Re: [PATCH] describe: use khash in finish_depth_computation()

> Jeff King <peff@peff.net> writes:
> 
> > So out of curiosity I tried replacing a slab that should be pretty
> > densely filled, using a khash based on oidhash/ptr along with some
> > quality-of-life wrappers.  Patch is below.
> >
> > It performs...very badly. Not sure if I've screwed something up, but
> > it's about 7x slower to run "git rev-list --author-date-order HEAD" in
> > the kernel. So maybe slabs really are worth it overall.
> 
> Hmph.  It is the best case scenario for the slab code, as you'd need
> author date for each and every commit object in this use case, and
> the comparison function called by prio-queue would be called for the
> same object many times.
> 
> But the hash function being oidhash(), I am a bit surprised.  It
> shouldn't be so much more expensive to peek at the first 4 bytes and
> then do the usual hashtable thing than looking at the in-object
> commit->index.  Is it a sign that the range of oidhash() is a bit
> too small for a real workload?
> 
> Nah, 4 byte unsigned integer should be sufficient for the number of
> objects in the kernel.