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Re: [PATCH 4/4] mark_parents_uninteresting(): avoid most allocation

On 5/14/2018 9:09 AM, Jeff King wrote:
On Mon, May 14, 2018 at 08:47:33AM -0400, Derrick Stolee wrote:

On 5/11/2018 2:03 PM, Jeff King wrote:
Commit 941ba8db57 (Eliminate recursion in setting/clearing
marks in commit list, 2012-01-14) used a clever double-loop
to avoid allocations for single-parent chains of history.
However, it did so only when following parents of parents
(which was an uncommon case), and _always_ incurred at least
one allocation to populate the list of pending parents in
the first place.

We can turn this into zero-allocation in the common case by
iterating directly over the initial parent list, and then
following up on any pending items we might have discovered.
This change appears to improve performance, but I was unable to measure any
difference between this commit and the one ahead, even when merging
ds/generation-numbers (which significantly reduces the other costs). I was
testing 'git status' and 'git rev-list --boundary master...origin/master' in
the Linux repo with my copy of master 70,000+ commits behind origin/master.
I think you'd want to go the other way: this is marking uninteresting
commits, so you'd want origin/master..master, which would make those 70k
commits uninteresting.

Thanks for the tip. Running 'git rev-list origin/master..master' 100 times gave the following times, on average (with ds/generation-numbers):

PATCH 3/4: 0.19 s
PATCH 4/4: 0.16 s
    Rel %: -16%

I still doubt it will create that much difference, though. It's
more or less saving a single malloc per commit we traverse. Certainly
without the .commits file that's a drop in the bucket compared to
inflating the object data, but I wouldn't be surprised if we end up with
a few mallocs even after your patches.

Anyway, thanks for poking at it. :)