On Sun, Nov 24, 2019 at 10:10:21PM +0100, SZEDER Gábor wrote: > To notice a changed todo file the sequencer stores the file's stat > data in its 'struct todo_list' instance, and compares it with the > file's current stat data after 'reword', 'squash' and 'exec' > instructions. If the two stat data doesn't match, it re-reads the > todo file. > > Sounds simple, but there are some subtleties going on here: > > - The 'struct todo_list' holds the stat data from the time when the > todo file was last read. > > - This stat data in 'struct todo_list' is not updated when the > sequencer itself writes the todo file. > > - Before executing each instruction during an interactive rebase, > the sequencer always updates the todo file by removing the > just-about-to-be-executed instruction. This changes the file's > size and inode [1]. > > Consequently, when the sequencer looks at the stat data after a > 'reword', 'squash' or 'exec' instruction, it most likely finds that > they differ, even when the user didn't modify the todo list at all! > This is not an issue in practice, it just wastes a few cycles on > re-reading the todo list that matches what the sequencer already has > in memory anyway. It can be much more than just a few cycles, because the total number of parsed instructions from all the todo file re-reads can go quadratic with the number of rebased commits. The simple test below runs 'git rebase -i -x' on 1000 commits, which takes over 14seconds to run. If it doesn't re-read the todo file at all (I simply deleted the whole condition block checking the stat data and re-reading) it runs for only ~2.5secs. Just another angle to consider... --- >8 --- test_expect_success 'test' ' num_commits=1000 && test_commit_bulk --filename=file $num_commits && /usr/bin/time -f "Elapsed time: %e" \ git rebase -i --root -x true 2>out && grep "Executing: true" out >actual && test_line_count = $num_commits actual && # show the elapsed time tail -n2 out ' --- >8 ---
