Red Hat Bugzilla – Bug 171351
strftime is unreasonably slow because of tzset
Last modified: 2016-11-24 11:10:30 EST
Description of problem: While analyzing a performance problem in PostgreSQL, I found out that each call to strftime() resulted in a stat64("/etc/localtime") kernel call. The source code says that POSIX.1 requires that strftime act as if it called tzset, which is fine, but surely POSIX.1 doesn't require stat'ing an external file. Couldn't this be fixed so that it behaves more like localtime(), ie, we only have to pay a strcmp() to decide that the info is up to date? Specifically I think strftime ought to result in the equivalent of tzset_internal(1, 0) not tzset_internal(1, 1). Version-Release number of selected component (if applicable): glibc-2.3.5-10.3 How reproducible: 100% Steps to Reproduce: 1. Write tight loop involving strftime() call. 2. strace the test program, having made sure TZ envar is not set. Actual results: Large number of stat64 syscalls. Expected results: Once into the loop, no syscalls should be needed. Additional info: It looks like mktime() has the same issue, though I didn't test it explicitly.
If strftime called tzset_internal(1, 0), it would violate POSIX, as it would no longer act as if it called tzset. The reason for the stat is to allow for timezone changes. If you don't want that, you can just use TZ=/etc/localtime or TZ=Europe/Prague or whatever timezone you want to use.