Re: [PATCH v1] diffcore-rename: speed up register_rename_src

[Jeff King <peff@peff.net>]

On Thu, Apr 20, 2017 at 02:08:46PM -0400, Jeff Hostetler wrote:

> > That's not the minimal change you were going for, but I think the end
> > result is simpler and more consistent.
> 
> OK, let me take a stab at something like that and
> see where it takes me.

Thanks.

I set the patch as a lump, but I think there are a few things going on
there:

  - the return value of register_rename_src() is actively dangerous (it
    points to memory which may be reallocated), so it's good that it
    goes away in favor of an "int"

  - we already refuse to do rename detection when there are duplicate
    dsts. This adds the same for srcs. I don't know if the same safety
    rules apply there, but it certainly seems like a reasonable and
    consistent precaution to say "this tree looks broken, let's skip
    rename detection". But it does mean a potential change in
    functionality in that corner case.

  - this patch probably adds "unsorted tree" to the list of breakages
    that would cause us to skip rename detection. I don't know if that's
    actually possible in practice (i.e., do we end up sorting the
    diffq elsewhere anyway?). I also wondered if it might run afoul of
    diffcore_order(), but that is applied after rename detection, so
    we're OK.

> WRT your earlier comment about how often we add or delete 4M
> files and then run status.  The use case that started this was a
> 1% sparse-checkout followed by a read-tree (which reset the
> skip-worktree bits) and then status (which thought 99% of the
> worktree had been deleted or maybe renamed).  There are probably
> other ways to get into this state, but that's how this started.

Right, that sounds plausible. I guess I just wondered if this is
something an average developer runs daily, or something that they would
run into once a year. Shaving 4s of CPU off of a once-a-year operation
is less exciting.

> The more subtle point is that -- for these obscenely large
> values of n -- any time I see an O(n log n) operation that could
> or should be O(n), I want to stop and look at it.

Heh. I spent a fair bit of time in Git's past turning O(n^2) operations
into O(n log n), so I feel your pain. I do think it's important to pay
attention to whole-operation numbers, though. Quite often you have an
O(n log n) with a small constant (like a single strcmp) coupled with
something linear but with a huge constant (like loading blob contents),
and micro-optimizations to the former get drowned out by the latter.

-Peff