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Message-ID: <alpine.DEB.2.00.0902102224510.10665@blackhole.kfki.hu>
Date: Tue, 10 Feb 2009 23:03:11 +0100 (CET)
From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To: Scott Feldman <scofeldm@...co.com>
cc: davem@...emloft.net, netdev@...r.kernel.org,
netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] Update jhash.h with the new version of Jenkins' hash
On Tue, 10 Feb 2009, Scott Feldman wrote:
> Jozsef Kadlecsik wrote:
> > The patch replaces the lookup2() implementation of the 'jhash*' functions
> > with that of lookup3().
>
> Should the lookup3() be added to rather than replacing lookup2()? In case
> some hardware vendor used the lookup2() version for weird things like flow
> classification.
Do you imagine a case where a system with lookup3() communicates with
another one with lookup2() and they want to exchange data identified by
the hash keys? But the normal approach is to use a random seed (initval),
so even two systems with lookup2() won't generate the same hash value for
the same key.
Grepping through the whole kernel, at a lot of places jhash used with zero
initval. :-(.
Sure, it's safer to add a new header file instead, see below.
> > /* The golden ration: an arbitrary value */
> > -#define JHASH_GOLDEN_RATIO 0x9e3779b9
> > +#define JHASH_GOLDEN_RATIO 0xdeadbeef
>
> The #define seems mis-named now.
It's the new arbitrary value, taken from the original source :-).
----
The current jhash.h implements the lookup2() hash function by Bob Jenkins.
However, lookup2() is outdated as Bob wrote a new hash function called
lookup3(). The new hash function
- mixes better than lookup2(): it passes the check that every input bit
changes every output bit 50% of the time, while lookup2() failed it.
- performs better: compiled with -O2 on Core2 Duo, lookup3() 20-40% faster
than lookup2() depending on the key length.
The patch adds jhash3.h which implements the lookup3() function(s) for
the Linux kernel.
Signed-off-by: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
---
include/linux/jhash3.h | 157 ++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 157 insertions(+), 0 deletions(-)
create mode 100644 include/linux/jhash3.h
diff --git a/include/linux/jhash3.h b/include/linux/jhash3.h
new file mode 100644
index 0000000..99a203d
--- /dev/null
+++ b/include/linux/jhash3.h
@@ -0,0 +1,157 @@
+#ifndef _LINUX_JHASH3_H
+#define _LINUX_JHASH3_H
+
+/* jhash3.h: Jenkins hash support.
+ *
+ * Copyright (C) 2006. Bob Jenkins (bob_jenkins@...tleburtle.net)
+ *
+ * http://burtleburtle.net/bob/hash/
+ *
+ * These are the credits from Bob's sources:
+ *
+ * lookup3.c, by Bob Jenkins, May 2006, Public Domain.
+ *
+ * These are functions for producing 32-bit hashes for hash table lookup.
+ * hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final()
+ * are externally useful functions. Routines to test the hash are included
+ * if SELF_TEST is defined. You can use this free for any purpose. It's in
+ * the public domain. It has no warranty.
+ *
+ * Copyright (C) 2009 Jozsef Kadlecsik (kadlec@...ckhole.kfki.hu)
+ *
+ * I've modified Bob's hash to be useful in the Linux kernel, and
+ * any bugs present are my fault. Jozsef
+ */
+
+#define __rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))
+
+/* __jhash3_mix - mix 3 32-bit values reversibly. */
+#define __jhash3_mix(a,b,c) \
+{ \
+ a -= c; a ^= __rot(c, 4); c += b; \
+ b -= a; b ^= __rot(a, 6); a += c; \
+ c -= b; c ^= __rot(b, 8); b += a; \
+ a -= c; a ^= __rot(c,16); c += b; \
+ b -= a; b ^= __rot(a,19); a += c; \
+ c -= b; c ^= __rot(b, 4); b += a; \
+}
+
+/* __jhash3_final - final mixing of 3 32-bit values (a,b,c) into c */
+#define __jhash3_final(a,b,c) \
+{ \
+ c ^= b; c -= __rot(b,14); \
+ a ^= c; a -= __rot(c,11); \
+ b ^= a; b -= __rot(a,25); \
+ c ^= b; c -= __rot(b,16); \
+ a ^= c; a -= __rot(c,4); \
+ b ^= a; b -= __rot(a,14); \
+ c ^= b; c -= __rot(b,24); \
+}
+
+/* The golden ration: an arbitrary value */
+#define JHASH3_GOLDEN_RATIO 0xdeadbeef
+
+/* The most generic version, hashes an arbitrary sequence
+ * of bytes. No alignment or length assumptions are made about
+ * the input key. The result depends on endianness.
+ */
+static inline u32 jhash3(const void *key, u32 length, u32 initval)
+{
+ u32 a,b,c;
+ const u8 *k = key;
+
+ /* Set up the internal state */
+ a = b = c = JHASH3_GOLDEN_RATIO + length + initval;
+
+ /* all but the last block: affect some 32 bits of (a,b,c) */
+ while (length > 12) {
+ a += (k[0] + ((u32)k[1]<<8) + ((u32)k[2]<<16) + ((u32)k[3]<<24));
+ b += (k[4] + ((u32)k[5]<<8) + ((u32)k[6]<<16) + ((u32)k[7]<<24));
+ c += (k[8] + ((u32)k[9]<<8) + ((u32)k[10]<<16) + ((u32)k[11]<<24));
+ __jhash3_mix(a, b, c);
+ length -= 12;
+ k += 12;
+ }
+
+ /* last block: affect all 32 bits of (c) */
+ /* all the case statements fall through */
+ switch (length) {
+ case 12: c += (u32)k[11]<<24;
+ case 11: c += (u32)k[10]<<16;
+ case 10: c += (u32)k[9]<<8;
+ case 9 : c += k[8];
+ case 8 : b += (u32)k[7]<<24;
+ case 7 : b += (u32)k[6]<<16;
+ case 6 : b += (u32)k[5]<<8;
+ case 5 : b += k[4];
+ case 4 : a += (u32)k[3]<<24;
+ case 3 : a += (u32)k[2]<<16;
+ case 2 : a += (u32)k[1]<<8;
+ case 1 : a += k[0];
+ __jhash3_final(a, b, c);
+ case 0 :
+ break;
+ }
+
+ return c;
+}
+
+/* A special optimized version that handles 1 or more of u32s.
+ * The length parameter here is the number of u32s in the key.
+ */
+static inline u32 jhash3_words(const u32 *k, u32 length, u32 initval)
+{
+ u32 a, b, c;
+
+ /* Set up the internal state */
+ a = b = c = JHASH3_GOLDEN_RATIO + (length<<2) + initval;
+
+ /* handle most of the key */
+ while (length > 3) {
+ a += k[0];
+ b += k[1];
+ c += k[2];
+ __jhash3_mix(a, b, c);
+ length -= 3;
+ k += 3;
+ }
+
+ /* handle the last 3 u32's */
+ /* all the case statements fall through */
+ switch (length) {
+ case 3: c += k[2];
+ case 2: b += k[1];
+ case 1: a += k[0];
+ __jhash3_final(a, b, c);
+ case 0: /* case 0: nothing left to add */
+ break;
+ }
+
+ return c;
+}
+
+/* A special ultra-optimized versions that knows they are hashing exactly
+ * 3, 2 or 1 word(s).
+ */
+static inline u32 jhash3_3words(u32 a, u32 b, u32 c, u32 initval)
+{
+ a += JHASH3_GOLDEN_RATIO + initval;
+ b += JHASH3_GOLDEN_RATIO + initval;
+ c += JHASH3_GOLDEN_RATIO + initval;
+
+ __jhash3_final(a, b, c);
+
+ return c;
+}
+
+static inline u32 jhash3_2words(u32 a, u32 b, u32 initval)
+{
+ return jhash3_3words(0, a, b, initval);
+}
+
+static inline u32 jhash3_1word(u32 a, u32 initval)
+{
+ return jhash3_3words(0, 0, a, initval);
+}
+
+#endif /* _LINUX_JHASH3_H */
--
1.5.4.3
Best regards,
Jozsef
-
E-mail : kadlec@...ckhole.kfki.hu, kadlec@...l.kfki.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : KFKI Research Institute for Particle and Nuclear Physics
H-1525 Budapest 114, POB. 49, Hungary
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