| Message ID | pull.520.git.1578621570180.gitgitgadget@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | unpack-trees: correctly compute result count | expand |
On Fri, Jan 10, 2020 at 01:59:30AM +0000, Derrick Stolee via GitGitGadget wrote: > Here is a very small fix to the cone-mode pattern-matching in > unpack-trees.c. Johannes found this while running a Coverity scan for > other issues. He alerted me to the problem and I could immediately see > my error here. In creating this patch, most of my time was spent asking > "how did this work before?" and "why didn't this hurt performance?" > Hopefully my commit message explains this thoroughly enough. Yes, it makes perfect sense (and as soon as I saw the explanation of the problem, my immediate response was also "wait, how did this even work"). And the patch itself looks good. > As for making it into the release, I don't know. The change is small, it > has a very limited scope, but this flaw is also not really hurting > anything in a major way. I could go either way. This counts as something small and obvious enough that I'd consider slipping it in at the last minute if it were fixing a bad bug. But given how minor the bug is, being conservative makes sense to me, if only because it's good to exercise our release discipline muscles. :) -Peff
On 1/10/2020 5:37 AM, Jeff King wrote: > On Fri, Jan 10, 2020 at 01:59:30AM +0000, Derrick Stolee via GitGitGadget wrote: >> >> As for making it into the release, I don't know. The change is small, it >> has a very limited scope, but this flaw is also not really hurting >> anything in a major way. > > I could go either way. > > This counts as something small and obvious enough that I'd consider > slipping it in at the last minute if it were fixing a bad bug. But given > how minor the bug is, being conservative makes sense to me, if only > because it's good to exercise our release discipline muscles. :) Perhaps this should be an example for future release windows. Thanks, -Stolee
On 1/10/2020 6:24 AM, Derrick Stolee wrote: > On 1/10/2020 5:37 AM, Jeff King wrote: >> On Fri, Jan 10, 2020 at 01:59:30AM +0000, Derrick Stolee via GitGitGadget wrote: >>> >>> As for making it into the release, I don't know. The change is small, it >>> has a very limited scope, but this flaw is also not really hurting >>> anything in a major way. >> >> I could go either way. >> >> This counts as something small and obvious enough that I'd consider >> slipping it in at the last minute if it were fixing a bad bug. But given >> how minor the bug is, being conservative makes sense to me, if only >> because it's good to exercise our release discipline muscles. :) > > Perhaps this should be an example for future release windows. (Forgive me for fat-fingering and accidentally using "gmx.net" instead of "gmx.de" for Johannes' email address. Fixed in this message.) -Stolee
Jeff King <peff@peff.net> writes: > On Fri, Jan 10, 2020 at 01:59:30AM +0000, Derrick Stolee via GitGitGadget wrote: > >> Here is a very small fix to the cone-mode pattern-matching in >> unpack-trees.c. Johannes found this while running a Coverity scan for >> other issues. He alerted me to the problem and I could immediately see >> my error here. In creating this patch, most of my time was spent asking >> "how did this work before?" and "why didn't this hurt performance?" >> Hopefully my commit message explains this thoroughly enough. > > Yes, it makes perfect sense (and as soon as I saw the explanation of the > problem, my immediate response was also "wait, how did this even work"). > > And the patch itself looks good. > >> As for making it into the release, I don't know. The change is small, it >> has a very limited scope, but this flaw is also not really hurting >> anything in a major way. > > I could go either way. > > This counts as something small and obvious enough that I'd consider > slipping it in at the last minute if it were fixing a bad bug. But given > how minor the bug is, being conservative makes sense to me, if only > because it's good to exercise our release discipline muscles. :) Heh. On one hand, it is obvious, even to a mindless compiler, that the author meant to count how many elements of cache[] array have been processed in the loop, so it is clear that the patch makes the code reflect the author's original intention better. On the other hand, the code that reflects the author's original intention has never been tested in the field---it could be possible that the author thought cache[0] thru cache_end[0] have been processed, but for some subtlety, only a very early part of the range was correctly processed, and returning smaller range may have been hiding that subtle bug ;-) I do not see any such subtle bug in this particular case, so I am somewhat tempted to say "it is clear that the 'fix' makes the code do what the author wanted to do, *and* what the author wanted to do seems sane, so let's apply it". Having said that, the earlier part of the patch, i.e. if (pl->use_cone_patterns && orig_ret == MATCHED_RECURSIVE) { struct cache_entry **ce = cache; - rc = (cache_end - cache) / sizeof(struct cache_entry *); + rc = cache_end - cache; while (ce < cache_end) { (*ce)->ce_flags &= ~clear_mask; ce++; } wouldn't have been needed any fix if it were actually *counting*, which is what clear_ce_flags_dir() promises its callers it does, instead of cheating with a subtraction. i.e. if (... RECURSIVE) { rc = 0; while (ce < cache_end) { clear; ce++; rc++; } } and that may have been more future-proof way, in that the body of the while loop may in the future decide to leave early etc. and actually counting how far the processing progressed would be less error prone. ALso, the computation of cache_end earlier in the function looks suspiciously similar to the code Emily recently touched to avoid segfaulting against inconsistent index state, where it used the entry count in the cache_tree structure (when it is valid) to determine how many entries to skip over. We may want to see if we can apply the same optimization here. Thanks. Will queue but will not merge to 'master' ;-)
diff --git a/unpack-trees.c b/unpack-trees.c index 2399b6818b..3dba7f9f09 100644 --- a/unpack-trees.c +++ b/unpack-trees.c @@ -1305,14 +1305,14 @@ static int clear_ce_flags_dir(struct index_state *istate, if (pl->use_cone_patterns && orig_ret == MATCHED_RECURSIVE) { struct cache_entry **ce = cache; - rc = (cache_end - cache) / sizeof(struct cache_entry *); + rc = cache_end - cache; while (ce < cache_end) { (*ce)->ce_flags &= ~clear_mask; ce++; } } else if (pl->use_cone_patterns && orig_ret == NOT_MATCHED) { - rc = (cache_end - cache) / sizeof(struct cache_entry *); + rc = cache_end - cache; } else { rc = clear_ce_flags_1(istate, cache, cache_end - cache, prefix,
The clear_ce_flags_dir() method processes the cache entries within a common directory. The returned int is the number of cache entries processed by that directory. When using the sparse-checkout feature in cone mode, we can skip the pattern matching for entries in the directories that are entirely included or entirely excluded. eb42feca (unpack-trees: hash less in cone mode, 2019-11-21) introduced this performance feature. The old mechanism relied on the counts returned by calling clear_ce_flags_1(), but the new mechanism calculated the number of rows by subtracting "cache_end" from "cache" to find the size of the range. However, the equation is wrong because it divides by sizeof(struct cache_entry *). This is not how pointer arithmetic works! A coverity build of Git for Windows in preparation for the 2.25.0 release found this issue with the warning, "Pointer differences, such as cache_end - cache, are automatically scaled down by the size (8 bytes) of the pointed-to type (struct cache_entry *). Most likely, the division by sizeof(struct cache_entry *) is extraneous and should be eliminated." This warning is correct. This leaves us with the question "how did this even work?" The problem that occurs with this incorrect pointer arithmetic is a performance-only bug, and a very slight one at that. Since the entry count returned by clear_ce_flags_dir() is reduced by a factor of 8, the loop in clear_ce_flags_1() will re-process entries from those directories. By inserting global counters into unpack-tree.c and tracing them with trace2_data_intmax() (in a private change, for testing), I was able to see count how many times the loop inside clear_ce_flags_1() processed an entry and how many times clear_ce_flags_dir() was called. Each of these are reduced by at least a factor of 8 with the current change. A factor larger than 8 happens when multiple levels of directories are repeated. Specifically, in the Linux kernel repo, the command git sparse-checkout set LICENSES restricts the working directory to only the files at root and in the LICENSES directory. Here are the measured counts: clear_ce_flags_1 loop blocks: Before: 11,520 After: 1,621 clear_ce_flags_dir calls: Before: 7,048 After: 606 While these are dramatic counts, the time spent in clear_ce_flags_1() is under one millisecond in each case, so the improvement is not measurable as an end-to-end time. Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> --- unpack-trees: correctly compute result count Here is a very small fix to the cone-mode pattern-matching in unpack-trees.c. Johannes found this while running a Coverity scan for other issues. He alerted me to the problem and I could immediately see my error here. In creating this patch, most of my time was spent asking "how did this work before?" and "why didn't this hurt performance?" Hopefully my commit message explains this thoroughly enough. As for making it into the release, I don't know. The change is small, it has a very limited scope, but this flaw is also not really hurting anything in a major way. Thanks, -Stolee Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-520%2Fderrickstolee%2Funpack-trees-division-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-520/derrickstolee/unpack-trees-division-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/520 unpack-trees.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) base-commit: 7a6a90c6ec48fc78c83d7090d6c1b95d8f3739c0