Hello again, This is the second version my patch series that refactors the reftable compaction strategy to instead follow a geometric sequence. Changes compared to v1: * Added GIT_TEST_REFTABLE_NO_AUTOCOMPACTION environment variable to disable reftable compaction when testing. * Refactored worktree tests in t0610-reftable-basics.sh to properly assert git-pack-refs(1) works as expected. * Added test to validate that alternating table sizes are compacted. * Added benchmark to compare compaction strategies. * Moved change that made compaction segment end inclusive to its own commit. * Added additional explanation in commits and comments and fixed typos. Thanks for taking a look! Justin Justin Tobler (3): reftable/stack: add env to disable autocompaction reftable/stack: use geometric table compaction reftable/segment: make segment end inclusive reftable/stack.c | 113 ++++++++++++++++--------------------- reftable/stack.h | 3 - reftable/stack_test.c | 66 +++++----------------- reftable/system.h | 1 + t/t0610-reftable-basics.sh | 43 +++++++++----- 5 files changed, 94 insertions(+), 132 deletions(-) base-commit: 3bd955d26919e149552f34aacf8a4e6368c26cec Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1683%2Fjltobler%2Fjt%2Freftable-geometric-compaction-v2 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1683/jltobler/jt/reftable-geometric-compaction-v2 Pull-Request: https://github.com/gitgitgadget/git/pull/1683 Range-diff vs v1: -: ----------- > 1: cb6b152e5c8 reftable/stack: add env to disable autocompaction 1: 7a518853a10 ! 2: def70084523 reftable/stack: use geometric table compaction @@ Commit message occurring until a separate operation produces a table matching the previous table log value. - To avoid unbounded growth of the table list, walk through each table and - evaluate if it needs to be included in the compaction segment to restore - a geometric sequence. + Instead, to avoid unbounded growth of the table list, the compaction + strategy is updated to ensure tables follow a geometric sequence after + each operation. This is done by walking the table list in reverse index + order to identify the compaction segment start and end. The compaction + segment end is found by identifying the first table which has a + preceding table size less than twice the current table. Next, the + compaction segment start is found iterating through the remaining tables + in the list checking if the previous table size is less than twice the + cumulative of tables from the segment end. This ensures the correct + segment start is found and that the newly compacted table does not + violate the geometric sequence. + + When creating 10 thousand references, the new strategy has no + performance impact: + + Benchmark 1: update-ref: create refs sequentially (revision = HEAD~) + Time (mean ± σ): 26.516 s ± 0.047 s [User: 17.864 s, System: 8.491 s] + Range (min … max): 26.447 s … 26.569 s 10 runs + + Benchmark 2: update-ref: create refs sequentially (revision = HEAD) + Time (mean ± σ): 26.417 s ± 0.028 s [User: 17.738 s, System: 8.500 s] + Range (min … max): 26.366 s … 26.444 s 10 runs + + Summary + update-ref: create refs sequentially (revision = HEAD) ran + 1.00 ± 0.00 times faster than update-ref: create refs sequentially (revision = HEAD~) Some tests in `t0610-reftable-basics.sh` assert the on-disk state of tables and are therefore updated to specify the correct new table count. @@ reftable/stack.c: static int segment_size(struct segment *s) + * until a valid segment end is found. If the preceding table is smaller + * than the current table multiplied by the geometric factor (2), the + * current table is set as the compaction segment end. ++ * ++ * Tables after the ending point are not added to the byte count because ++ * they are already valid members of the geometric sequence. Due to the ++ * properties of a geometric sequence, it is not possible for the sum of ++ * these tables to exceed the value of the ending point table. + */ + for (i = n - 1; i > 0; i--) { + if (sizes[i - 1] < sizes[i] * 2) { -+ seg.end = i; ++ seg.end = i + 1; + bytes = sizes[i]; break; + } @@ reftable/stack.c: static int segment_size(struct segment *s) + + /* + * Find the starting table of the compaction segment by iterating -+ * through the remaing tables and keeping track of the accumulated size -+ * of all tables seen from the segment end table. ++ * through the remaining tables and keeping track of the accumulated ++ * size of all tables seen from the segment end table. + * + * Note that we keep iterating even after we have found the first -+ * first starting point. This is because there may be tables in the -+ * stack preceding that first starting point which violate the geometric ++ * starting point. This is because there may be tables in the stack ++ * preceding that first starting point which violate the geometric + * sequence. + */ + for (; i > 0; i--) { @@ reftable/stack.c: static int segment_size(struct segment *s) } static uint64_t *stack_table_sizes_for_compaction(struct reftable_stack *st) -@@ reftable/stack.c: int reftable_stack_auto_compact(struct reftable_stack *st) - suggest_compaction_segment(sizes, st->merged->stack_len); - reftable_free(sizes); - if (segment_size(&seg) > 0) -- return stack_compact_range_stats(st, seg.start, seg.end - 1, -+ return stack_compact_range_stats(st, seg.start, seg.end, - NULL); - - return 0; ## reftable/stack.h ## @@ reftable/stack.h: int read_lines(const char *filename, char ***lines); @@ reftable/stack_test.c: static void test_reftable_stack_hash_id(void) - EXPECT(min.start == 2); - EXPECT(min.end == 7); + EXPECT(min.start == 1); -+ EXPECT(min.end == 9); ++ EXPECT(min.end == 10); } static void test_suggest_compaction_segment_nothing(void) @@ t/t0610-reftable-basics.sh: test_expect_success 'ref transaction: writes cause a test_commit -C repo --no-tag B && test_line_count = 1 repo/.git/reftable/tables.list + ' + ++test_expect_success 'ref transaction: alternating table sizes are compacted' ' ++ test_when_finished "rm -rf repo" && ++ git init repo && ++ test_commit -C repo A && ++ for i in $(test_seq 20) ++ do ++ git -C repo branch -f foo && ++ git -C repo branch -d foo || return 1 ++ done && ++ test_line_count = 2 repo/.git/reftable/tables.list ++' ++ + check_fsync_events () { + local trace="$1" && + shift && @@ t/t0610-reftable-basics.sh: test_expect_success 'ref transaction: writes are synced' ' git -C repo -c core.fsync=reference \ -c core.fsyncMethod=fsync update-ref refs/heads/branch HEAD && @@ t/t0610-reftable-basics.sh: do git -C repo pack-refs && test_expect_perms "-rw-rw-r--" repo/.git/reftable/tables.list && @@ t/t0610-reftable-basics.sh: test_expect_success 'worktree: pack-refs in main repo packs main refs' ' + test_when_finished "rm -rf repo worktree" && + git init repo && test_commit -C repo A && - git -C repo worktree add ../worktree && +- git -C repo worktree add ../worktree && ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C repo worktree add ../worktree && ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C worktree update-ref refs/worktree/per-worktree HEAD && - test_line_count = 3 repo/.git/worktrees/worktree/reftable/tables.list && - test_line_count = 4 repo/.git/reftable/tables.list && -+ test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && -+ test_line_count = 1 repo/.git/reftable/tables.list && ++ test_line_count = 4 repo/.git/worktrees/worktree/reftable/tables.list && ++ test_line_count = 3 repo/.git/reftable/tables.list && git -C repo pack-refs && - test_line_count = 3 repo/.git/worktrees/worktree/reftable/tables.list && -+ test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && ++ test_line_count = 4 repo/.git/worktrees/worktree/reftable/tables.list && test_line_count = 1 repo/.git/reftable/tables.list ' @@ t/t0610-reftable-basics.sh: test_expect_success 'worktree: pack-refs in worktree packs worktree refs' ' + test_when_finished "rm -rf repo worktree" && + git init repo && test_commit -C repo A && - git -C repo worktree add ../worktree && +- git -C repo worktree add ../worktree && ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C repo worktree add ../worktree && ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C worktree update-ref refs/worktree/per-worktree HEAD && - test_line_count = 3 repo/.git/worktrees/worktree/reftable/tables.list && - test_line_count = 4 repo/.git/reftable/tables.list && -+ test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && -+ test_line_count = 1 repo/.git/reftable/tables.list && ++ test_line_count = 4 repo/.git/worktrees/worktree/reftable/tables.list && ++ test_line_count = 3 repo/.git/reftable/tables.list && git -C worktree pack-refs && test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && - test_line_count = 4 repo/.git/reftable/tables.list -+ test_line_count = 1 repo/.git/reftable/tables.list ++ test_line_count = 3 repo/.git/reftable/tables.list ' test_expect_success 'worktree: creating shared ref updates main stack' ' + test_when_finished "rm -rf repo worktree" && + git init repo && + test_commit -C repo A && ++ test_commit -C repo B && + + git -C repo worktree add ../worktree && + git -C repo pack-refs && @@ t/t0610-reftable-basics.sh: test_expect_success 'worktree: creating shared ref updates main stack' ' + test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && + test_line_count = 1 repo/.git/reftable/tables.list && - git -C worktree update-ref refs/heads/shared HEAD && +- git -C worktree update-ref refs/heads/shared HEAD && ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C worktree update-ref refs/heads/shared HEAD && test_line_count = 1 repo/.git/worktrees/worktree/reftable/tables.list && -- test_line_count = 2 repo/.git/reftable/tables.list -+ test_line_count = 1 repo/.git/reftable/tables.list + test_line_count = 2 repo/.git/reftable/tables.list ' - - test_expect_success 'worktree: creating per-worktree ref updates worktree stack' ' -: ----------- > 3: a23e3fc6972 reftable/segment: make segment end inclusive -- gitgitgadget
