Re: [PATCH] have `date` cache the system timezone

[Pádraig Brady]

On 26/11/2024 03:34, Michael Greenberg wrote:
> If TZ is unset, the date utility will call newfstat("/etc/localtime")
> each time it reads a time using date --date or date -f. If reading from
> /etc is expensive, you can experience significant slowdowns.
>
> I didn't write a new test---you can observe the phenomenon by running
> the following:
>
> $ cat dates  # 4 isn't enough for slowness, but enough to make the trace clear
> Mon Nov 25 09:14:54 PM EST 2024
> Mon Nov 25 09:15:01 PM EST 2024
> Mon Nov 25 09:15:01 PM EST 2024
> Mon Nov 25 09:15:01 PM EST 2024
> $ strace date -f dates +%a
> ... see 4 calls to newfstatat on /etc/localtime
> $ strace coreutils/src/date -f dates +%a # use the patched version
> ... see 1 call to newfstatat on /usr/share/zoneinfo/YOURTIMEZONEHERE ...

Indeed:

  $ yes 1 | head -n100 | strace -e newfstatat -c date -f- > /dev/null
  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
  100.00    0.000071           0       100           newfstatat

Also if specifying TZ in the date, it gets worse!

  $ yes 'TZ="UTC" 1' | head -n100 | strace -e newfstatat -c date -f- > /dev/null
  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
  100.00    0.005097           5       999           newfstatat

> It might be possible to achieve the same effect by patching gnulib, but
> I haven't dug into it.

Thanks for the info. It does seem like we could improve here.
I won't be able to dig into this, this week at least.

thanks,
Pádraig.

> Cheers,
> Michael
>
> ---
>   src/date.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/src/date.c b/src/date.c
> index ee32403ef..45b111c4e 100644
> --- a/src/date.c
> +++ b/src/date.c
> @@ -575,6 +575,14 @@ main (int argc, char **argv)
>     char *format_copy = adjust_resolution (format);
>     char const *format_res = format_copy ? format_copy : format;
>     char const *tzstring = getenv ("TZ");
> +  /* If TZ is unset, set it to the system timezone. This caches the
> +     value, so batch_convert won't have to do it over and over
> +     again. */
> +  if (tzstring == NULL) {
> +    tzset();
> +    tzstring = tzname[0];
> +    setenv("TZ", tzstring, 0);
> +  }
>     timezone_t tz = tzalloc (tzstring);
>   
>     if (batch_file != nullptr)