On Thu, Dec 22, 2016 at 02:10:33PM +0100, Jason A. Donenfeld wrote:
> On Thu, Dec 22, 2016 at 1:47 PM, Hannes Frederic Sowa
> <han...@stressinduktion.org> wrote:
> > following up on what appears to be a random subject: ;)
> >
> > IIRC, ext4 code by default still uses half_md4 for hashing of filenames
> > in the htree. siphash seems to fit this use case pretty good.
>
> I saw this too. I'll try to address it in v8 of this series.
This is a separate issue, and this series is getting a bit too
complex. So I'd suggest pushing this off to a separate change.
Changing the htree hash algorithm is an on-disk format change, and so
we couldn't roll it out until e2fsprogs gets updated and rolled out
pretty broadley. In fact George sent me patches to add siphash as a
hash algorithm for htree a while back (for both the kernel and
e2fsprogs), but I never got around to testing and applying them,
mainly because while it's technically faster, I had other higher
priority issues to work on --- and see previous comments regarding
pixel peeping. Improving the hash algorithm by tens or even hundreds
of nanoseconds isn't really going to matter since we only do a htree
lookup on a file creation or cold cache lookup, and the SSD or HDD I/O
times will dominate. And from the power perspective, saving
microwatts of CPU power isn't going to matter if you're going to be
spinning up the storage device....
- Ted
