From f66e0401abaa884aa91100b4c155c4d042c47e0d Mon Sep 17 00:00:00 2001 From: Jeff King Date: Mon, 11 Nov 2019 06:12:49 -0500 Subject: pack-objects: avoid pointless oe_map_new_pack() calls This patch fixes an extreme slowdown in pack-objects when you have more than 1023 packs. See below for numbers. Since 43fa44fa3b (pack-objects: move in_pack out of struct object_entry, 2018-04-14), we use a complicated system to save some per-object memory. Each object_entry structs gets a 10-bit field to store the index of the pack it's in. We map those indices into pointers using packing_data->in_pack_by_idx, which we initialize at the start of the program. If we have 2^10 or more packs, then we instead create an array of pack pointers, one per object. This is packing_data->in_pack. So far so good. But there's one other tricky case: if a new pack arrives after we've initialized in_pack_by_idx, it won't have an index yet. We solve that by calling oe_map_new_pack(), which just switches on the fly to the less-optimal in_pack mechanism, allocating the array and back-filling it for already-seen objects. But that logic kicks in even when we've switched to it already (whether because we really did see a new pack, or because we had too many packs in the first place). The result doesn't produce a wrong outcome, but it's very slow. What happens is this: - imagine you have a repo with 500k objects and 2000 packs that you want to repack. - before looking at any objects, we call prepare_in_pack_by_idx(). It starts allocating an index for each pack. On the 1024th pack, it sees there are too many, so it bails, leaving in_pack_by_idx as NULL. - while actually adding objects to the packing list, we call oe_set_in_pack(), which checks whether the pack already has an index. If it's one of the packs after the first 1023, then it doesn't have one, and we'll call oe_map_new_pack(). But there's no useful work for that function to do. We're already using in_pack, so it just uselessly walks over the complete list of objects, trying to backfill in_pack. And we end up doing this for almost 1000 packs (each of which may be triggered by more than one object). And each time it triggers, we may iterate over up to 500k objects. So in the absolute worst case, this is quadratic in the number of objects. The solution is simple: we don't need to bother checking whether the pack has an index if we've already converted to using in_pack, since by definition we're not going to use it. So we can just push the "does the pack have a valid index" check down into that half of the conditional, where we know we're going to use it. The current test in p5303 sadly doesn't notice this problem, since it maxes out at 1000 packs. If we add a new test to it at 2000 packs, it does show the improvement: Test HEAD^ HEAD ---------------------------------------------------------------------- 5303.12: repack (2000) 26.72(39.68+0.67) 15.70(28.70+0.66) -41.2% However, these many-pack test cases are rather expensive to run, so adding larger and larger numbers isn't appealing. Instead, we can show it off more easily by using GIT_TEST_FULL_IN_PACK_ARRAY, which forces us into the absolute worst case: no pack has an index, so we'll trigger oe_map_new_pack() pointlessly for every single object, making it truly quadratic. Here are the numbers (on git.git) with the included change to p5303: Test HEAD^ HEAD ---------------------------------------------------------------------- 5303.3: rev-list (1) 2.05(1.98+0.06) 2.06(1.99+0.06) +0.5% 5303.4: repack (1) 33.45(33.46+0.19) 2.75(2.73+0.22) -91.8% 5303.6: rev-list (50) 2.07(2.01+0.06) 2.06(2.01+0.05) -0.5% 5303.7: repack (50) 34.21(35.18+0.16) 3.49(4.50+0.12) -89.8% 5303.9: rev-list (1000) 2.87(2.78+0.08) 2.88(2.80+0.07) +0.3% 5303.10: repack (1000) 41.26(51.30+0.47) 10.75(20.75+0.44) -73.9% Again, those improvements aren't realistic for the 1-pack case (because in the real world, the full-array solution doesn't kick in), but it's more useful to be testing the more-complicated code path. While we're looking at this issue, we'll tweak one more thing: in oe_map_new_pack(), we call REALLOC_ARRAY(pack->in_pack). But we'd never expect to get here unless we're back-filling it for the first time, in which case it would be NULL. So let's switch that to ALLOC_ARRAY() for clarity, and add a BUG() to document the expectation. Unfortunately this code isn't well-covered in the test suite because it's inherently racy (it only kicks in if somebody else adds a new pack while we're in the middle of repacking). Signed-off-by: Jeff King Reviewed-by: Derrick Stolee Signed-off-by: Junio C Hamano diff --git a/pack-objects.c b/pack-objects.c index 5256029..24ef42f 100644 --- a/pack-objects.c +++ b/pack-objects.c @@ -123,7 +123,10 @@ void oe_map_new_pack(struct packing_data *pack) { uint32_t i; - REALLOC_ARRAY(pack->in_pack, pack->nr_alloc); + if (pack->in_pack) + BUG("packing_data has already been converted to pack array"); + + ALLOC_ARRAY(pack->in_pack, pack->nr_alloc); for (i = 0; i < pack->nr_objects; i++) pack->in_pack[i] = oe_in_pack(pack, pack->objects + i); diff --git a/pack-objects.h b/pack-objects.h index 857d438..9f55ac9 100644 --- a/pack-objects.h +++ b/pack-objects.h @@ -253,12 +253,21 @@ static inline void oe_set_in_pack(struct packing_data *pack, struct object_entry *e, struct packed_git *p) { - if (!p->index) + if (pack->in_pack_by_idx) { + if (p->index) { + e->in_pack_idx = p->index; + return; + } + /* + * We're accessing packs by index, but this pack doesn't have + * an index (e.g., because it was added since we created the + * in_pack_by_idx array). Bail to oe_map_new_pack(), which + * will convert us to using the full in_pack array, and then + * fall through to our in_pack handling. + */ oe_map_new_pack(pack); - if (pack->in_pack_by_idx) - e->in_pack_idx = p->index; - else - pack->in_pack[e - pack->objects] = p; + } + pack->in_pack[e - pack->objects] = p; } static inline struct object_entry *oe_delta( diff --git a/t/perf/p5303-many-packs.sh b/t/perf/p5303-many-packs.sh index 3779851..a369152 100755 --- a/t/perf/p5303-many-packs.sh +++ b/t/perf/p5303-many-packs.sh @@ -77,6 +77,7 @@ do # actual pack generation, without smudging the on-disk setup # between trials. test_perf "repack ($nr_packs)" ' + GIT_TEST_FULL_IN_PACK_ARRAY=1 \ git pack-objects --keep-true-parents \ --honor-pack-keep --non-empty --all \ --reflog --indexed-objects --delta-base-offset \ -- cgit v0.10.2-6-g49f6