Re: [Ext2-devel] Re: Shrinking ext3 directories

Helge Hafting
2002-06-21T07:03:48Z

Daniel Phillips wrote:

> I ran a bakeoff between your new half-md4 and dx_hack_hash on Ext2. As
> predicted, half-md4 does produce very even bucket distributions. For 200,000
> creates:
>
> half-md4: 2872 avg bytes filled per 4k block (70%)
> dx_hack_hash: 2853 avg bytes filled per 4k block (69%)
>
> but guess which was faster overall?
>
> half-md4: user 0.43 system 6.88 real 0:07.33 CPU 99%
> dx_hack_hash: user 0.43 system 6.40 real 0:06.82 CPU 100%
>
> This is quite reproducible: dx_hack_hash is always faster by about 6%. This
> must be due entirely to the difference in hashing cost, since half-md4
> produces measurably better distributions. Now what do we do?

No surprise that the worse distribution is faster - you get less
io when fewer blocks are used. Which means a bad distribution beats
a good one _until_ blocks start to really fill up and collide. 2.8K per
4K block is only 70% full. I guess the better hash wins
if you force a higher fill rate?

Helge Hafting