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[84.236.109.79]) by smtp.gmail.com with ESMTPSA id f11sm24484684ejd.40.2021.01.05.01.47.41 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 05 Jan 2021 01:47:41 -0800 (PST) Date: Tue, 5 Jan 2021 10:47:40 +0100 From: SZEDER =?utf-8?B?R8OhYm9y?= To: Abhishek Kumar via GitGitGadget Cc: git@vger.kernel.org, Derrick Stolee , Jakub =?utf-8?B?TmFyxJlic2tp?= , Taylor Blau , Abhishek Kumar Subject: Re: [PATCH v5 01/11] commit-graph: fix regression when computing Bloom filters Message-ID: <20210105094740.GO8396@szeder.dev> References: <20210105094535.GN8396@szeder.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210105094535.GN8396@szeder.dev> User-Agent: Mutt/1.5.24 (2015-08-30) Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Tue, Jan 05, 2021 at 10:45:35AM +0100, SZEDER Gábor wrote: > > But since c49c82aa4c (commit: move members graph_pos, generation to a > > slab, 2020-06-17), this optimization is broken, since asking for a > > 'commit_graph_generation()' directly returns GENERATION_NUMBER_INFINITY > > while writing. > > I wouldn't say that c49c82aa4c broke this optimisation, because: > > did not break that optimization. Though, sadly, it's not > mentioned in 3d11275505's commit message, when commit_gen_cmp() > compares two commits with identical generation numbers, then it > doesn't leave them unsorted, but falls back to use their committer > date as a tie-braker. This means that after c49c82aa4c the commits > are sorted by committer date, which appears to be so good a heuristic > for Bloom filter computation that there is barely any slowdown > compared to sorting by generation numbers: Gaah, scratch this paragraph; I first misunderstood what you wrote in the paragraph below, but then forgot to remove it. > > Not all hope is lost, though: 'commit_graph_generation()' falls back to > > You mean commit_gen_cmp() here. > > > comparing commits by their date when they have equal generation number, > > and so since c49c82aa4c is purely a date comparision function. This > > heuristic is good enough that we don't seem to loose appreciable > > performance while computing Bloom filters.