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https://github.com/uber-archive/cpustat

high frequency performance measurements for Linux. This project is deprecated and not maintained.
https://github.com/uber-archive/cpustat

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high frequency performance measurements for Linux. This project is deprecated and not maintained.

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# cpustat - high(er) frequency stats sampling



[![Join the chat at https://gitter.im/uber-common/cpustat](https://badges.gitter.im/uber-common/cpustat.svg)](https://gitter.im/uber-common/cpustat?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)



(This project is deprecated and not maintained.)



`cpustat` is a tool for Linux systems to measure performance. You can think of it like a

fancy sort of `top` that does different things. This project is motivated by Brendan Gregg's

[USE Method](http://www.brendangregg.com/usemethod.html) and tries to expose CPU

utilization and saturation in a helpful way.



Most performance tools average CPU usage over a few seconds or even a minute. This can

create the illusion of excess capacity because brief spikes in resource usage are blended

in with less busy periods. `cpustat` takes higher frequency samples of every process running

on the machine and then summarizes these samples at a lower frequency. For example, it can

measure every process every 200ms and summarize these samples every 5 seconds, including

min/average/max values for some metrics.



There are two ways of displaying this data: a pure text list of the summary interval

and a colorful scrolling dashboard of each sample.



Here are examples of both modes observing the same workload:



![Text Mode](http://imgur.com/vu4LrBD.gif)



![Demo](http://i.imgur.com/mbasDlZ.gif)



## Installation



To install the latest version of `cpustat` and all of its dependencies, try this:



```

go get github.com/uber-common/cpustat

```



If you have `glide`, you can use `glide install` to get consistent dependencies.



## Usage



This program uses Linux taskstats, which requires root.



Here are the command line flags most users will want:



Flag | Description | Default

-----|-------------|--------

`-i` | sample interval in milliseconds | 200

`-s` | summarize after this many samples | 10

`-n` | display top n processes | 10

`-maxprocs` | truncate process list if it exceed this | 2048

`-p` | only measure processes in this list of pids | none

`-u` | only measure processes owned by this list of users | none

`-t` | use fancy termui mode | false



There are also a few less common options:



Flag | Description | Default

-----|-------------|--------

`-jiffy` | set the Linux clock tick duration time in milliseconds | 100

`-cpuprofile` | write CPU pprof data of cpustat itself to this file | none

`-memprofile` | write memory pprof data of cpustat itself to this file | none



Examples:



```

sudo cpustat -s 500 -s 10 -n 20

```



This will take a sample of all processes every 500ms and summarize this data after 10

samples, which is every 5 seconds.



```

sudo cpustat -u mjr,mranney

```



Only measure processes that are owned by either user `mjr` or user

`mranney`. The overall system stats will still be measured.



```

sudo ./cpustat -p $(pgrep -d, vim\|emacs)

```



Only measure processes that pgrep thinks are called "vim" or "emacs". The `-p`

option to `cpustat` takes a list of process ids to measure, and `pgrep` is a

handy way to get this list. The `-d,` option to `pgrep` prints the list of

matching pids with a comma separator.



## Displayed Values



In pure text mode, there are some system-wide summary metrics that come from /proc/stat:



Name | Description

-----|------------

usr | min/avg/max user mode run time as a percentage of a CPU

sys | min/avg/max system mode run time as a percentage of a CPU

nice | min/avg/max user mode low priority run time as a percentage of a CPU

idle | min/avg/max user mode run time as a percentage of a CPU

iowait | min/avg/max delay time waiting for disk IO

prun | min/avg/max count of processes in a runnable state (load average)

pblock | min/avg/max count of processes blocked on disk IO

pstart | number of processes/threads started in this summary interval



In fancy scrolling dashboard mode, the unique panes are as follows:



In the top right, labeled "total usr/sys time", the system-wide measurements for user time

and system time are displayed. User time is cyan, and system time is red. The X axis is

labeled in seconds, the Y axis is percentage of a CPU. Each dot represents an individual

sample. The data is not summarized.



In the top left, labeled "top procs", the combined user+system time of the topN processes

is displayed. Each process gets a separate line with a color that matches the list

below. The X and Y axes are the same as the top right graph. Due to limitations of a

terminal UI, overlapping lines are drawn in the same cell, potentially obscuring each

other.



Both modes display the same per-process summary data. The fields are:



Name | Description

-----|------------

`name` | common Name from /proc/pid/stat or /proc/pid/cmdline. There is some logic to resolve common patterns into more useful names for common things.

`pid` | Top level process id, sometimes referred to as "tgid"

`min` | lowest sample of combined user and system time for this pid, measured from /proc/pid/stat. Scale is a percentage of a CPU.

`max` | highest sample of combined user and system time for this pid, measured from /proc/pid/stat.

`usr` | average user time for this pid over the summary period, measured from /proc/pid/stat. This plus `sys` should be similar to what "top" reports.

`sys` | average system time for this pid over the summary period, measured from /proc/pid/stat. This plus `usr` should be similar to what "top" reports.

`nice` | current "nice" value for this process, measured from /proc/pid/stat. Higher is "nicer".

`runq` | time this process and all of its threads spent runnable but waiting to run, measured from taskstats via netlink. Scale is a percentage of a CPU.

`iow` | time this process and all of its threads spent blocked by disk IO, measured from taskstats via netlink. Scale is a percentage of a CPU, averaged over the summary interval.

`swap` | time this process and all of its threads spent waiting to be swapped in, measured from taskstats via netlink. Scale is a percentage of a CPU, averaged over the summary interval.

`vcx` | total number of voluntary context switches by this process and all of its threads over the summary interval, measured from taskstats via netlink.

`icx` | total number of involuntary context switches by this process and all of its threads over the summary interval, measured from taskstats via netlink.

`rss` | current RSS value measured from /proc/pid/stat. This is the amount of memory this process is using.

`ctime` | total user+sys CPU time consumed by waited for children that exited during this summary interval, measured from /proc/pid/stat. Long running child processes can often confuse this measurement, because the time is reported only when the child process exits. However, this is useful for measuring the impact of frequent cron jobs and health checks where the CPU time is often consumed by many child processes.

`thrd` | Number of threads at the end of the summary interval, measured from /proc/pid/stat.

`sam` | number of samples for this process included in the summary interval. Processes that have recently started or exited may have been visible for fewer samples than the summary interval.



## Understanding the Output



Here are a few examples of running `cpustat` on a 4 processor vm on my laptop.



The first is a mostly idle system where the only thing really running is `cpustat` itself:



![Idle](https://ranney.com/cpustat_images/1__ssh.png)



The `idle` min/avg/max shows that for most of the 20 samples, the system was almost completely idle.

We can tell that because we know this is a 4 processor system, so the maximum value for `idle` is 400.

It's perhaps a little surprising then that `prun` would show 1.0/1.1/3.0, meaning that we never woke up

to find fewer than 1 process running. On an idle system, surely that number should be lower. What's

happening is that `cpustat` ends up measuring itself as the single running process.



So let's put this computer to work:



![Idle](https://ranney.com/cpustat_images/1__ssh 2.png)



I'm running a single instance of "CPU Burn-In" that shows up as `burnP6`. This process uses a single

CPU.  We can see that the overall system now reports only about 300% idle, and that `burnP6` is only

using a single thread from the `thrd` column.



We can also see that the `runq` column for `burnP6` shows 0.4 on the first summary interval. This

means that for the duration of the summary interval, 0.4% of an effective CPU's time was asked for

by a process, but that process wasn't scheduled for whatever reason. During the same interval, `icx`

is a lot higher than normal.  `icx` is "involuntary context switches". It's hard to say exactly what

caused this, but we can also see that several other processes have non-0 `iow` or "IO wait". During

that interval, `prun` had a max value of 5.0, so a few things probably woke up at the same time to

do some brief work, caused a bit of interference, then went back to sleep.



During the second summary interval, the `runq` is back to 0, so `burnP6` is getting all of the time

it wants. It's also curious to note that the `burnP6` pretty clearly reports its CPU usage as 100%

`usr` time, the overall system `usr` doesn't line up with this at 60.0/77.2/95.0, but the system

`idle` does. I'm not exactly sure what causes this, but it's something about how `burnP6` works and

Linux accounts for it. Many other single threaded programs in a tight loop do not exhibit this

behavior.



I have another program to generate a more irregular an in my experience more realistic workload

called `wastetime`. This program uses many threads, tries to wake them up at the same time to do

some work, then sleeps for a bit.



![Idle](https://ranney.com/cpustat_images/1__ssh 3.png)



For some of the samples, we wake up and find `wastetime` using 0 CPU, and sometimes we find it 300%

because `burnP6` is using the other 100%. `wastetime` reports a `runq` time of around 230%. This

means that we'd need approximately 2.3 more CPUs on average to do all of the work and avoid

delay. This is the average delay, but we can see from the `prun` max that we pretty regularly need

35 CPUs to avoid absolutely all delays.



We also see that `wastetime` is causing a little bit of `runq` interference for `burnP6`.



This is what it looks like to run 10 parallel instances of `go build` in this vm on the `cpustat`

source with:



```

for C in {0..10}; do echo $C ; (go build &) ; done

```



![Idle](https://ranney.com/cpustat_images/1__ssh 4.png)



This is obviously way more work than my little vm can manage, so we've triggered some major CPU

saturation.



Nearly every process running on the machine is spending more time in the `runq` than we'd like. Even

so, there is still some idle time left in the system. That can be explained by the `swap` time and

`iowait` time. Also note that the `sam` column is all less than 20. That means these are all short

lived processes, which are often hard to account for.



## Data Sources



Every sample interval, the following data sources are checked:



* The directory /proc is scanned to get the current list of process ids

* For each pid, read /proc/pid/stat, compute difference from previous sample

* If this is a new pid, read /proc/pid/cmdline

* For each pid, send a netlink message to fetch the taskstats, compute difference from

  previous sample.

* Fetch /proc/stat to get the overall system stats



Each sleep interval is adjusted to account for the amount of time spent fetching all of

these stats. Each sample also records the time it was taken to scale each measurement by

the actual elapsed time between samples. This attempts to account for delays in `cpustat`

itself.



## Run within a Docker container



```

 docker run --rm -ti --privileged --pid=host --net=host user/cpustat -s=200 -n=20

```



## Limitations



There are many important limitations to understand before drawing conclusions from

`cpustat`'s output.



The first is that all sampling systems are subject to

[Nyquist](https://en.wikipedia.org/wiki/Nyquist_rate), which is both a practical reality

and a fun concept that will lend gravitas to many conversations. Even if we sample at

200ms, which is pretty fast compared to most tools, we'll not be able to accurately

measure many common performance problems.



Linux CPU time accounting is done in terms of whole "clock ticks", which are often

100ms. This can cause some strange values when sampling every 200ms. Common problems are

that CPU utilization will appear to slightly exceed the maximum number of CPUs on the

system. On a very lightly loaded system, sometimes processes will appear to oscillate

between 0 percent and some higher number.



There is no way to get a consistent snapshot of all processes on a Linux system. After we

scan /proc to get the process list, new processes could come and go. Processes could exit

at any time along the process of scanning /proc, reading /proc/cmdline, /proc/pid/stat, or

netlink taskstats.



`cpustat` itself can cause the very problems it was written to expose by doing a burst of

work on a regular interval. It would be nicer to the underlying system to spread the work

out evenly over the sampling interval instead of trying to do it all at once.



The Linux netlink taskstats interface can only be used by root, which means this program

must be run as root.



In spite of these limitations, this tool has already been useful in understanding

performance problems on production systems. I hope it's useful to you as well.



## Agent



In addition to the interactive version of `cpustat`, a long running measurement server is

available in the `agent` directory. This program uses only the measurement logic to record

the raw samples in a circular buffer. The data in the agent can be collected by the

`client` program. This can be used to summarize the system state for publishing metrics to

other systems. For example, instead of recording the average CPU utilization over a minute

like many metrics systems do, you could report the min/avg/max CPU utilization over a

minute or any other interval.



## Future Work



There is an almost an endless set of UI-type features that would be nice.