Elapsed time с linux

Measure elapsed time in Linux shell using time and date

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There are two main cases:

If you have a command and you’d like to know how much it takes you can use the time command. Measure time externally.

If you have a script and you’d like to measure how much various parts of it take you can use the date command. Measure time internally.

Measure elapsed time internally in a script

This is the other type of elapsed time measuring.

Measure elapsed time externally

Let’s say you have a command such as run_this_thing and you’d like to measure how long it takes to run. You can run it as

This will run the program and after the program has finished it will print something that looks like this:

It tells us that the job took 14 seconds real time to run. This is what is sometimes referred to as wallclock time. This is the time you could have measured with your personal stop-watch.

The user time is what the process spent in user mode (e.g. computing in memory).

The sys time (aka. system time) it the process spent using system calls. (e.g. the CPU time of accessing the file-system, the screen, or the network, etc.), but excluding the time the Operatng System was waiting for the disk or the network to send the data, or the screen to display it. Also excluding the time it was waiting for the Operating Sytem to allocate CPU time to it.

All the time while the process was not actually using CPU but was waiting for something is in the real time.

So the above results mean that the process I ran was mostly waiting for some IO to finish. (In fact this was an ssh command that executed something on a remote computer. So the computer I was running on hardly used its CPU. It was mostly waiting for the other computer to finish its job and send a response.

On a single core (single cpu) system this would be the formula:

So you could calculate the IO_time from the 3 numbers given by time .

On a multi-core system however both sys and user contain the combined time spent on all of the cores. This means that both sys and user might be higher than the real time.

e.g. this result on a 4-core system probably means that each core spent 1 sec in user mode and 1 sec in sys . So each core was used for 2 seconds, but that happened in parallel, so only 2 seconds of your life was used.

Of course this cannot really happen as there are other things running on your compuuter that take time so your process won’t be able to use 100% of each core. So the real time will be longer. The following seems to be possible:

Here, the 2 seconds per each CPU were executed within a 2.5 seconds timeframe.

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fast elapsed time on linux

I am looking for a fast way to get the elapsed time between two calls of a function in C.

I considered using jiffies, but they are not available in userland. So, should I use getimeofday() or is there any fastest way to do this.

I am only interested in the elasped time between two calls, to use in a benchmark tool.

7 Answers 7

Take a look at clock_gettime , which provides access to high-resolution timers.

I’d get the processor time via clock() from time.h . To get useful values, convert to milliseconds via CLOCKS_PER_SEC :

Yes, gettimeofday() would suffice if you want to find the actual elapsed. If you need to find the CPU time, then you can use clock(). In many cases, both approach would give similar result (unless there is a sleep or some sort of wait in code). An example of clock() can be found here:
http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_19.html

If you’re on an x86/x64 architecture, and you’re running on a single CPU, you might consider reading the time-stamp counter on the CPU to get a cycle-count. Wikipedia has more information. Note that this approach is less useful if your application is running on multiple CPUs, since each CPU will have its own TSC. Also be wary of frequency scaling if you decide that you want to convert cycles -> time units.

If your kernel supports gettimeofday() as a vsyscall (virtual system call), then using this will be faster than calling a regular system call such as clock() . See Andrea Arcangeli’s presentation for some information on how vsyscalls work.

Bit of a late addition, however the time uility available on any linux/unix may be a lighter weight way of achieving what you want. Here are 2 examples

The following works for me on CentOS 6:

. although it does require librt :

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Measuring elapsed time in linux for a c program

I am trying to measure elapsed time in Linux. My answer keeps returning zero which makes no sense to me. Below is the way i measure time in my program.

6 Answers 6

clock_gettime() goes upto nanosecond accuracy and it supports 4 clocks.

CLOCK_REALTIME

System-wide realtime clock. Setting this clock requires appropriate privileges.

CLOCK_MONOTONIC

Clock that cannot be set and represents monotonic time since some unspecified starting point.

CLOCK_PROCESS_CPUTIME_ID

High-resolution per-process timer from the CPU.

CLOCK_THREAD_CPUTIME_ID

Thread-specific CPU-time clock.

You can use it as

Note: The clock() function returns CPU time for your process, not wall clock time. I believe this is what the OP was interested in. If wall clock time is desired, then gettimeofday() is a good choice as suggested by an earlier answer. clock_gettime() can do either one if your system supports it; on my linux embedded system clock_gettime() is not supported, but clock() and gettimeofday() are.

Below is the code for getting wall clock time using gettimeofday()

To start with you need to use floating point arithmetics. Any integer value divided by a larger integer value will be zero, always.

And of course you should actually do something between getting the start and end times.

By the way, if you have access to gettimeofday it’s normally preferred over clock as it has higher resolution. Or maybe clock_gettime which has even higher resolution.

There are two issues with your code as written.

According to man 3 clock , the resolution of clock() is in CLOCKS_PER_SEC increments per second. On a recent-ish Cygwin system, it’s 200. Based on the names of your variables, you are expecting the value to be 1,000,000.

will compute the quotient as an integer, because both operands are integers. The promotion to a floating-point type occurs at the time of the assignment to millis , at which point the fractional part has already been discarded.

To compute the number of seconds elapsed using clock() , you need to do something like this:

However, you will almost certainly not get millisecond accuracy. For that, you would need to use gettimeofday() or clock_gettime() . Further, you probably want to use double instead of float , because you are likely going to wind up subtracting very large numbers with a very tiny difference. The example using clock_gettime() would be:

Since NANO_PER_SEC is a floating-point value, the division operations are carried out in floating-point.

Sources:
man pages for clock(3) , gettimeofday(3) , and clock_gettime(3) .
The C Programming Language, Kernighan and Ritchie

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linux — [Solved-5 Solutions] Easily measure elapsed time in Linux — ubuntu — red hat — debian — linux server — linux pc

Linux — Problem :

To use time() to measure various points of your program.

click below button to copy the code. By — Linux tutorial — team

click below button to copy the code. By — Linux tutorial — team

How to read a result of **time taken = 0 26339? Does that mean 26,339 nanoseconds = 26.3 msec?

What about **time taken = 4 45025, does that mean 4 seconds and 25 msec?

Linux — Solution 1:

click below button to copy the code. By — Linux tutorial — team

The time() function is only accurate to within a second, but there are CLOCKS_PER_SEC «clocks» within a second.

Linux — Solution 2:

You can abstract the time measuring mechanism and have each callable’s run time measured with minimal extra code, just by being called through a timer structure. At compile time you can parametrize the timing type (milliseconds, nanoseconds etc).

click below button to copy the code. By — Linux tutorial — team

The above class could give the user the choice to call count manually by providing an extra static method

click below button to copy the code. By — Linux tutorial — team

The most useful for clients that

If C++17’s std::invoke is available, the invocation of the callable in execution could be done like this :

click below button to copy the code. By — Linux tutorial — team

To provide for callables that are pointers to member functions.

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C Elapsed time linux

Sorry I am new to C. But what am I doing wrong? Tried almost everything, but still can not calculate seconds passed to execute the code between t1 and t2, always returns me Finished in 0.00 seconds. Thanks for your patience 🙂

3 Answers 3

If coarse granularity (whole second) is ok, perhaps time(2) and time_t are good enough.

It seems like clock_t does not take input account the «wall time», but CPU time. The CPU is not effectively executing the code when it sleeps for 5 seconds.

clock() is unsuitable for measuring elapsed time. It’s resolution is generally very low and it only measures CPU time, not wall time. If you’re targeting Unix you should use clock_gettime(CLOCK_MONOTONIC, . ) . On Windows QueryPerformanceCounter() is the API to use.

It is worth noting that gettimeofday() is also unsuitable because it is affected by leap seconds and adjustments by the network time protocol.

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