From: Junio C Hamano <gitster@pobox.com>
To: Karsten Blees <karsten.blees@gmail.com>
Cc: Git List <git@vger.kernel.org>
Subject: Re: broken racy detection and performance issues with nanosecond file times
Date: Mon, 28 Sep 2015 11:17:19 -0700 [thread overview]
Message-ID: <xmqqtwqecssw.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: <5605D88A.20104@gmail.com> (Karsten Blees's message of "Sat, 26 Sep 2015 01:28:10 +0200")
Karsten Blees <karsten.blees@gmail.com> writes:
> Ideas for potential solutions:
> ==============================
>
> Performance issues:
> -------------------
>
> 1. Compare file times in minimum supported precision
> When comparing file times, use the minimum precision supported by
> both the writing and reading git implementations.
> 1a. Simplest variant: Don't compare nanoseconds if the field in the
> cached index entry is 0. JGit already does this [5], but at the
> same time it is very unfriendly to USE_NSEC-enabled git by storing
> only milliseconds in the nanosecond field. This "simple" solution
> implies that git implementations that cannot provide full
> nanosecond precision must leave the nanosecond field empty.
> 1b. More involved: Store the precision in the index entry.
> We only need 30 bits to encode nanoseconds, so the high 2 bits of
> the nanosecond field could be used as follows:
> 00: second precision (i.e. ignore, for backward compatibility)
> 01: millisecond precision
> 10: microsecond precision
> 11: nanosecond precision
> When reading the index, USE-NSEC-enabled git implementations would
> do dirty checks with the minimum precision supported by themselves
> and the creator of the index entry.
Yeah, my gut feeling is that we should make sure that at least 1a is
done by all implementations.
I agree that 1b. is a bit more involved in that all binary that was
built with USE_NSEC that is not aware of these 2-bits need to be
eradicated for a new version to be deployed --- the transition for
users who use multiple implementations will be a pain (those that
use just one implementation of Git can just say "rm -f .git/index &&
git reset --hard" or something after updating to the new version of
Git).
> 2. Don't use ctime in dirty checks if ctime.sec == 0.
OK. That is slightly less drastic than !trust_ctime, I guess.
> Racy detection:
> ---------------
>
> 3. Minimal racy solution
> * Do all racy checks with second-precision only.
> * When committing an index.lock file, reset mtime to the time
> before git started reading the old index (i.e. time(null) when
> calling read_cache()).
>
> I believe this should fix all three racy problems described above,
> although restraining ourselves to second-precision somewhat
> thwarts the ability to track nanoseconds in the first place.
>
> The problem with this solution is that files changed by git itself
> will appear racy to the next git process, thus increasing the
> performance penalty after e.g. a large checkout. Although I think
> that re-reading the file after the file's mtime is the only way to
> be really sure it hasn't been changed.
... the last of which is what is done anyway, so I think the above,
especally the second bullet-point, is all sensible.
prev parent reply other threads:[~2015-09-28 18:17 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-25 23:28 broken racy detection and performance issues with nanosecond file times Karsten Blees
2015-09-28 10:39 ` [PATCH/RFC] read-cache: fix file time comparisons with different precisions Karsten Blees
2015-09-28 12:52 ` Johannes Schindelin
2015-09-29 10:23 ` Karsten Blees
2015-09-29 13:42 ` Johannes Schindelin
2015-09-28 17:38 ` broken racy detection and performance issues with nanosecond file times Junio C Hamano
2015-09-29 11:28 ` Karsten Blees
2015-09-28 18:17 ` Junio C Hamano [this message]
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