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From: Jeff Hostetler <>
To: Junio C Hamano <>
Cc: Anh Le via GitGitGadget <>,, Timothy Jones <>,
	Jeff Hostetler <>, Anh Le <>
Subject: Re: [PATCH] index: add trace2 region for clear skip worktree
Date: Wed, 26 Oct 2022 14:29:09 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <xmqq8rl2lgl3.fsf@gitster.g>

On 10/26/22 12:01 PM, Junio C Hamano wrote:
> Jeff Hostetler <> writes:
>> In the worst case, we walk the entire index and lstat() for a
>> significant number of skipped-and-not-present files, then near
>> the end of the loop, we find a skipped-but-present directory
>> and have to restart the loop.  The second pass will still run
>> the full loop again.  Will the second pass actually see any
>> skipped cache-entries?  Will it re-lstat() them?  Could the
>> `goto restart` just be a `break` or `return`?
>> I haven't had time to look under the hood here, but I was
>> hoping that these two counters would help the series author
>> collect telemetry over many runs and gain more insight into
>> the perf problem.
> Without being able to answer these questions, would we be able to
> interpret the numbers reported from these counters?
>> Continuing the example from above, if we've already paid the
>> costs to lstat() the 95% sparse files AND THEN near the bottom
>> of the loop we have to do a restart, then we should expect
>> this loop to be doubly slow.  It was my hope that this combination
>> of counters would help us understand the variations in perf.
> Yes, I understand that double-counting may give some clue to detect
> that, but it just looked roundabout way to do that.  Perhaps
> counting the path inspected during the first iteration and the path
> inspected during the second iteration, separately, without the "how
> many times did we repeat?", would give you a better picture?  "After
> inspecting 2400 paths, we need to go back and then ended up scanning
> 3000 paths in the flattened index in the second round" would be
> easier to interpret than "We needed flattening, and scanned 5400
> paths in total in the two iterations".

Good point and I was wondering about whether we might see "2 x 95%"
values for path_count in really slow cases.  And yes, it would be
better to have `int path_count[2]` and tally each loop pass

I guess I was looking for a first-order "where is the pain?" knowing
that we could always dig deeper later.  That is, is the lstat's or
is it the ensure-full and index rewrite?  Or both?

We have other places that do lstat() over the cache-entries and have
added code to spread the loop across n threads.  I doubt that is needed
here and I didn't want to lead with it.

>> WRT the `intmax_t` vs just `int`: either is fine.
> I thought "int" was supposed to be natural machine word, while
> incrementing "intmax_t" is allowed to be much slower than "int".
> Do we expect more than 2 billion paths?

You're right.  An `int` would be fine here.


  reply	other threads:[~2022-10-26 18:30 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-26  0:05 [PATCH] index: add trace2 region for clear skip worktree Anh Le via GitGitGadget
2022-10-26  3:16 ` Junio C Hamano
2022-10-26 14:13   ` Jeff Hostetler
2022-10-26 16:01     ` Junio C Hamano
2022-10-26 18:29       ` Jeff Hostetler [this message]
2022-10-27  0:04         ` Anh Le
2022-10-28  0:46 ` [PATCH v2] " Anh Le via GitGitGadget
2022-10-28 15:49   ` Derrick Stolee
2022-10-28 17:17     ` Junio C Hamano
2022-10-30 23:28       ` Anh Le
2022-10-28 16:50   ` Jeff Hostetler
2022-10-31  0:56   ` [PATCH v3] " Anh Le via GitGitGadget
2022-10-31 22:34     ` Taylor Blau
2022-11-03 23:04     ` [PATCH v4 0/2] " Anh Le via GitGitGadget
2022-11-03 23:05       ` [PATCH v4 1/2] " Anh Le via GitGitGadget
2022-11-03 23:05       ` [PATCH v4 2/2] index: raise a bug if the index is materialised more than once Anh Le via GitGitGadget
2022-11-05  0:29       ` [PATCH v4 0/2] index: add trace2 region for clear skip worktree Taylor Blau
2022-11-07 20:50         ` Derrick Stolee

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