So this article[1] from 2017 popped up again on the tech radar via hackernews[2] and a few other sites[3]. It
annotates how if the envvar TZ is undefined on a Linux system, it causes glibc to generate a number of
additional syscalls, mainly stat-related calls (in my tests, newfstatat()). If defined to an actual value,
such as ":/etc/localtime" (or even an empty string), glibc will instead generate far fewer, if any at all, of
these stat-related syscalls.
Apparently, TZ is accessed quite frequently, so this has a compound effect, according to the article, in glibc
making thousands of unnecessary stat-related syscalls to /etc/localtime (which must be hard-coded somewhere in
glibc for this case). Given the article's age (five years old), I tested the example C program out, and it
does appear to still be accurate on a modern glibc-based system. When TZ is undefined, I get exactly nine
newfstatat calls on /etc/localtime. If I define TZ to ":/etc/localtime", I do not get any of these newfstatat
calls, and if I set TZ to an empty string, glibc will call openat() against "/usr/share/zoneinfo/Universal"
and then generate exactly two newfstatat syscalls on that handle to read it.
I ran strace() against the undefined TZ case and the ":/etc/localtime" case, normalized the hex addresses to
get a clean diff, and this is what it looks like:
--- a 2023-01-18 20:30:36.826805343 -0500
+++ b 2023-01-18 20:30:45.106983600 -0500
@@ -1,4 +1,4 @@
-# strace ./tz_test
+# TZ=":/etc/localtime" strace ./tz_test
execve("./tz_test", ["./tz_test"], 0xhhhhhhhhhhhh /* XX vars */) = 0
brk(NULL) = 0xhhhhhhhhhhhh
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xhhhhhhhhhhhh
@@ -61,15 +61,6 @@ read(3, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0
lseek(3, -2260, SEEK_CUR) = 1292
read(3, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\6\0\0\0\6\0\0\0\0"...,
3584) = 2260
close(3) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
-newfstatat(AT_FDCWD, "/etc/localtime", {st_mode=S_IFREG|0644,
st_size=3552, ...}, 0) = 0
write(1, "Godspeed, dear friend!\n", 23Godspeed, dear friend!
) = 23
exit_group(0) = ?
For comparison, I tested the same program on FreeBSD and it does not exhibit this behavior at all, regardless
of whether TZ is undefined, a value, or an empty string. I have yet to make a similar test on a mips/musl
chroot to see how musl handles this.
There is a rather old (2010) StackOverflow question[4] about it as well, and someone left an answer in March
of last year about the specific code in glibc that handles TZ if it is set or is an empty string.
So is adding a default definition of TZ to our base system /etc/profile something we want to look at? I
haven't tried any other methods of benchmarking to see if not making those additional syscalls is just placebo
or if there are actual impacts. Given how long this oddity has been around, I can't tell if it's a genuine
bug in glibc, an unoptimized corner case, or just a big nothingburger.
1.
https://blog.packagecloud.io/set-environment-variable-save-thousands-of-system-calls/
2. https://news.ycombinator.com/item?id=34346346
3. https://vermaden.wordpress.com/posts/
4.
https://stackoverflow.com/questions/4554271/how-to-avoid-excessive-stat-etc-localtime-calls-in-strftime-on-linux
Thoughts?
--
Joshua Kinard
Gentoo/MIPS
ku...@gentoo.org
rsa6144/5C63F4E3F5C6C943 2015-04-27
177C 1972 1FB8 F254 BAD0 3E72 5C63 F4E3 F5C6 C943
"The past tempts us, the present confuses us, the future frightens us. And our lives slip away, moment by
moment, lost in that vast, terrible in-between."
--Emperor Turhan, Centauri Republic
