Is gettimeofday() guaranteed to be of microsecond resolution?

+5 votes
asked Aug 1, 2008 by bernard

So I find myself porting a game, that was originally written for the Win32 API, to Linux (well, porting the OS X port of the Win32 port to Linux). I have implemented QueryPerformanceCounter by giving the uSeconds since the process start up:

BOOL QueryPerformanceCounter(LARGE_INTEGER* performanceCount)
{
    gettimeofday(&currentTimeVal, NULL);
    performanceCount->QuadPart = (currentTimeVal.tv_sec - startTimeVal.tv_sec);
    performanceCount->QuadPart *= (1000 * 1000);
    performanceCount->QuadPart += (currentTimeVal.tv_usec - startTimeVal.tv_usec);

    return true;
}

This, coupled with QueryPerformanceFrequency() giving a constant 1000000 as the frequency, works well on my machine, giving me a 64 bit variable that contains uSeconds since the program's start up. So is this portable? I don't want to discover it works differently if the kernel was compiled in a certain way or anything like that. I am fine with it being non-portable to something other than Linux, however.

16 Answers

0 votes
answered Aug 1, 2008 by codingwithoutcomment

From my experience, and from what I've read across the internet, the answer is "No," it is not guaranteed. It depends on CPU speed, operating system, flavor of Linux, etc.

0 votes
answered Aug 1, 2008 by louis-brandy

Maybe. But you have bigger problems. gettimeofday() can result in incorrect timings if there are processes on your system that change the timer (ie, ntpd). On a "normal" linux, though, I believe the resolution of gettimeofday() is 10us. It can jump forward and backward and time, consequently, based on the processes running on your system. This effectively makes the answer to your question no.

You should look into clock_gettime(CLOCK_MONOTONIC) for timing intervals. It suffers from several less issues due to things like multi-core systems and external clock settings.

Also, look into the clock_getres() function.

0 votes
answered Aug 1, 2008 by codingwithoutcomment

The actual resolution of gettimeofday() depends on the hardware architecture. Intel processors as well as SPARC machines offer high resolution timers that measure microseconds. Other hardware architectures fall back to the system’s timer, which is typically set to 100 Hz. In such cases, the time resolution will be less accurate.

I obtained this answer from High Resolution Time Measurement and Timers, Part I

0 votes
answered Aug 2, 2008 by mark-harrison

High Resolution, Low Overhead Timing for Intel Processors

If you're on Intel hardware, here's how to read the CPU real-time instruction counter. It will tell you the number of CPU cycles executed since the processor was booted. This is probably the finest-grained counter you can get for performance measurement.

Note that this is the number of CPU cycles. On linux you can get the CPU speed from /proc/cpuinfo and divide to get the number of seconds. Converting this to a double is quite handy.

When I run this on my box, I get

11867927879484732
11867927879692217
it took this long to call printf: 207485

Here's the Intel developer's guide that gives tons of detail.

#include <stdio.h>
#include <stdint.h>

inline uint64_t rdtsc() {
    uint32_t lo, hi;
    __asm__ __volatile__ (
      "xorl %%eax, %%eax\n"
      "cpuid\n"
      "rdtsc\n"
      : "=a" (lo), "=d" (hi)
      :
      : "%ebx", "%ecx");
    return (uint64_t)hi << 32 | lo;
}

main()
{
    unsigned long long x;
    unsigned long long y;
    x = rdtsc();
    printf("%lld\n",x);
    y = rdtsc();
    printf("%lld\n",y);
    printf("it took this long to call printf: %lld\n",y-x);
}
0 votes
answered Aug 2, 2008 by joe-shaw

So it says microseconds explicitly, but says the resolution of the system clock is unspecified. I suppose resolution in this context means how the smallest amount it will ever be incremented?

The data structure is defined as having microseconds as a unit of measurement, but that doesn't mean that the clock or operating system is actually capable of measuring that finely.

Like other people have suggested, gettimeofday() is bad because setting the time can cause clock skew and throw off your calculation. clock_gettime(CLOCK_MONOTONIC) is what you want, and clock_getres() will tell you the precision of your clock.

0 votes
answered Aug 4, 2008 by mark-harrison

@Bernard:

I have to admit, most of your example went straight over my head. It does compile, and seems to work, though. Is this safe for SMP systems or SpeedStep?

That's a good question... I think the code's ok. From a practical standpoint, we use it in my company every day, and we run on a pretty wide array of boxes, everything from 2-8 cores. Of course, YMMV, etc, but it seems to be a reliable and low-overhead (because it doesn't make a context switch into system-space) method of timing.

Generally how it works is:

  • declare the block of code to be assembler (and volatile, so the optimizer will leave it alone).
  • execute the CPUID instruction. In addition to getting some CPU information (which we don't do anything with) it synchronizes the CPU's execution buffer so that the timings aren't affected by out-of-order execution.
  • execute the rdtsc (read timestamp) execution. This fetches the number of machine cycles executed since the processor was reset. This is a 64-bit value, so with current CPU speeds it will wrap around every 194 years or so. Interestingly, in the original Pentium reference, they note it wraps around every 5800 years or so.
  • the last couple of lines store the values from the registers into the variables hi and lo, and put that into the 64-bit return value.

Specific notes:

  • out-of-order execution can cause incorrect results, so we execute the "cpuid" instruction which in addition to giving you some information about the cpu also synchronizes any out-of-order instruction execution.

  • Most OS's synchronize the counters on the CPUs when they start, so the answer is good to within a couple of nano-seconds.

  • The hibernating comment is probably true, but in practice you probably don't care about timings across hibernation boundaries.

  • regarding speedstep: Newer Intel CPUs compensate for the speed changes and returns an adjusted count. I did a quick scan over some of the boxes on our network and found only one box that didn't have it: a Pentium 3 running some old database server. (these are linux boxes, so I checked with: grep constant_tsc /proc/cpuinfo)

  • I'm not sure about the AMD CPUs, we're primarily an Intel shop, although I know some of our low-level systems gurus did an AMD evaluation.

Hope this satisfies your curiosity, it's an interesting and (IMHO) under-studied area of programming. You know when Jeff and Joel were talking about whether or not a programmer should know C? I was shouting at them, "hey forget that high-level C stuff... assembler is what you should learn if you want to know what the computer is doing!"

0 votes
answered Aug 4, 2008 by vincent-robert

Wine is actually using gettimeofday() to implement QueryPerformanceCounter() and it is known to make many Windows games work on Linux and Mac.

Starts http://source.winehq.org/source/dlls/kernel32/cpu.c#L312

leads to http://source.winehq.org/source/dlls/ntdll/time.c#L448

0 votes
answered Aug 18, 2008 by doug

Reading the RDTSC is not reliable in SMP systems, since each CPU maintains their own counter and each counter is not guaranteed to by synchronized with respect to another CPU.

I might suggest trying clock_gettime(CLOCK_REALTIME). The posix manual indicates that this should be implemented on all compliant systems. It can provide a nanosecond count, but you probably will want to check clock_getres(CLOCK_REALTIME) on your system to see what the actual resolution is.

0 votes
answered Aug 18, 2008 by david-schlosnagle
+4 votes
answered Jun 26, 2010 by strae

Before you start thinking about how to make the plugins system, you have to define what exactly a plugin is for you application, what the plugins can do and what data the plugins will be able to access (for example, posts table but not users table).

Then, take a look at Drupal, i guess it's module system based on hooks is really powerfull and 'simple' to use for developers.

Basically, the idea is that when a module or plugin is installed, everythings you do on your 'core' code, search if there is some module/plugins hooked to that action.

Example:

//your code
$modules_enabled = array('foo', 'bar');
//example action, lets say insert a new blog post. Obviously, nothings prevent you
//to do that in OOP style (i'd never really understood why drupal is almost all procedural).
function create_new_post($modules_enabled, $other_args){ //looks if some modules need to be called before create the blog post foreach($modules_enables AS $module){ if(function_exists("after_create_new_post_$module")){ $func = "before_create_new_post_$module"; $func($params); } } //do your stuff here //looks if some modules need to be called after the post is been created foreach($modules_enables AS $module){ if(function_exists("after_create_new_post_$module")){ $func = "after_create_new_post_$module"; $func($params); } }
}
//the module file should look like $hooks_name . $module_name:
function after_create_new_post_foo($args){ //do your job
}

This is a very very sintetic example (and doesnt work!), but should give you the idea.

The only problem here comes with the args that you pass to each hook_function, that have to be documented really good, however the documentation is important whatever path you'll choose.

Some reference: Drupal hooks, hook_insert

Welcome to Q&A, where you can ask questions and receive answers from other members of the community.
Website Online Counter

...