| Message ID | pull.845.git.1614484707.gitgitgadget@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Optimization batch 9: avoid detecting irrelevant renames | expand |
On Sat, Feb 27, 2021 at 7:58 PM Elijah Newren via GitGitGadget <gitgitgadget@gmail.com> wrote: > > This series depends textually on ort-perf-batch-8, but semantically it's > almost completely unrelated and can be reviewed without any familiarity with > any of the previous patch series. > > === Basic Optimization idea === > > This series determines paths which meet special cases where detection of > renames is irrelevant, where the irrelevance is due to the fact that the > merge machinery will arrive at the same result regardless of whether a > rename is detected for any of those paths. This series represents > "Optimization #2" from my Git Merge 2020 talk[1], though this series has > some improvements on the optimization relative to what I had at that time. > > The basic idea here is that if side A of history: > > * only modifies/adds/deletes a few files > * adds new files to few if any of the directories that side B deleted or > renamed > > then when we do rename detection on side B we can avoid even looking at most > (and perhaps even all) paths that side B deleted. Since commits being > rebased or cherry-picked tend to only modify a few files, this optimization > tends to be particularly effective for rebases and cherry-picks. > > Basing rename detection on what the other side of history did to a file > means that extra information needs to be fed from merge-ort to > diffcore-rename in order to take advantage of such an optimization. > > === Comparison to previous series === > > This series differs from my two previous optimizations[2][3] (focusing on > basename-guided rename detection) in two important aspects: > > * there are no behavioral changes (there is no heuristic involved) > > * this optimization is merge specific (it does not help the diff/status/log > family of commands, just merge/rebase/cherry-pick and such) > > === Results === > > For the testcases mentioned in commit 557ac0350d ("merge-ort: begin > performance work; instrument with trace2_region_* calls", 2020-10-28), the > changes in just this series improves the performance as follows: > > Before Series After Series > no-renames: 12.596 s ± 0.061 s 5.680 s ± 0.096 s > mega-renames: 130.465 s ± 0.259 s 13.812 s ± 0.162 s > just-one-mega: 3.958 s ± 0.010 s 506.0 ms ± 3.9 ms > > > However, interestingly, if we had ignored the basename-guided rename > detection optimizations[2][3], then this optimization series would have > improved the performance as follows: > > Before Basename Series After Just This Series > no-renames: 13.815 s ± 0.062 s 5.728 s ± 0.104 s > mega-renames: 1799.937 s ± 0.493 s 18.213 s ± 0.139 s > just-one-mega 51.289 s ± 0.019 s 891.9 ms ± 7.0 ms > > > As a reminder, before any merge-ort/diffcore-rename performance work, the > performance results we started with (as noted in the same commit message) > were: > > no-renames-am: 6.940 s ± 0.485 s > no-renames: 18.912 s ± 0.174 s > mega-renames: 5964.031 s ± 10.459 s > just-one-mega: 149.583 s ± 0.751 s > > > === Competition between optimizations === > > We now have three major rename-related optimizations: > > * exact rename detection > * basename-guided rename detection[2][3] > * skip-because-unnecessary (this series) > > It is possible for all three to potentially apply for specific paths (they > do for the majority of renamed paths in our testcases), but we cannot use > more than one for any given path. It turns out that the priority we give > each optimization is very important and can drastically affect performance. > We get best results by prioritizing them as follows: > > 1. exact rename detection > 2. skip-because-unnecessary > 3. basename-guided rename detection > > The third-to-last patch of this series also discusses this ordering and > another minor variant of the skip-because-unnecessary optimization that was > tried (and resulted in less effective performance gains than reported here), > as well as some of the preparatory work over the past few years that this > series relies on in order to enable this optimization. Oops. When I restructured the series I carefully re-read the commit messages to make sure I didn't forget to update one...but I apparently forgot to update the cover letter. The discussion was actually split across a few patches by the refactoring, and what is now the third-to-last patch doesn't contain any of that discussion. > === Near optimal? === > > You may remember that there was a row labelled "everything else" from the > commit message of 557ac0350d that represented the maximum possible speed-up > from accelerating rename detection alone; as stated in that commit, those > rows represented how fast the code could be if we had somehow infinitely > parallelized the inexact rename detection. However, if you compare those > "maximum speedup" numbers to what we have above, you'll note that the > infinitely parallelized inexact rename detection would have been slightly > slower than the results we have now achieved. (The reason this is possible, > despite the fact that we still spend time in rename detection after our > optimizations, is because we implemented two optimizations outside of > diffcore_rename() along the way.) However, this good news does also come > with a downside -- it means that our remaining optimization potential is > somewhat limited, and subsequent optimization series will have to fight for > much smaller gains. > > [1] > https://github.com/newren/presentations/blob/pdfs/merge-performance/merge-performance-slides.pdf > [2] > https://lore.kernel.org/git/pull.843.git.1612651937.gitgitgadget@gmail.com/ > [3] > https://lore.kernel.org/git/pull.844.git.1613289544.gitgitgadget@gmail.com/ > > Elijah Newren (8): > diffcore-rename: enable filtering possible rename sources > merge-ort: precompute subset of sources for which we need rename > detection > merge-ort: add data structures for an alternate tree traversal > merge-ort: introduce wrappers for alternate tree traversal > merge-ort: precompute whether directory rename detection is needed > merge-ort: use relevant_sources to filter possible rename sources > merge-ort: skip rename detection entirely if possible > diffcore-rename: avoid doing basename comparisons for irrelevant > sources > > diffcore-rename.c | 63 ++++++++++--- > diffcore.h | 1 + > merge-ort.c | 236 +++++++++++++++++++++++++++++++++++++++++++++- > 3 files changed, 285 insertions(+), 15 deletions(-) > > > base-commit: 4be565c472088d4144063b736308bf2a57331f45 > Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-845%2Fnewren%2Fort-perf-batch-9-v1 > Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-845/newren/ort-perf-batch-9-v1 > Pull-Request: https://github.com/gitgitgadget/git/pull/845 > -- > gitgitgadget
Hi, On Sat, Feb 27, 2021 at 7:58 PM Elijah Newren via GitGitGadget <gitgitgadget@gmail.com> wrote: > > This series depends textually on ort-perf-batch-8, but semantically it's > almost completely unrelated and can be reviewed without any familiarity with > any of the previous patch series. > > === Basic Optimization idea === > > The basic idea here is that if side A of history: > > * only modifies/adds/deletes a few files > * adds new files to few if any of the directories that side B deleted or > renamed > > then when we do rename detection on side B we can avoid even looking at most > (and perhaps even all) paths that side B deleted. Since commits being > rebased or cherry-picked tend to only modify a few files, this optimization > tends to be particularly effective for rebases and cherry-picks. After thinking it over a bit more, there's a much better way to put this summary: We only need expensive rename detection on the subset of files changed on *both* sides (for the most part). This is because: 1. The primary reason for rename detection in merges is enabling three-way content merges 2. The purpose of three-way content merges is reconciling changes when *both* sides of history modified some file 3. If a file was only modified by the side that renamed the file, then detecting the rename is irrelevant; we'll get the same answer without knowing about the rename. 4. (Well...there are rare cases where we need the rename for reasons other than three-way content merges. Patch 5 explains those.)
This series depends textually on ort-perf-batch-8, but semantically it's almost completely unrelated and can be reviewed without any familiarity with any of the previous patch series. === Basic Optimization idea === This series determines paths which meet special cases where detection of renames is irrelevant, where the irrelevance is due to the fact that the merge machinery will arrive at the same result regardless of whether a rename is detected for any of those paths. This series represents "Optimization #2" from my Git Merge 2020 talk[1], though this series has some improvements on the optimization relative to what I had at that time. The basic idea here is that if side A of history: * only modifies/adds/deletes a few files * adds new files to few if any of the directories that side B deleted or renamed then when we do rename detection on side B we can avoid even looking at most (and perhaps even all) paths that side B deleted. Since commits being rebased or cherry-picked tend to only modify a few files, this optimization tends to be particularly effective for rebases and cherry-picks. Basing rename detection on what the other side of history did to a file means that extra information needs to be fed from merge-ort to diffcore-rename in order to take advantage of such an optimization. === Comparison to previous series === This series differs from my two previous optimizations[2][3] (focusing on basename-guided rename detection) in two important aspects: * there are no behavioral changes (there is no heuristic involved) * this optimization is merge specific (it does not help the diff/status/log family of commands, just merge/rebase/cherry-pick and such) === Results === For the testcases mentioned in commit 557ac0350d ("merge-ort: begin performance work; instrument with trace2_region_* calls", 2020-10-28), the changes in just this series improves the performance as follows: Before Series After Series no-renames: 12.596 s ± 0.061 s 5.680 s ± 0.096 s mega-renames: 130.465 s ± 0.259 s 13.812 s ± 0.162 s just-one-mega: 3.958 s ± 0.010 s 506.0 ms ± 3.9 ms However, interestingly, if we had ignored the basename-guided rename detection optimizations[2][3], then this optimization series would have improved the performance as follows: Before Basename Series After Just This Series no-renames: 13.815 s ± 0.062 s 5.728 s ± 0.104 s mega-renames: 1799.937 s ± 0.493 s 18.213 s ± 0.139 s just-one-mega 51.289 s ± 0.019 s 891.9 ms ± 7.0 ms As a reminder, before any merge-ort/diffcore-rename performance work, the performance results we started with (as noted in the same commit message) were: no-renames-am: 6.940 s ± 0.485 s no-renames: 18.912 s ± 0.174 s mega-renames: 5964.031 s ± 10.459 s just-one-mega: 149.583 s ± 0.751 s === Competition between optimizations === We now have three major rename-related optimizations: * exact rename detection * basename-guided rename detection[2][3] * skip-because-unnecessary (this series) It is possible for all three to potentially apply for specific paths (they do for the majority of renamed paths in our testcases), but we cannot use more than one for any given path. It turns out that the priority we give each optimization is very important and can drastically affect performance. We get best results by prioritizing them as follows: 1. exact rename detection 2. skip-because-unnecessary 3. basename-guided rename detection The third-to-last patch of this series also discusses this ordering and another minor variant of the skip-because-unnecessary optimization that was tried (and resulted in less effective performance gains than reported here), as well as some of the preparatory work over the past few years that this series relies on in order to enable this optimization. === Near optimal? === You may remember that there was a row labelled "everything else" from the commit message of 557ac0350d that represented the maximum possible speed-up from accelerating rename detection alone; as stated in that commit, those rows represented how fast the code could be if we had somehow infinitely parallelized the inexact rename detection. However, if you compare those "maximum speedup" numbers to what we have above, you'll note that the infinitely parallelized inexact rename detection would have been slightly slower than the results we have now achieved. (The reason this is possible, despite the fact that we still spend time in rename detection after our optimizations, is because we implemented two optimizations outside of diffcore_rename() along the way.) However, this good news does also come with a downside -- it means that our remaining optimization potential is somewhat limited, and subsequent optimization series will have to fight for much smaller gains. [1] https://github.com/newren/presentations/blob/pdfs/merge-performance/merge-performance-slides.pdf [2] https://lore.kernel.org/git/pull.843.git.1612651937.gitgitgadget@gmail.com/ [3] https://lore.kernel.org/git/pull.844.git.1613289544.gitgitgadget@gmail.com/ Elijah Newren (8): diffcore-rename: enable filtering possible rename sources merge-ort: precompute subset of sources for which we need rename detection merge-ort: add data structures for an alternate tree traversal merge-ort: introduce wrappers for alternate tree traversal merge-ort: precompute whether directory rename detection is needed merge-ort: use relevant_sources to filter possible rename sources merge-ort: skip rename detection entirely if possible diffcore-rename: avoid doing basename comparisons for irrelevant sources diffcore-rename.c | 63 ++++++++++--- diffcore.h | 1 + merge-ort.c | 236 +++++++++++++++++++++++++++++++++++++++++++++- 3 files changed, 285 insertions(+), 15 deletions(-) base-commit: 4be565c472088d4144063b736308bf2a57331f45 Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-845%2Fnewren%2Fort-perf-batch-9-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-845/newren/ort-perf-batch-9-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/845