On 7/31/2018 11:31 AM, Duy Nguyen wrote:
On Mon, Jul 30, 2018 at 8:10 PM Ben Peart <peartben@xxxxxxxxx> wrote:
I ran "git checkout" on a large repo and averaged the results of 3 runs.
This clearly demonstrates the benefit of the optimized unpack_trees()
as even the final "diff-index" is essentially a 3rd call to unpack_trees().
baseline new
----------------------------------------------------------------------
0.535510167 0.556558733 s: read cache .git/index
0.3057373 0.3147105 s: initialize name hash
0.0184082 0.023558433 s: preload index
0.086910967 0.089085967 s: refresh index
7.889590767 2.191554433 s: unpack trees
0.120760833 0.131941267 s: update worktree after a merge
2.2583504 2.572663167 s: repair cache-tree
0.8916137 0.959495233 s: write index, changed mask = 28
3.405199233 0.2710663 s: unpack trees
0.000999667 0.0021554 s: update worktree after a merge
3.4063306 0.273318333 s: diff-index
16.9524923 9.462943133 s: git command:
'c:\git-sdk-64\usr\src\git\git.exe' checkout
The first call to unpack_trees() saves 72%
The 2nd and 3rd call save 92%
By the 3rd I guess you meant "diff-index" line. I think it's the same
with the second call. diff-index triggers the second unpack-trees but
there's no indent here and it's misleading to read this as diff-index
and unpack-trees execute one after the other.
Total time savings for the entire command was 44%
Wow.. I guess you have more trees since I could only save 30% on gcc.git.
Yes, with over 500K trees, this optimization really pays off for us. I
can't wait to see how this works out in the wild (vs my "lab" based
performance testing).
Thank you! I definitely owe you lunch. :)
In the performance game of whack-a-mole, that call to repair cache-tree
is now looking quite expensive...
Yeah and I think we can whack that mole too. I did some measurement.
Best case possible, we just need to scan through two indexes (one with
many good cache-tree, one with no cache-tree), compare and copy
cache-tree over. The scanning takes like 1% time of current repair
step and I suspect it's the hashing that takes most of the time. Of
course real world won't have such nice numbers, but I guess we could
maybe half cache-tree update/repair time.
I have some great profiling tools available so will take a look at this
next and see exactly where the time is being spent.
