NAME¶

multitime — time command execution over multiple executions

SYNOPSIS¶

DESCRIPTION¶

Unix's time(1) utility is a simple and often effective way of measuring how long a command takes to execute. Unfortunately, executing a command once can give misleading timings: the process may create a cache on its first execution, running faster subsequently; other processes may cause the command to be starved of CPU or IO time; etc. It is common to see people execute time(1) several times and take whichever values they feel most comfortable with. Inevitably, this causes problems.

multitime is, in essence, a simple extension to time(1) which executes command multiple times and prints the timing means, standard deviations, mins, medians, and maxes having done so. This can give a much better understanding of the command's performance. multitime also has a number of options to help advanced uses. For basic uses, multitime can replace time(1) by using the -n option to specifying how many times command should be executed. e.g. if we want to time awk(1):

The full set of options is as follows:

-b batchfile

Execute multiple commands from batchfile. See the BATCHFILES section for more details.

-f liketime | rusage

If called as time, the default output style of multitime is POSIX.2 compatible, showing means for real, user, and sys readings. -f liketime can be used to force POSIX.2 compatibility in all cases. Otherwise, its default output style is an incompatible extension that shows means, standard deviations, mins, medians, and maxes. -f rusage additionally shows the entire output of the rusage structure.

-I replstr

Instances of replstr found in inputcmd, outputcmd, and precmd are replaced with an integer denoting the current execution run number, from 1 to numruns (both inclusive).

-i stdincmd

Before the timing of each execution of command, stdincmd is executed and its output piped to a temporary file. That temporary file is then used as stdin for command, allowing the user to ensure that each execution of command sees exactly the input on stdin expected. stdincmd is a full shell command which is passed to popen(3).

-l

Same as -f rusage, for compatibility with time(1).

-n numruns

Specify how many times command should be executed. Defaults to 1.

-o stdoutcmd

When executing command, its output is piped to a temporary file. After execution has finished, stdoutcmd is then executed, with the temporary file being its stdin. If stdoutcmd returns an exit code (i.e. non-zero), multitime stops executing. This can be used as a sanity check that command is executing as per expectations. stdoutcmd is a full shell command which is passed to popen(3). This option is mutually exclusive with -q.

-p

Same as -f liketime, for compatibility with time(1).

-r precmd

Before each execution of command -- and, if it is specified, before stdincmd -- precmd is executed by calling system(3). This can be used to set the system to a known good state. If precmd returns an exit code (i.e. non-zero), multitime stops executing.

-q

If specified once, -q suppresses stdout output; if specified twice, -qq suppresses both stdout and stderr. This can be useful for programs which produce voluminous output, which can lead to one unintentionally measuring the output speed of the terminal being used, rather than command itself. This option is mutually exclusive with -o.

-s sleep

multitime pauses a random length of time between 0 and sleep seconds between each command execution. Particularly for short-running commands, this can smooth out temporary peaks and troughs. If not specified, sleep defaults to 3 seconds; if set to 0, multitime does not sleep at all between executions.

-v

Causes verbose output (e.g. which commands are being executed).

Note that multitime exits immediately if any execution of command fails, returning the failed commands error code.

BATCHFILES¶

Batchfiles are only needed for advanced uses of multitime. One important use is when multitime is being used to compare the performance of multiple commands. The obvious way to do this is to execute multitime for each command and record its output. However, it is possible that one command is unduly affected by issues elsewhere in the machine (e.g. a cron(8) job running in the background), distorting the comparison. Batchfiles allow multiple completely different commands to be executed, with each iteration running a random command. Assuming that numruns is set sufficiently high, batchfiles tend to better spread timing problems over the whole set of commands rather than a single command.

The format of batchfiles is relatively simple being, more or less, a cut-down version of the normal multitime arguments without having to specify multitime itself. Each line specifies a command to be executed. Each line has the format:

[-I replstr] [-i stdincmd] [-o stdoutcmd] [-q] [-r precmd] command [arg1, ..., argn]

The -f, -n, -s, and -v options are global and can not be specified in the batch file.

EXAMPLES¶

A basic invocation of multitime is as follows:

command will produce its output as normal; multitime will then produce output such as the following on stderr:

As an example of more complex uses of multitime, one could time the overall performance of sort(1) on different sequences of random data using -i:

If you are timing sort(1) against pre-defined batches of data (called data1, data2, ..., data10):

If you want to cache the output of each execution of command use -o:

An example batch file bf is as follows:

LIMITATIONS¶

Though multitime goes out of its way not to colour timings, ultimately the operating system and tasks executing in the system can significantly affect timing measurements. For example, multitime timings include the time to fork(2) a process and execvp(3) a command, which are entirely outside its hands. Short-running tasks can be particularly affected by seemingly minor blips in system activity.

There are methods which can increase the likely accuracy of timing measurements. For example, raising numruns (and, depending on your circumstances, sleep) reduces the likelihood of temporary blips distorting timing measurements. If comparing the execution times of multiple commands, the use of batchfiles can spread blips out rather than concentrating them on a single command. Increasing the process priority of multitime can decrease the likelihood of other tasks interfering with timings. Ultimately, however, there can never be absolute guarantees of accuracy. Instead, such methods should be thought of as increasing the likelihood that the numbers returned are indicative of the 'true' measurements. By presenting means and standard deviations, multitime encourages the use of confidence intervals, a statistical technique which encourages this mode of thinking.

AUTHORS¶

multitime was written by Laurence Tratt ⟨http://tratt.net/laurie/⟩.