Subject: Re: Re: Shrinking ext3 directories
On Jun 21, 2002 05:28 +0200, 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
> 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?
While I normally advocate the "cheapest" way of implementing a given
solution (and dx_hack_hash is definitely the lowest-cost hash function
we could reasonably have), I would still be inclined to go with half-MD4
for this. A few reasons for that:
1) CPUs are getting faster all the time
2) it is a well-understood algorithm that has very good behaviour
3) it is much harder to spoof MD4 than dx_hack_hash
4) it is probably better to have the most uniform hash function we can
find than to do lots more block split/coalesce operations, so the
extra cost of half-MD4 may be a benefit overall
It would be interesting to re-run this test to create a few million
entries, but with periodic deletes.
Hmm, now that I think about it, split/coalesce operations are only
important on create and delete, while the hash cost is paid for each
lookup as well. It would be interesting to see the comparison with
a test something like this (sorry, don't have a system which has both
hash functions working righ...
for d in `seq -f "directory_name_%05g" 1 10000` ; do
for f in `seq -f "file_name_%05g" 1 10000` ; do
mount -o noatime $DEV $TESTDIR
for d in `seq -f "directory_name_%05g" 10000 -1 1` ; do
for f in `seq -f "file_name_%05g" 10000 -1 1` ; do
stat $TESTDIR/$d/$d_$f > /dev/null
Having the longer filenames will put more load on the hash function,
so we will see if we are really paying a big price for the overhead,
and the stat test will remove all of the creation time and disk dirtying
and just leave us with the pure lookup costs hopefully.
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