Messages in this thread | | | Date | Fri, 21 Jun 2002 16:06:59 +0100 | From | "Stephen C. Tweedie" <> | Subject | Re: [Ext2-devel] Re: Shrinking ext3 directories |
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Hi,
On Fri, Jun 21, 2002 at 05:28:28AM +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 > 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?
I want to get this thing tested!
There are far too many factors for this to be resolved very quickly. In reality, there will be a lot of disk cost under load which you don't see in benchmarks, too. We also know for a fact that the early hashes used in Reiserfs were quick but were vulnerable to terribly bad behaviour under certain application workloads. With the half-md4, at least we can expect decent worst-case behaviour unless we're under active attack (ie. only maliscious apps get hurt).
I think the md4 is a safer bet until we know more, so I'd vote that we stick with the ext3 cvs code which uses hash version #1 for that, and defer anything else until we've seen more --- the hash versioning lets us do that safely.
> By the way, I'm running about 37 usec per create here, on a 1GHz/1GB PIII, > with Ext2. I think most of the difference vs your timings is that your test > code is eating a lot of cpu.
I was getting nearer to 50usec system time, but on an athlon k7-700, so those timings are pretty comparable. Mine was ext3, too, which accounts for a bit. The difference between that and wall-clock time was all just idle time, which I think was due to using "touch"/"rm" --- ie. there was a lot of inode table write activity due to the files being created/deleted, and that was forcing a journal wrap before the end of the test. That effect is not visible on ext2, of course.
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