Here is a short (but dense) series that I worked on towards the end of
2021 with Peff.
Its goal is to improve the bitmap traversal we implement in
prepare_bitmap_walk(). Specifically, it avoids building up a complete
bitmap of the "haves" side, and instead uses a combination of (a)
UNINTERESTING commits between the tips and boundary, and (b) the
boundary itself.
The gory details are laid out in 3/3, but the high-level overview of the
new algorithm to compute the set of objects between "haves" and "wants"
using bitmaps is:
1. Build a (partial) bitmap of the haves side by first OR-ing any
bitmap(s) that already exist for UNINTERESTING commits between the
haves and the boundary.
2. For each commit along the boundary, add it as a fill-in traversal
tip (where the traversal terminates once an existing bitmap is
found), and perform fill-in traversal.
3. Build up a complete bitmap of the wants side as usual, stopping any
time we intersect the (partial) haves side.
This improves many cases where using bitmaps was significantly *slower*
than regular, non-bitmap traversal. In some instances, using bitmaps is
still slower than the non-bitmap case, but it is a substantial
improvement over the classic bitmap walk.
$ ours="$(git branch --show-current)"
argv="--count --objects $ours --not --exclude=$ours --branches"
hyperfine \
-n 'no bitmaps' "git.compile rev-list $argv" \
-n 'existing bitmap traversal' "git rev-list --use-bitmap-index $argv" \
-n 'this commit' "git.compile rev-list --use-bitmap-index $argv"
Benchmark 1: no bitmaps
Time (mean ± σ): 15.1 ms ± 4.1 ms [User: 8.1 ms, System: 6.9 ms]
Range (min … max): 7.4 ms … 21.8 ms 131 runs
Benchmark 2: existing bitmap traversal
Time (mean ± σ): 82.6 ms ± 9.2 ms [User: 63.6 ms, System: 19.0 ms]
Range (min … max): 73.8 ms … 105.4 ms 28 runs
Benchmark 3: this commit
Time (mean ± σ): 19.8 ms ± 3.1 ms [User: 13.0 ms, System: 6.8 ms]
Range (min … max): 17.7 ms … 38.6 ms 158 runs
Summary
'no bitmaps' ran
1.31 ± 0.41 times faster than 'this commit'
5.49 ± 1.62 times faster than 'existing bitmap traversal'
In a large repository on GitHub.com, the timings to compute the objects
unique to "master", as in:
$ git rev-list --count --objects master --not --exclude=master --branches
improve as follows:
- 1.012 sec (without bitmaps)
- 29.571 sec (with the existing bitmap walk)
- 2.279 sec (with these patches)
The first couple of patches are preparatory, and the third patch is the
substantive one.
Please have a look and poke any holes you can find in the new approach
:-). Thanks in advance for your review.
Taylor Blau (3):
revision: support tracking uninteresting commits
pack-bitmap.c: extract `fill_in_bitmap()`
pack-bitmap.c: use commit boundary during bitmap traversal
pack-bitmap.c | 252 ++++++++++++++++++++++++++++++++++++++------------
revision.c | 10 +-
revision.h | 5 +
3 files changed, 205 insertions(+), 62 deletions(-)
--
2.40.0.380.gda896aa358.dirty
