tool monkey

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Interaction between mktime() and tm_isdst - a compute cycle landmine

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I was recently profiling the Condor collector, and was a bit stunned to discover that the standard C library function mktime() was burning 60% of the collector’s cycles.

Matt helpfully attempted to reproduce, but his profile showed mktime() using almost none of the cycles, which is exactly the sane result one would expect.

In the code, I noticed that tm_isdst was set to 1, in other words “assert that DST is in effect.” This made my eye twitch, because I live in Arizona, where we boldy do not observe DST. I created a little test rig to help confirm my suspicion that time zone might have something to do with it:

#include <stdlib.h>
#include <time.h>
#include <iostream>

using std::cout;

time_t mktA(struct tm* tmp) {
    tmp->tm_isdst = -1;
    return mktime(tmp);

time_t mktB(struct tm* tmp) {
    tmp->tm_isdst = 0;
    return mktime(tmp);

time_t mktC(struct tm* tmp) {
    tmp->tm_isdst = 1;
    return mktime(tmp);

int main(int argc, char** argv) {
    struct tm stm;
    stm.tm_year = 2012 - 1900;
    stm.tm_mon = 3-1;
    stm.tm_mday = 17-1;
    stm.tm_hour = 0;
    stm.tm_min = 0;
    stm.tm_sec = 0;

    // this gets altered for each testing function:
    stm.tm_isdst = 0;

    cout << mktA(&stm) << "\n";
    cout << mktB(&stm) << "\n";
    cout << mktC(&stm) << "\n";

    return 0;

Then I built the test rig, which I expertly named test_mktime, and profiled it using valgrind/callgrind:

# build the test rig
$ make test_mktime
g++     test_mktime.cpp   -o test_mktime

# profile using valgrind/callgrind:
$ valgrind --tool=callgrind ./test_mktime
==2671== Callgrind, a call-graph generating cache profiler
==2671== Copyright (C) 2002-2009, and GNU GPL'd, by Josef Weidendorfer et al.
==2671== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==2671== Command: ./test_mktime
==2671== For interactive control, run 'callgrind_control -h'.
==2671== Events    : Ir
==2671== Collected : 4125723
==2671== I   refs:      4,125,723

# massage the raw output into something (more or less) human readable:
$ callgrind_annotate --inclusive=yes --tree=calling callgrind.out.2671 > mktprof.txt

Examining the massaged output in mktprof.txt, I observed that calling mktime() with tm_isdst = {-1|0} (mktA() and mktB()) takes the small amount of time one would expect, calling with tm_isdst = 1 (mktC()) uses a completely insane number of cycles, and clearly nearly all of the cycles burned by the test rig:

2,749,933  *  ???:main [/home/eje/mktime/test_mktime]
2,655,457  >   ???:mktC(tm*) (1x) [/home/eje/mktime/test_mktime]
    4,428  >   ???:std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (3x) [/usr/lib64/]
   11,260  >   ???:std::ostream::operator<<(long) (3x) [/usr/lib64/]
    4,064  >   ???:mktB(tm*) (1x) [/home/eje/mktime/test_mktime]
    3,989  >   ???:_dl_runtime_resolve (2x) [/lib64/]
   74,212  >   ???:mktA(tm*) (1x) [/home/eje/mktime/test_mktime]

Again, Matt verified that he could reproduce the weird behavior if he set his timezone to “Arizona”.

The bottom line appears to be that invoking mktime() with tm_isdst = 1, in a time zone that does not observe DST, can set off a nuclear cycle-stealing land mine of inefficiency and horror.