# A cross-platform monotonic timer

I've been working on writing a memory bandwidth benchmark for a while and needed to use a monotonic timer to compute accurate timings. I have since learned that this is more challenging to do that I initially expected and each platform has a different way of doing it.

# The problem

I was trying to determine the run time of a function and wanted the most precise and accurate information possible. First, I started by using gettimeofday:

struct timeval before, after;

gettimeofday(&before, NULL);
function();
gettimeofday(&after, NULL);


Unfortunately, this will not always work since it is dependent on the system clock. If some other process changes the system time between the two calls to gettimeofday, it could report inaccurate results. We need a function that returns a monotonically increasing value.

# A solution?

Luckily, such a function exists on Linux. We can use clock_gettime with CLOCK_MONOTONIC:

struct timespec before, after, total;

clock_gettime(CLOCK_MONOTONIC, &before);
function();
clock_gettime(CLOCK_MONOTONIC, &after);


# Other platforms

Unfortunately, this doesn't work everywhere! Each platform has its own way accessing a high resolution monotonic counter. On Mac OS X we use mach_absolute_time and on Windows we use QueryPerformanceCounter.

## rdtsc

On x86 machines where none of these are available, we can resort directly to rdtsc. This is a special instruction that returns the Time Stamp Counter, the number of cycles since reset. Unfortunately, we have to be very careful when using this instruction. This white paper offers a lot of good advice on how to use it, but in short we have to take care to prevent instruction reordering. In the following code, the reordering of the fdiv after the rdtsc would lead to inaccurate timing results:

rdtsc
fdiv # Or another slow instruction
rdtsc


The instruction rdtscp prevents instructions that occur before the rdtsc from being reordered afterwards. Unfortunately, instructions that occur after the rdtscp can still be reordered before it. The following code could have fdiv reordered before the rdtscp, leading to inaccurate results:

rdtscp
call function
rdtscp
fdiv


The suggested way to avoid the reordering is to use the cpuid instruction, which has the effect of preventing all instruction reordering around it. While this is a slow instruction, we can be a bit clever and ensure that we never have to execute it while between the times when we query the counter.
The ideal timing code looks something like this:

cpuid
rtdsc
# Save %edx and %eax (the output of rtdsc).
call function
rdtscp
# Save %edx and %eax.
cpuid


# A cross platform timer

Assembling all this information, I attempted to write a cross-platform utility for fine grained timing. A few late nights and a file full of #ifdefs later, I have the start of such a utility. Currently, it supports the function monotonic_seconds which returns the seconds from some unspecified start point as a double precision floating point number. In the future, I'll add support for monotonic_cycles as a static inline function in the header and cycles_to_seconds as a way to convert cycles to seconds. Check it out here!