From: Junio C Hamano <gitster@pobox.com>
To: Eric Wong <e@80x24.org>
Cc: git@vger.kernel.org, Paul Eggert <eggert@cs.ucla.edu>
Subject: Re: [PATCH 2/2] git-compat-util: use gettimeofday for current time
Date: Mon, 20 Mar 2023 09:33:01 -0700 [thread overview]
Message-ID: <xmqqh6ufo01u.fsf@gitster.g> (raw)
In-Reply-To: <20230319193449.M629601@dcvr> (Eric Wong's message of "Sun, 19 Mar 2023 19:34:49 +0000")
Eric Wong <e@80x24.org> writes:
> Paul Eggert <eggert@cs.ucla.edu> wrote:
>> Use gettimeofday instead of time(NULL) to get current time.
>> This avoids clock skew on glibc 2.31+ on Linux, where in the
>> first 1 to 2.5 ms of every second, time(NULL) returns a
>> value that is one less than the tv_sec part of
>> higher-resolution timestamps such as those returned by
>> gettimeofday or timespec_get, or those in the file system.
>> There are similar clock skew problems on AIX and MS-Windows,
>> which have problems in the first 5 ms of every second.
>
> Wow, this is enlightening... and unfortunate :<
>
> I decided to check glibc archives to find more discussion on it.
> So far, I've found:
>
> https://inbox.sourceware.org/libc-alpha/20230306160321.2942372-1-adhemerval.zanella@linaro.org/T/
>
> and the original bug:
> https://sourceware.org/bugzilla/show_bug.cgi?id=30200
>
> And this is due to the time64 changes in glibc 2.31+?
> (<= 2.30 isn't affected?)
>
> <snip>
>
>> Yet another possibility would be to fix the Linux kernel so
>> that the time syscall is consistent with the other timestamp
>> syscalls. I suppose this has not been done due to
>> performance implications. (Git's use of timestamps is rare
>> enough that performance is not a significant consideration
>> for git.) However, this wouldn't fix Git's problem on older
>> Linux kernels, or on AIX or MS-Windows.
>
> Agreed on the older kernels and other OSes part.
Yeah, this is interesting. I however wonder if we should follow our
usual pattern of implementing git_time() with the identical function
signature as what we replace (i.e. system's time()), and #undef/#define
the symbol we replace with git_time, though. Wouldn't it make [1/2]
a lot smaller and future-proof?
next prev parent reply other threads:[~2023-03-20 16:39 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-19 6:43 [PATCH 0/2] use gettimeofday for current time Paul Eggert
2023-03-19 6:43 ` [PATCH 1/2] git-compat-util: time_now " Paul Eggert
2023-03-19 6:43 ` [PATCH 2/2] git-compat-util: use gettimeofday " Paul Eggert
2023-03-19 19:34 ` Eric Wong
2023-03-20 16:33 ` Junio C Hamano [this message]
2023-03-20 17:01 ` Junio C Hamano
2023-03-20 19:00 ` Paul Eggert
2023-03-20 19:40 ` Junio C Hamano
2023-03-20 20:36 ` Taylor Blau
2023-03-20 20:35 ` Taylor Blau
2023-03-20 23:05 ` [PATCH v2] git-compat-util: use gettimeofday(2) for time(2) Junio C Hamano
2023-03-20 23:21 ` Paul Eggert
2023-03-21 16:20 ` Junio C Hamano
2023-03-21 17:44 ` Konstantin Khomoutov
2023-03-21 18:22 ` Junio C Hamano
2023-03-21 18:22 ` Jeff King
2023-03-21 19:06 ` Taylor Blau
2023-03-21 20:04 ` Junio C Hamano
2023-03-22 17:11 ` Jeff King
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