Jeff King <peff@xxxxxxxx> writes: > On Thu, May 14, 2015 at 10:37:39AM -0700, Junio C Hamano wrote: > ... >> 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. >> 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. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html