--or provisioning
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hardware according to workload demand.
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As psi aggregates this information in realtime, systems can be managed
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dynamically using techniques such as load shedding, migrating jobs to
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other systems or data centers, or strategically pausing or killing low
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priority or restartable batch jobs.
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This allows maximizing hardware utilization without sacrificing
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workload health or risking major disruptions such as OOM kills.
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Pressure interface
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==================
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Pressure information for each resource is exported through the
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respective file in /proc/pressure/ -- cpu, memory, and io.
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The format for CPU is as such:
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some avg10=0.00 avg60=0.00 avg300=0.00 total=0
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and for memory and IO:
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some avg10=0.00 avg60=0.00 avg300=0.00 total=0
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full avg10=0.00 avg60=0.00 avg300=0.00 total=0
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The "some" line indicates the share of time in which at least some
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tasks are stalled on a given resource.
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The "full" line indicates the share of time in which all non-idle
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tasks are stalled on a given resource simultaneously. In this state
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actual CPU cycles are going to waste, and a workload that spends
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extended time in this state is considered to be thrashing. This has
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severe impact on performance, and it's useful to distinguish this
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situation from a state where some tasks are stalled but the CPU is
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still doing productive work. As such, time spent in this subset of the
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stall state is tracked separately and exported in the "full" averages.
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The ratios are tracked as recent trends over ten, sixty, and three
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hundred second windows, which gives insight into short term events as
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well as medium and long term trends. The total absolute stall time is
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tracked and exported as well, to allow detection of latency spikes
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which wouldn't necessarily make a dent in the time averages, or to
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average trends over custom time frames.
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Cgroup2 interface
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=================
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In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem
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mounted, pressure stall information is also tracked for tasks grouped
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into cgroups. Each subdirectory in the cgroupfs mountpoint contains
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cpu.pressure, memory.pressure, and io.pressure files; the format is
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the same as the /proc/pressure/ files.
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