Linux Clocks

At ZipRecruiter we have an awesome access log that includes information about each request, like a measure of the response time, the increase in rss, and lots of other details. Before I joined we had a measure of how much CPU was used, but it was a little coarse. Read about how I increased the granularity here!

times

Originally we were using the times system call. This is especially useful since Perl has a builtin for times. It’s easy, and it gives a pretty good set of information (specifically how much work was done in kernel, out of kernel, in kernel for child processes, and out of kernel for child processes.

The only thing that was a little annoying, as alluded to above, is that the time granularity was low. To be clear, the minimum time slice was 10ms, or 0.01s. Peter Rabbitson mentioned to me at some point that getrusage would be worth looking into, because it has higher time precision.

getrusage

So I decided, once we saw that we needed to start focusing on CPU more, to get more precision by using getrusage. The following is the data I wanted, documented in the linked man page:

ru_utime: This is the total amount of time spent executing in user mode, expressed in a timeval structure (seconds plus microseconds).

I migrated our Perl code to getrusage, but then in the logs I noticed that our times all looked like:

 1.123000
21.242000
 2.930000
 2.494000

Strange that the microseconds would be getting truncated to milliseconds. I was sure that it was a bug in Unix::Getrusage and tried BSD::Resource. It had the same problem, but I was still confident that they were both marshalling the data incorrectly. I decided to write a C program to test the system call directly:

#include <stdio.h>
#include <sys/time.h>
#include <sys/resource.h>


int main(void) {

    long int x = 2;
    for (int y = 0; y < 5322332; y++) {
        x = x * 2;
    }

    struct rusage usage;
    struct rusage *p = &usage;
    getrusage(RUSAGE_SELF, p);

    printf("%ld.%06ld\n", usage.ru_utime.tv_sec, usage.ru_utime.tv_usec);
    return 0;
}

On my Linux laptop, this program prints out 0.008000. I had a friend run it on OSX and it printed 1.763319. So basically the Linux kernel (version 4.8) does not actually have the fine grained accuracy that is documented! From what I gather, older versions of Linux do, but the accuracy was lost with the migration to tickless operation.

clock_gettime

Generally, when I’m doing some kind of ad hoc timing of things in Perl I use Time::HiRes, which has super fine grained precision; after a brief source-dive into it I came upon the clock_gettime system call. It allows you to request the time for a number of clocks, but in this case the one that’s useful is CLOCK_PROCESS_CPUTIME_ID. I’m not sure why it has _ID in the name; I can only assume that it’s an artifact of the history of the syscall, and related to the fact that glibc used to implement this by reading special registers directly.

The general usage is as follows:

use Time::HiRes qw( clock_gettime CLOCK_PROCESS_CPUTIME_ID );
my $t0 = clock_gettime(CLOCK_PROCESS_CPUTIME_ID);

# do some complicated stuff

my $total_seconds = clock_gettime(CLOCK_PROCESS_CPUTIME_ID) - $t0;

$total_seconds will have how many seconds (with lots of precision) was spent on the CPU, both in user code and in kernel code. If we cared to have them split apart we’d be stuck with the accuracy discussed before, but this gives us much finer grained details and really, is as much information as we need.


This was a fun little issue to track down, especially the diversion into C. I look forward to verifying issues at such a low level in the future.

Addendum: I use precision and accuracy interchangeably above. Technically all I mean is higher resolution values. It is beyond the scope of this article to discuss accuracy and precision.

Posted Thu, Oct 13, 2016
Updated Fri, Oct 14, 2016