[Bug libc/5548] New: localtime/strftime calling tzset unconditionally

["bazsi at balabit dot hu" <sourceware-bugzilla at sourceware dot org>]

X-Bugzilla-Reason: CC

I have an application (syslog-ng) that calls localtime() related functions a
lot. After some performance profiling localtime/strftime seems to be a root of a
lot of CPU consumption because of the fact that they call tzset() unconditionally.

I understand that POSIX requires that localtime()/strftime() notice that
timezone information was changed, but calling tzset() unconditionally to do so
seems overly  expensive to me.

What I did to work around the problem was to cache the previous results of
localtime() and not call localtime() if the same second was requested. This
results in about one localtime() call per second.

But still, tzset is on the profile list:

-------------------------------------------------------------------------------
  759      100.000  libc-2.6.1.so            libc-2.6.1.so            __tz_convert
2948      1.5846  libc-2.6.1.so            libc-2.6.1.so            getenv
  2948     100.000  libc-2.6.1.so            libc-2.6.1.so            getenv [self]
-------------------------------------------------------------------------------
  924      100.000  libc-2.6.1.so            libc-2.6.1.so            __tz_convert
2686      1.4438  libc-2.6.1.so            libc-2.6.1.so            strcmp
  2686     100.000  libc-2.6.1.so            libc-2.6.1.so            strcmp [self]
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
  2         0.1980  libc-2.6.1.so            libc-2.6.1.so            tzset
  1008     99.8020  libc-2.6.1.so            libc-2.6.1.so            __tz_convert
1065      0.5725  libc-2.6.1.so            libc-2.6.1.so            tzset_internal
  1065     99.8126  libc-2.6.1.so            libc-2.6.1.so           
tzset_internal [self]
  2         0.1874  libc-2.6.1.so            libc-2.6.1.so            __tzfile_read
-------------------------------------------------------------------------------

Hopefully the profile data can be recognized in this bugzilla entry. To sum it
up, the CPU use might be decreased somewhat if this unconditional call to
tzset() would be tightened a bit:

 * only call tzset if /etc/localtime or the TZ environment variable changed
 * the latter can still be optimized somewhat as putenv() reallocates the
environment area, thus any change in TZ will result in a new value of the
__environ pointer

-- 
           Summary: localtime/strftime calling tzset unconditionally
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: bazsi at balabit dot hu
                CC: glibc-bugs at sources dot redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=5548

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