Jeff King [off-list ref] writes:
Yeah. We tolerate a certain amount of skew (24 hours for --name-rev, and
5 broken commits in a row for --since). But the big ones are usually
software bugs (the big kernel ones were from broken "guilt", I think) or
broken imports (when I published a bunch of skew statistics last year,
the interesting ones were all imports; I don't know if they were
software bugs, or just garbage in, garbage out).
I was hoping that 2e6bdd3 (test-generation: compute generation
numbers and clock skews, 2012-09-04) may be a good first step to
come up with a practical and cheap solution on top of it.
The traversal can be fooled by clock skews when it sees a commit
that has a timestamp that is older than it should, causing it to
give up, incorrectly thinking that there won't be newer commits that
it is interested in behind the problematic commit.
The logic implemented by the change is to identify these problematic
commits, and we could record these commits with the value of the
timestamps they should have had (e.g. the timestamp of the newest
ancestor for each of these commits) in a notes tree. Then the
traversal logic (commit-list-insert-by-date) could be updated use
that "corrected" timestamp instead not to be fooled by the clock
skew.
Such a notes tree can be built once and updated by only "appending",
as a commit will never acquire more ancestors in its parents chain
once it is made.
Is it too simplistic, or too costly? In git.git we have three such
commits whose timestamp need to be corrected, while in the Linux
kernel there were 2.2k skewed commits when I counted them a few
months ago.
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On Mon, Nov 12, 2012 at 08:51:37PM -0800, Junio C Hamano wrote:
Jeff King [off-list ref] writes:
quoted
Yeah. We tolerate a certain amount of skew (24 hours for --name-rev, and
5 broken commits in a row for --since). But the big ones are usually
software bugs (the big kernel ones were from broken "guilt", I think) or
broken imports (when I published a bunch of skew statistics last year,
the interesting ones were all imports; I don't know if they were
software bugs, or just garbage in, garbage out).
I was hoping that 2e6bdd3 (test-generation: compute generation
numbers and clock skews, 2012-09-04) may be a good first step to
come up with a practical and cheap solution on top of it.
The traversal can be fooled by clock skews when it sees a commit
that has a timestamp that is older than it should, causing it to
give up, incorrectly thinking that there won't be newer commits that
it is interested in behind the problematic commit.
I wrote a similar skew-finding tool last year, though some of the
numbers it came up with were different (I remember having many fewer
skewed commits in the kernel repo).
One problem is that it identifies commits which behave badly with
certain algorithms, but it does not identify commits which are wrong.
If I skew backwards, it will find my commit. But if I skew forwards, it
will label my children as wrong.
The logic implemented by the change is to identify these problematic
commits, and we could record these commits with the value of the
timestamps they should have had (e.g. the timestamp of the newest
ancestor for each of these commits) in a notes tree. Then the
traversal logic (commit-list-insert-by-date) could be updated use
that "corrected" timestamp instead not to be fooled by the clock
skew.
Such a notes tree can be built once and updated by only "appending",
as a commit will never acquire more ancestors in its parents chain
once it is made.
Is it too simplistic, or too costly? In git.git we have three such
commits whose timestamp need to be corrected, while in the Linux
kernel there were 2.2k skewed commits when I counted them a few
months ago.
This came up in the big generations discussion last summer, and I think
I even implemented a proof of concept. I couldn't find the actual code,
though but only that I got "pleasing performance results using a notes
tree to store a list of commits with bogus timestamps":
http://article.gmane.org/gmane.comp.version-control.git/161101
It is a little wasteful in space if you have a lot of skewed commits
(the notes tree stores a 160-bit hash pointing to a blob object storing
a 32-bit integer).
My personal preference at this point would be:
1. introduce an auxiliary metadata file that would live alongside the
pack index and contain generation numbers
2. generate the metadata file during pack indexing.
3. If we have a generation metadata file, but a particular object is
not in it, compute the generation; this should be quick because we
will hit a file with a stored generation eventually
4. If we do not have any generation metadata files, or if grafts or
replace objects are in use, do not use cutoffs in algorithms. Be
safe but slow.
On the other hand, just switching to doing a single traversal instead of
one merge-base computation per tag already got rid of the really awful
performance cases. Nobody has complained since that went in, so maybe
nobody cares about shaving a few seconds per operation down to a few
tens of milliseconds. The real win was shaving tens of seconds down to a
few seconds.
-Peff
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