* [GSOC] Blog about weeks 4, 5 [not found] <20200706182213.GA51227@Abhishek-Arch> @ 2020-07-07 2:24 ` Abhishek Kumar 2020-07-13 20:00 ` Jakub Narębski 0 siblings, 1 reply; 3+ messages in thread From: Abhishek Kumar @ 2020-07-07 2:24 UTC (permalink / raw) To: abhishekkumar8222; +Cc: git, stolee, jnareb Hello everyone! Over the last two weeks, I have worked on refining the performance report on generation numbers. Here are our conclusions: - Corrected Commit Dates With Monotonically Offset (i.e. generation number v5) performs better than topological levels but is still walks too many commits when compared with Corrected Commit Dates. Number of commits walked (git merge-base v4.8 v4.9, on linux repository): Topological Level : 635579 Corrected Commit Date : 167468 Corrected Commit Date With Monotonic Offset: 506577 As such, I am expecting that we will store Corrected Commit Date in an additional chunk (called "generation data chunk") and store topological levels into CDAT. Thus, old Git clients can operate as expected, with new Git clients using the better generation number. - Using a new chunk does affect the locality of reference but did not impact the performance appreciably. - This does increase the size of commit graph file by nearly 5%. You can read more in my report [1] and the pull request with instructions to replicate the results [2]. [1]: https://lore.kernel.org/git/20200703082842.GA28027@Abhishek-Arch/T/#mda33f6e13873df55901768e8fd6d774282002146 [2]: https://github.com/abhishekkumar2718/git/pull/1 I talk a bit more about a patch I worked on, trying to improve performance of commit graph write using buffers which ultimately did not work and is dropped. Up next is actually implementing the generation number and take care of all little details. https://abhishekkumar2718.github.io/programming/2020/07/05/gsoc-weeks-4-5.html Feedback and suggestions welcome! Thanks - Abhishek -------- Re-sending this email as I forgot to cc git@vger.kernel.org ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [GSOC] Blog about weeks 4, 5 2020-07-07 2:24 ` [GSOC] Blog about weeks 4, 5 Abhishek Kumar @ 2020-07-13 20:00 ` Jakub Narębski 2020-07-14 6:23 ` Abhishek Kumar 0 siblings, 1 reply; 3+ messages in thread From: Jakub Narębski @ 2020-07-13 20:00 UTC (permalink / raw) To: Abhishek Kumar; +Cc: git, Derrick Stolee Abhishek Kumar <abhishekkumar8222@gmail.com> writes: > Hello everyone! > > Over the last two weeks, I have worked on refining the performance > report on generation numbers. Here are our conclusions: > > - Corrected Commit Dates With Monotonically Offset (i.e. generation > number v5) performs better than topological levels but is still walks > too many commits when compared with Corrected Commit Dates. Thank you for your work examining different approaches to introducing generation number v2. > Number of commits walked (git merge-base v4.8 v4.9, on linux repository): > > Topological Level : 635579 > Corrected Commit Date : 167468 > Corrected Commit Date With Monotonic Offset: 506577 It is a bit strange that requiring monotonic offsets leads to so much of a difference in performance (in commits walked). > > As such, I am expecting that we will store Corrected Commit Date in an > additional chunk (called "generation data chunk") and store topological > levels into CDAT. Thus, old Git clients can operate as expected, with > new Git clients using the better generation number. > > - Using a new chunk does affect the locality of reference but did not > impact the performance appreciably. > - This does increase the size of commit graph file by nearly 5%. All right, it seems like it is the way to go. > You can read more in my report [1] and the pull request with > instructions to replicate the results [2]. > > [1]: https://lore.kernel.org/git/20200703082842.GA28027@Abhishek-Arch/T/#mda33f6e13873df55901768e8fd6d774282002146 > [2]: https://github.com/abhishekkumar2718/git/pull/1 > > I talk a bit more about a patch I worked on, trying to improve > performance of commit graph write using buffers which ultimately did not > work and is dropped. Up next is actually implementing the generation > number and take care of all little details. > > https://abhishekkumar2718.github.io/programming/2020/07/05/gsoc-weeks-4-5.html > > Feedback and suggestions welcome! Some comments about the blog entry contents: AK> Dr. Stolee pointed out ... [to] use the number of commits as a AK> metric instead of wall clock timing (which can be influenced by other AK> factors like CPU usage at the time). There are a few factors. If we compare similar algorithms, that might be a good decision. First, one can try to reduce the influence of random factors on the wall clock timing by using statistics. For example one can try to detect and remove outliers by using robust statistics measures to detect them, like tools like for example Dumbbench [3], hyperfine [4] or bench [5]. After warmup, one approach is to compute the robust estimate of value, e.g. median, and robust estimate of dispersion, e.g. MAD = median absolute deviation, and use those to detect outliers, e.g. rescale MAD and mark as outlier and remove entries that are more than "three sigma" of robust dispersion away from robust estimate of value. Dumbbench [3] has good explanation. [3]: https://metacpan.org/pod/Dumbbench#HOW-IT-WORKS-AND-WHY-IT-DOESN'T [4]: https://github.com/sharkdp/hyperfine [5]: https://github.com/Gabriel439/bench Second, because of pecularities of current processor architecture (caches, data prefetching, branch prediction) performing more operations might in admittedly rare cases be faster than doing less operations. One such example can be found in the CppCon 2019 talk by Andrei Alexandrescu "Speed Is Found In The Minds of People" [6][7] about 'small sort', where doing more operations results in, on average, faster sort. This of course has a possibility to happen only if difference with the number of operations is small enough... nevertheless it might be a good idea to at least check that the wall clock time agrees with conclusions from the number of commits walked, for at least a few examples. [6]: https://www.youtube.com/watch?v=FJJTYQYB1JQ [7]: https://github.com/CppCon/CppCon2019/blob/master/Presentations/speed_is_found_in_the_minds_of_people/speed_is_found_in_the_minds_of_people__andrei_alexandrescu__cppcon_2019.pdf AK> With the second report, storing corrected commit date in GDAT as AK> well as computing topological levels seems like a no-brainer. I have AK> started working on the patch and will push to the mailing list after AK> some discussion on the report. Do you have any numbers how much does providing backward compatibility cost at `git commit-graph write`, that is how much more time it takes to computer topological levels during computation of corrected committerdate compared to storing GENERATION_NUMBER_MAX in place of topological level, and whether having topological level (as tie-breaker) helps with Git performance when using commit-graphh for querying? Does having topological levels as tie-breaker or secondary negative-cut reachability index helps at all? Thank you for your work and for the report. P.S. Would it be possible to put GSoC entries into separate 'GSoC' category instead of generic 'Programming' one, or add a 'GSoC' tag? Best, -- Jakub Narębski ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [GSOC] Blog about weeks 4, 5 2020-07-13 20:00 ` Jakub Narębski @ 2020-07-14 6:23 ` Abhishek Kumar 0 siblings, 0 replies; 3+ messages in thread From: Abhishek Kumar @ 2020-07-14 6:23 UTC (permalink / raw) To: Jakub Narębski; +Cc: git, stolee, abhishekkumar8222 On Mon, Jul 13, 2020 at 10:00:03PM +0200, Jakub Narębski wrote: > Abhishek Kumar <abhishekkumar8222@gmail.com> writes: > > > Hello everyone! > > > > Over the last two weeks, I have worked on refining the performance > > report on generation numbers. Here are our conclusions: > > > > - Corrected Commit Dates With Monotonically Offset (i.e. generation > > number v5) performs better than topological levels but is still walks > > too many commits when compared with Corrected Commit Dates. > > Thank you for your work examining different approaches to introducing > generation number v2. > > > Number of commits walked (git merge-base v4.8 v4.9, on linux repository): > > > > Topological Level : 635579 > > Corrected Commit Date : 167468 > > Corrected Commit Date With Monotonic Offset: 506577 > > It is a bit strange that requiring monotonic offsets leads to so much > of a difference in performance (in commits walked). > > > > > As such, I am expecting that we will store Corrected Commit Date in an > > additional chunk (called "generation data chunk") and store topological > > levels into CDAT. Thus, old Git clients can operate as expected, with > > new Git clients using the better generation number. > > > > - Using a new chunk does affect the locality of reference but did not > > impact the performance appreciably. > > - This does increase the size of commit graph file by nearly 5%. > > All right, it seems like it is the way to go. > > > You can read more in my report [1] and the pull request with > > instructions to replicate the results [2]. > > > > [1]: https://lore.kernel.org/git/20200703082842.GA28027@Abhishek-Arch/T/#mda33f6e13873df55901768e8fd6d774282002146 > > [2]: https://github.com/abhishekkumar2718/git/pull/1 > > > > I talk a bit more about a patch I worked on, trying to improve > > performance of commit graph write using buffers which ultimately did not > > work and is dropped. Up next is actually implementing the generation > > number and take care of all little details. > > > > https://abhishekkumar2718.github.io/programming/2020/07/05/gsoc-weeks-4-5.html > > > > Feedback and suggestions welcome! > > Some comments about the blog entry contents: > > AK> Dr. Stolee pointed out ... [to] use the number of commits as a > AK> metric instead of wall clock timing (which can be influenced by other > AK> factors like CPU usage at the time). > > There are a few factors. If we compare similar algorithms, that might > be a good decision. > > First, one can try to reduce the influence of random factors on the wall > clock timing by using statistics. For example one can try to detect and > remove outliers by using robust statistics measures to detect them, like > tools like for example Dumbbench [3], hyperfine [4] or bench [5]. After > warmup, one approach is to compute the robust estimate of value, e.g. > median, and robust estimate of dispersion, e.g. MAD = median absolute > deviation, and use those to detect outliers, e.g. rescale MAD and mark > as outlier and remove entries that are more than "three sigma" of robust > dispersion away from robust estimate of value. Dumbbench [3] has good > explanation. > > [3]: https://metacpan.org/pod/Dumbbench#HOW-IT-WORKS-AND-WHY-IT-DOESN'T > [4]: https://github.com/sharkdp/hyperfine > [5]: https://github.com/Gabriel439/bench That's interesting. When you think about it, medians are a better measure than average because medians are robust to the outliers. > > Second, because of pecularities of current processor architecture > (caches, data prefetching, branch prediction) performing more operations > might in admittedly rare cases be faster than doing less operations. One > such example can be found in the CppCon 2019 talk by Andrei Alexandrescu > "Speed Is Found In The Minds of People" [6][7] about 'small sort', where > doing more operations results in, on average, faster sort. This of > course has a possibility to happen only if difference with the number of > operations is small enough... nevertheless it might be a good idea to at > least check that the wall clock time agrees with conclusions from the > number of commits walked, for at least a few examples. > > [6]: https://www.youtube.com/watch?v=FJJTYQYB1JQ > [7]: https://github.com/CppCon/CppCon2019/blob/master/Presentations/speed_is_found_in_the_minds_of_people/speed_is_found_in_the_minds_of_people__andrei_alexandrescu__cppcon_2019.pdf > > AK> With the second report, storing corrected commit date in GDAT as > AK> well as computing topological levels seems like a no-brainer. I have > AK> started working on the patch and will push to the mailing list after > AK> some discussion on the report. > > Do you have any numbers how much does providing backward compatibility > cost at `git commit-graph write`, that is how much more time it takes to > computer topological levels during computation of corrected > committerdate compared to storing GENERATION_NUMBER_MAX in place of > topological level, and whether having topological level (as tie-breaker) > helps with Git performance when using commit-graphh for querying? Does > having topological levels as tie-breaker or secondary negative-cut > reachability index helps at all? > We do have timings comparing the time to compute topological levels as compared to storing GENERATION_NUMBER_MAX in place [1]: Writing GENERATION_NUMBER_MAX to commit-graph: 14.175s Writing topological levels to commit-graph: 14.331s That's around 160ms and 1% percent faster. I do think there's a case to be made for GENERATION_NUMBER_MAX because the performance degradation for old Git would help in faster adoption (Junio was in favor of this, the last time we discussed alternatives [2]). It is a double-edged sword as we force people who cannot upgrade git into worse performance. I do not have anything for using topological level as a tie-breaker. Will benchmark and get back to you. [1]: https://lore.kernel.org/git/20200630150056.GA4111@Abhishek-Arch/ [2]: https://lore.kernel.org/git/xmqq8sjp1mnz.fsf@gitster.c.googlers.com/ > > Thank you for your work and for the report. > > P.S. Would it be possible to put GSoC entries into separate 'GSoC' > category instead of generic 'Programming' one, or add a 'GSoC' tag? > Great idea! Try this out: https://abhishekkumar2718.github.io/gsoc/ > Best, > -- > Jakub Narębski ^ permalink raw reply [flat|nested] 3+ messages in thread
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