Hello again, This is the third version my patch series that refactors the reftable compaction strategy to instead follow a geometric sequence. Changes compared to v2: * Added test to validate the GIT_TEST_REFTABLE_NO_AUTOCOMPACTION environment variable works as expected. * Added additional clarifying comments and examples to explain how the new compaction strategy works. * Removed outdated comment from stack_test.c test Thanks for taking a look! -Justin Justin Tobler (3): reftable/stack: add env to disable autocompaction reftable/stack: use geometric table compaction reftable/stack: make segment end inclusive reftable/stack.c | 124 ++++++++++++++++++------------------- reftable/stack.h | 3 - reftable/stack_test.c | 67 +++++--------------- reftable/system.h | 1 + t/t0610-reftable-basics.sh | 58 ++++++++++++----- 5 files changed, 120 insertions(+), 133 deletions(-) base-commit: c75fd8d8150afdf836b63a8e0534d9b9e3e111ba Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1683%2Fjltobler%2Fjt%2Freftable-geometric-compaction-v3 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1683/jltobler/jt/reftable-geometric-compaction-v3 Pull-Request: https://github.com/gitgitgadget/git/pull/1683 Range-diff vs v2: 1: cb6b152e5c8 ! 1: 2fdd8ea1133 reftable/stack: add env to disable autocompaction @@ reftable/stack.c: int reftable_addition_commit(struct reftable_addition *add) ## reftable/system.h ## @@ reftable/system.h: license that can be found in the LICENSE file or at - #include "strbuf.h" + #include "tempfile.h" #include "hash-ll.h" /* hash ID, sizes.*/ #include "dir.h" /* remove_dir_recursively, for tests.*/ +#include "parse.h" int hash_size(uint32_t id); + + ## t/t0610-reftable-basics.sh ## +@@ t/t0610-reftable-basics.sh: test_expect_success 'ref transaction: writes cause auto-compaction' ' + test_line_count = 1 repo/.git/reftable/tables.list + ' + ++test_expect_success 'ref transaction: environment variable disables auto-compaction' ' ++ test_when_finished "rm -rf repo" && ++ ++ git init repo && ++ test_commit -C repo A && ++ for i in $(test_seq 20) ++ do ++ GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C repo update-ref branch-$i HEAD || return 1 ++ done && ++ test_line_count = 23 repo/.git/reftable/tables.list && ++ ++ git -C repo update-ref foo HEAD && ++ test_line_count = 1 repo/.git/reftable/tables.list ++' ++ + check_fsync_events () { + local trace="$1" && + shift && 2: def70084523 ! 2: 7e62c2286ae reftable/stack: use geometric table compaction @@ Commit message 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. + each operation by individually evaluating each table in reverse index + order. This strategy results in a much simpler and more robust algorithm + compared to the previous one while also maintaining a minimal ordered + set of tables on-disk. When creating 10 thousand references, the new strategy has no performance impact: @@ reftable/stack.c: static int segment_size(struct segment *s) + * 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. ++ * ++ * Example table size sequence requiring no compaction: ++ * 64, 32, 16, 8, 4, 2, 1 ++ * ++ * Example compaction segment end set to table with size 3: ++ * 64, 32, 16, 8, 4, 3, 1 + */ + for (i = n - 1; i > 0; i--) { + if (sizes[i - 1] < sizes[i] * 2) { @@ reftable/stack.c: static int segment_size(struct segment *s) break; + } + } -+ + +- min_seg.start = prev; +- min_seg.bytes += sizes[prev]; + /* + * Find the starting table of the compaction segment by iterating + * through the remaining tables and keeping track of the accumulated -+ * size of all tables seen from the segment end table. ++ * size of all tables seen from the segment end table. The previous ++ * table is compared to the accumulated size because the tables from the ++ * segment end are merged backwards recursively. + * + * Note that we keep iterating even after we have found the first + * starting point. This is because there may be tables in the stack + * preceding that first starting point which violate the geometric + * sequence. ++ * ++ * Example compaction segment start set to table with size 32: ++ * 128, 32, 16, 8, 4, 3, 1 + */ + for (; i > 0; i--) { + uint64_t curr = bytes; + bytes += sizes[i - 1]; - -- min_seg.start = prev; -- min_seg.bytes += sizes[prev]; ++ + if (sizes[i - 1] < curr * 2) { + seg.start = i - 1; + seg.bytes = bytes; @@ reftable/stack_test.c: static void test_reftable_stack_hash_id(void) static void test_suggest_compaction_segment(void) { - uint64_t sizes[] = { 128, 64, 17, 16, 9, 9, 9, 16, 16 }; +- /* .................0 1 2 3 4 5 6 */ + uint64_t sizes[] = { 512, 64, 17, 16, 9, 9, 9, 16, 2, 16 }; - /* .................0 1 2 3 4 5 6 */ struct segment min = suggest_compaction_segment(sizes, ARRAY_SIZE(sizes)); - EXPECT(min.start == 2); @@ 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 +@@ t/t0610-reftable-basics.sh: test_expect_success 'ref transaction: environment variable disables auto-compact + do + GIT_TEST_REFTABLE_NO_AUTOCOMPACTION=true git -C repo update-ref branch-$i HEAD || return 1 + done && +- test_line_count = 23 repo/.git/reftable/tables.list && ++ test_line_count = 22 repo/.git/reftable/tables.list && + + git -C repo update-ref foo HEAD && + test_line_count = 1 repo/.git/reftable/tables.list ' +test_expect_success 'ref transaction: alternating table sizes are compacted' ' 3: a23e3fc6972 ! 3: 9a33914c852 reftable/segment: make segment end inclusive @@ Metadata Author: Justin Tobler <jltobler@xxxxxxxxx> ## Commit message ## - reftable/segment: make segment end inclusive + reftable/stack: make segment end inclusive For a reftable segment, the start of the range is inclusive and the end is exclusive. In practice we increment the end when creating the -- gitgitgadget
