StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PONegative clock cycle measurements with back-to-back rdtsc?
text
Body
copied!<p>I am writing a C code for measuring the number of clock cycles needed to acquire a semaphore. I am using rdtsc, and before doing the measurement on the semaphore, I call rdtsc two consecutive times, to measure the overhead. I repeat this many times, in a for-loop, and then I use the average value as rdtsc overhead.</p> <p>Is this correct, to use the average value, first of all?</p> <p>Nonetheless, the big problem here is that sometimes I get negative values for the overhead (not necessarily the averaged one,but at least the partial ones inside the for loop).</p> <p>This also affects the consecutive calculation of the number of cpu cycles needed for the <code>sem_wait()</code> operation, which sometimes also turns out to be negative. If what I wrote is not clear, here there's a part of the code I am working on.</p> <p>Why am I getting such negative values?</p> <hr> <p>(editor's note: see <a href="https://stackoverflow.com/questions/13772567/get-cpu-cycle-count">Get CPU cycle count?</a> for a correct and portable way of getting the full 64-bit timestamp. An <code>"=A"</code> asm constraint will only get the low or high 32 bits when compiled for x86-64, depending on whether register allocation happens to pick RAX or RDX for the <code>uint64_t</code> output. It won't pick <code>edx:eax</code>.)</p> <p>(editor's 2nd note: oops, that's the answer to why we're getting negative results. Still worth leaving a note here as a warning not to copy this <code>rdtsc</code> implementation.)</p> <hr> <pre><code>#include <semaphore.h> #include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <inttypes.h> static inline uint64_t get_cycles() { uint64_t t; // editor's note: "=A" is unsafe for this in x86-64 __asm volatile ("rdtsc" : "=A"(t)); return t; } int num_measures = 10; int main () { int i, value, res1, res2; uint64_t c1, c2; int tsccost, tot, a; tot=0; for(i=0; i<num_measures; i++) { c1 = get_cycles(); c2 = get_cycles(); tsccost=(int)(c2-c1); if(tsccost<0) { printf("#### ERROR!!! "); printf("rdtsc took %d clock cycles\n", tsccost); return 1; } tot = tot+tsccost; } tsccost=tot/num_measures; printf("rdtsc takes on average: %d clock cycles\n", tsccost); return EXIT_SUCCESS; } </code></pre>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload