Re: [rfc] Near-constant time directory index for Ext2

From: Ed Tomlinson (tomlins@cam.org)
Date: Wed Feb 21 2001 - 19:35:40 EST


Linus Torvalds <torvalds@transmeta.com> wrote:
>
> Ed Tomlinson  <tomlins@cam.org> wrote:
> >The default in reiserfs is now the R5 hash, but you are right that lots of
> > efforts went into finding this hash.  This includes testing various
> > hashes on real directory structures to see which one worked best.  R5
> > won.
>
> That's interesting.  The R5 hash is easily also the only one of the
> reiser hashes that might be useable for the generic VFS hashing.  It's
> not so different in spirit from the current one, and if you've done the
> work to test it, it's bound to be a lot better.

It was not me personally.   I just remembered the thread (from june 2000) on
the reiserfs list...  I have summerized the results for you below.

For the program see: http://www.jedi.claranet.fr/hash_torture.tar.gz

Ed

PS.  I am still seeing hangs with (2.4.2pre2 then I switched to ac7 or so and
have had hangs with all pre and ac(s) tried and that is most of them)  ac20
plus the latest reiserfs fixes has stayed up 8 hours so far - it can take two
or three days  to trigger the hang though.  When it hangs it really dead,  a
UPS connected via a serial port cannot shut it down.   pings to the box fail.
A+SysRQ is dead, and the software watchdog does not trigger a reboot.  
ideas?

> (The current VFS name hash is probably _really_ stupid - I think it's
> still my original one, and nobody probably ever even tried to run it
> through any testing.  For example, I bet that using a shift factor of 4
> is really bad, because it evenly divides a byte, which together with the
> xor means that you can really easily generate trivial bad cases).
>
> What did you use for a test-case? Real-life directory contents? Did you
> do any worst-case analysis too?
>
>                Linus

some test results from june 2000 with Hans's summary first.
---------------------------------------------------------------
(reiserfs) Re: r5 hash
From: Hans Reiser <hans@reiser.to>
To: "Yury Yu. Rupasov" <yura@yura.polnet.botik.ru>
Cc: Jedi/Sector One <j@4u.net>, Petru Paler <ppetru@coltronix.com>,
"reiserfs@devlinux.com" <reiserfs@devlinux.com>, Yury Shevchuk
<sizif@botik.ru>

Ok, based on this benchmark let's put rupasov5 in, and warn users who choose
the
currently used rupasov1 hash that rupasov5 has obsoleted it.  Do this in both
3.6 and 3.5, and fix the the delimiting key check in 3.5 REISERFS_CHECK bug at
the same time.  Cut the patch, start testing, and see if you can release by
Monday.  Make rupasov5 the default.  sizif, review the documentation he
creates
for users.

Jedi, if you disagree with the benchmarks let me know.  You might try
concatenating two filenames together instead of adding a digit to them, or
running find on a really large FS, to improve these tests.  Thanks for helping
us with analyzing the different hash methods available Jedi.

Hans

---------------------------------------------------------------
(reiserfs) Re: r5 hash
From: "Yury Yu. Rupasov" <yura@yura.polnet.botik.ru>
To: Hans Reiser <hans@reiser.to>
Cc: Jedi/Sector One <j@4u.net>, Petru Paler <ppetru@coltronix.com>,
"reiserfs@devlinux.com" <reiserfs@devlinux.com>, Yury Shevchuk
<sizif@botik.ru>

Hans Reiser wrote:

> What is the speed of the real filenames, not just the number of collisions.

Ok, here is the results for real names :
# find / -type d -exec ls {} \; | sort | uniq > allfiles.txt

# wc -l allfiles.txt
161101 allfiles.txt

Collisions for 161 101 names:

tea_hash  : 784 total,  2 dangerous
jedi_hash2: 957 total,  2 dangerous
r5_hash   :1191 total,  2 dangerous
r7_hash   :8439 total, 18 dangerous

The speed for 161 101 real names :

create 161101 files of 10 bytes with names from allfiles.txt

# time create d1 allfiles.txt
# time cp d1 d2 -r
# time rm d1 -r

              create      copy        remove
             --------------------------------
tea_hash   : 1m27.223s   5m43.069s  2m33.449s
jedi_hash2 : 1m26.062s   5m40.872s  2m32.795s
r5_hash    : 1m16.729s   4m14.967s  1m53.037s
r7_hash    : 1m10.665s   3m34.950s  1m39.756s

As you can see the results are differ, but not too much. :)
The situation changes dramatically if we will test 1 million files.

The same test, but at the end of each name from allfiles.txt
added numbers from 0 to 6 (1 127 707 files):
 
              create      copy        remove
             --------------------------------
tea_hash   : 81m44.449s  
jedi_hash2 : 79m46.419s
r5_hash    : 15m56.037s
r7_hash    : 15m30.680s

Dual Celeron 500, 128 MB RAM, 8 GB scsi HDD
Reiserfs-3.5.21, Linux-2.2.15

Thanks,
Yura.
---------------------------------------------------------------
body { font-family: "helvetica" } p { font-size: 12pt } a { color: #0000ff;
text-decoration: none; }(reiserfs) Torture results
From: Jedi/Sector One <j@4u.net>
To: reiserfs@devlinux.com

  Here are the results of the hash torture on a Celeron 300.
  Once again, you can substract 1 from the dangerous collisions numbers.
  Xuan, can you provide a test for the case Rupasov hash was designed
for ?
  Anyway, I don't really see why large directories should have similar
file names, rather that keywords.

  Best regards,

-- 
         Frank DENIS aka Jedi/Sector One aka DJ Chrysalis <j@4u.net>
                 -> Software : http://www.jedi.claranet.fr <-
      If Bill Gates had a dime for every time a Windows box crashed...
                  ...oh, wait a minute -- he already does.

********************** /usr/dict/words test **********************

Trying with   45402 words

-------------[Benchmarking tea hash]-------------

Collisions : 45 Dangerous :       1      ffff980 Timing :

real     0m0.145s user     0m0.120s sys      0m0.010s

-------------[Benchmarking rupasov hash]-------------

Collisions : 553 Dangerous :       1      ffffe00 Timing :

real     0m0.297s user     0m0.260s sys      0m0.020s

-------------[Benchmarking r5 hash]-------------

Collisions : 185 Dangerous :       1      ffae000 Timing :

real     0m0.124s user     0m0.080s sys      0m0.030s

-------------[Benchmarking r7 hash]-------------

Collisions : 2528 Dangerous :       1      fffd400 Timing :

real     0m0.121s user     0m0.100s sys      0m0.000s

-------------[Benchmarking jedi hash]-------------

Collisions : 54 Dangerous :       1      fff9780 Timing :

real     0m0.122s user     0m0.100s sys      0m0.010s

-------------[Benchmarking jedi2 hash]-------------

Collisions : 93 Dangerous :       1      fff9780 Timing :

real     0m0.122s user     0m0.090s sys      0m0.020s

-------------[Benchmarking lookup2 hash]-------------

Collisions : 63 Dangerous :       1      ffff480 Timing :

real     0m0.123s user     0m0.100s sys      0m0.000s

********************** Squid names test **********************

Trying with  458752 squid cache entries

-------------[Benchmarking tea hash]-------------

Collisions : 6237 Dangerous :       1      fffff80 Timing :

real     0m1.138s user     0m1.090s sys      0m0.030s

-------------[Benchmarking rupasov hash]-------------

Collisions : 377520 Dangerous :       1      e32700 Timing :

real     0m2.588s user     0m2.550s sys      0m0.020s

-------------[Benchmarking r5 hash]-------------

Collisions : 309991 Dangerous :       1      55406b80 Timing :

real     0m0.940s user     0m0.880s sys      0m0.040s

-------------[Benchmarking r7 hash]-------------

Collisions : 449006 Dangerous :       2      22b16580 Timing :

real     0m0.928s user     0m0.840s sys      0m0.070s

-------------[Benchmarking jedi hash]-------------

Collisions : 2771 Dangerous :       1      fffef80 Timing :

real     0m0.928s user     0m0.860s sys      0m0.050s

-------------[Benchmarking jedi2 hash]-------------

Collisions : 0 Dangerous :       1      ffff80 Timing :

real     0m0.879s user     0m0.810s sys      0m0.050s

-------------[Benchmarking lookup2 hash]-------------

Collisions : 6203 Dangerous :       1      fffdc00 Timing :

real     0m0.930s user     0m0.840s sys      0m0.080s

********************** Real names test **********************

Trying with   89830 files

-------------[Benchmarking tea hash]-------------

Collisions : 237 Dangerous :       1      fff5580 Timing :

real     0m0.276s user     0m0.250s sys      0m0.000s

-------------[Benchmarking rupasov hash]-------------

Collisions : 6288 Dangerous :       1      ffee080 Timing :

real     0m0.582s user     0m0.560s sys      0m0.010s

-------------[Benchmarking r5 hash]-------------

Collisions : 3920 Dangerous :       1      fff4600 Timing :

real     0m0.230s user     0m0.190s sys      0m0.020s

-------------[Benchmarking r7 hash]-------------

Collisions : 11801 Dangerous :       1      fff580 Timing :

real     0m0.225s user     0m0.180s sys      0m0.030s

-------------[Benchmarking jedi hash]-------------

Collisions : 269 Dangerous :       1      fff9f80 Timing :

real     0m0.226s user     0m0.200s sys      0m0.010s

-------------[Benchmarking jedi2 hash]-------------

Collisions : 415 Dangerous :       1      fff9f80 Timing :

real     0m0.225s user     0m0.200s sys      0m0.010s

-------------[Benchmarking lookup2 hash]-------------

Collisions : 223 Dangerous :       1      ffff480 Timing :

real     0m0.230s user     0m0.210s sys      0m0.000s

----------------------------------------------------------------------------------------

body { font-family: "helvetica" } p { font-size: 12pt } a { color: #0000ff; text-decoration: none; }(reiserfs) hash torture results From: Petru Paler <ppetru@coltronix.com> To: reiserfs@devlinux.com

Machine: AMD Athlon/650MHz, 128Mb RAM, Quantum Fireball lct15 IDE hdd (UDMA/66 but that doesn't matter). Kernel 2.4.0-test1-ac10.

The results are interesting, but more interesting would be to see how fast reiserfs actually is with each of these hashes.

Script output:

********************** /usr/dict/words test **********************

Trying with   45402 words

-------------[Benchmarking tea hash]-------------

Collisions : 45 Dangerous :       1      ffff980 Timing : 0.00user 0.01system 0:00.08elapsed 11%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking rupasov hash]-------------

Collisions : 553 Dangerous :       1      ffffe00 Timing : 0.00user 0.00system 0:00.18elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r5 hash]-------------

Collisions : 185 Dangerous :       1      ffae000 Timing : 0.00user 0.00system 0:00.08elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r7 hash]-------------

Collisions : 2528 Dangerous :       1      fffd400 Timing : 0.00user 0.01system 0:00.07elapsed 12%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi hash]-------------

Collisions : 54 Dangerous :       1      fff9780 Timing : 0.00user 0.00system 0:00.08elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi2 hash]-------------

Collisions : 93 Dangerous :       1      fff9780 Timing : 0.00user 0.00system 0:00.07elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking lookup2 hash]-------------

Collisions : 63 Dangerous :       1      ffff480 Timing : 0.00user 0.00system 0:00.07elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

********************** Squid names test **********************

Trying with  262144 squid cache entries

-------------[Benchmarking tea hash]-------------

Collisions : 2019 Dangerous :       1      ffff880 Timing : 0.00user 0.01system 0:00.47elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking rupasov hash]-------------

Collisions : 210912 Dangerous :       1      a88f00 Timing : 0.00user 0.02system 0:01.03elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r5 hash]-------------

Collisions : 171912 Dangerous :       1      54ca7680 Timing : 0.00user 0.03system 0:00.41elapsed 7%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r7 hash]-------------

Collisions : 256171 Dangerous :       6      22aa0600 Timing : 0.00user 0.03system 0:00.41elapsed 7%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi hash]-------------

Collisions : 589 Dangerous :       1      fffda00 Timing : 0.00user 0.02system 0:00.42elapsed 4%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi2 hash]-------------

Collisions : 0 Dangerous :       1      ffff80 Timing : 0.00user 0.00system 0:00.40elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking lookup2 hash]-------------

Collisions : 2041 Dangerous :       1      fffdc00 Timing : 0.00user 0.01system 0:00.40elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

********************** Real names test **********************

find: /proc/31112/fd/4: No such file or directory Trying with   94836 files

-------------[Benchmarking tea hash]-------------

Collisions : 235 Dangerous :       1      fff5e80 Timing : 0.00user 0.00system 0:00.20elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking rupasov hash]-------------

Collisions : 2016 Dangerous :       1      fffab80 Timing : 0.01user 0.00system 0:00.46elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r5 hash]-------------

Collisions : 495 Dangerous :       1      fff8780 Timing : 0.00user 0.00system 0:00.17elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking r7 hash]-------------

Collisions : 8162 Dangerous :       1      fff580 Timing : 0.00user 0.02system 0:00.17elapsed 11%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi hash]-------------

Collisions : 331 Dangerous :       1      ffe400 Timing : 0.00user 0.00system 0:00.17elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking jedi2 hash]-------------

Collisions : 341 Dangerous :       1      ffe400 Timing : 0.00user 0.00system 0:00.17elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-------------[Benchmarking lookup2 hash]-------------

Collisions : 298 Dangerous :       1      fffb700 Timing : 0.00user 0.00system 0:00.17elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (83major+13minor)pagefaults 0swaps

-Petru

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