Re: [PATCH 2/2] log: do not shorten decoration names too early

Jeff King [off-list ref] writes:

On Thu, May 14, 2015 at 10:37:39AM -0700, Junio C Hamano wrote:
...

quoted

Wouldn't the hashtable used by decorate.[ch] with the max load
factor capped to 66% a better economy?

Good point. A slab would be less memory efficient, but cheaper to look
up (it is a direct memory reference, with no probing and no hashcmp()).
But cache effects matter, so it could even be slower.

Yes, that was what I meant by economy.  I do not think memory footprint
is free in that sense.

On the other hand, the slab makes it easy to actually store the real
type (struct name_decoration), whereas the decorate hash stores only
pointers. So we'd save an extra malloc/pointer in each case.

So with your slab_peek() below, I'd guess that the slab would actually
be faster. But I'd also be unsurprised if it makes no appreciable
difference to the overall runtime of "git log --decorate". I think we'd
have to build it and profile (and please feel free to say "eh, not worth
the time to think about further").

While I think *slabname##_peek() would be worth doing regardless of
this caller, I suspect that the major overhead of decorate code
would come from the for_each_ref() that jumps deep into the history
to parse old commits; it would trigger a lot of unpacking of objects
deep in the delta chain, which would be expensive than table look-up
in either scheme.

quoted

I notice that there is no API into commit_slab to ask "Does this
commit have data in the slab?"  *slabname##_at() may be the closest
thing, but that would allocate the space and then says "This is the
slot for that commit; go check if there is data there already."

Yes. I think it's not a big deal if your slab is reasonably full (you'll
extend it to the full size of your commit space eventually either way).
But for a sparse slab, it does make that query much more expensive than
it needs to be.

Yes, and I think that commit decoration is such a use case.