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SubjectRe: [PATCH 2/2] The new jhash implementation
On Thu, 25 Nov 2010, Eric Dumazet wrote:

> Le jeudi 25 novembre 2010 ? 15:41 +0100, Jozsef Kadlecsik a ?crit :
>
> > +/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
> > +#define __jhash_final(a, b, c) \
> > +{ \
> > + c ^= b; c -= rol32(b, 14); \
> > + a ^= c; a -= rol32(c, 11); \
> > + b ^= a; b -= rol32(a, 25); \
> > + c ^= b; c -= rol32(b, 16); \
> > + a ^= c; a -= rol32(c, 4); \
> > + b ^= a; b -= rol32(a, 14); \
> > + c ^= b; c -= rol32(b, 24); \
> > +}
> > +
>
> So we now have a special __jhash_final(a, b, c) thing for the last
> values.

Yes, and...

> > +/* jhash_3words - hash exactly 3, 2 or 1 word(s) */
> > +u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
> > +{
> > + a += JHASH_INITVAL;
> > + b += JHASH_INITVAL;
> > + c += initval;
> > +
> > + __jhash_mix(a, b, c);
> > +
> > + return c;
> > +}
> > +EXPORT_SYMBOL(jhash_3words);
>
> But you dont use it in jhash_3words().

... excellent spotting! That should be __jhash_final in jhash_3words.

> I do think jhash_3words() should stay inlined, unless maybe
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y
>
> We hit it several time per packet in network stack in RX path.
>
> Once in skb_get_rxhash() (unless device fills skb->rxhash)
> Once at least in conntrack (if used).
> Once in UDP or TCP stack

There follows the new version of the second patch: jhash_3words is moved
back to inlined form and the bug in it fixed. Thanks for your thorough
reviewing indeed!

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However, lookup2() is outdated as Bob wrote a new hash function called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 replaces the lookup2() implementation of the 'jhash*'
functions with that of lookup3().

You can read a longer comparison of the two and other hash functions at
http://burtleburtle.net/bob/hash/doobs.html.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
---
include/linux/jhash.h | 140 ++++++++-----------------------------------------
lib/Makefile | 2 +-
lib/jhash.c | 127 ++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 151 insertions(+), 118 deletions(-)
create mode 100644 lib/jhash.c
diff --git a/include/linux/jhash.h b/include/linux/jhash.h
index ced1159..963abad 100644
--- a/include/linux/jhash.h
+++ b/include/linux/jhash.h
@@ -1,131 +1,37 @@
#ifndef _LINUX_JHASH_H
#define _LINUX_JHASH_H

-/* jhash.h: Jenkins hash support.
- *
- * Copyright (C) 1996 Bob Jenkins (bob_jenkins@burtleburtle.net)
- *
- * http://burtleburtle.net/bob/hash/
- *
- * These are the credits from Bob's sources:
- *
- * lookup2.c, by Bob Jenkins, December 1996, Public Domain.
- * hash(), hash2(), hash3, and mix() 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 has no warranty.
- *
- * Copyright (C) 2003 David S. Miller (davem@redhat.com)
- *
- * I've modified Bob's hash to be useful in the Linux kernel, and
- * any bugs present are surely my fault. -DaveM
- */
-
-/* NOTE: Arguments are modified. */
-#define __jhash_mix(a, b, c) \
-{ \
- a -= b; a -= c; a ^= (c>>13); \
- b -= c; b -= a; b ^= (a<<8); \
- c -= a; c -= b; c ^= (b>>13); \
- a -= b; a -= c; a ^= (c>>12); \
- b -= c; b -= a; b ^= (a<<16); \
- c -= a; c -= b; c ^= (b>>5); \
- a -= b; a -= c; a ^= (c>>3); \
- b -= c; b -= a; b ^= (a<<10); \
- c -= a; c -= b; c ^= (b>>15); \
-}
-
-/* The golden ration: an arbitrary value */
-#define JHASH_GOLDEN_RATIO 0x9e3779b9
-
-/* The most generic version, hashes an arbitrary sequence
- * of bytes. No alignment or length assumptions are made about
- * the input key.
- */
-static inline u32 jhash(const void *key, u32 length, u32 initval)
-{
- u32 a, b, c, len;
- const u8 *k = key;
-
- len = length;
- a = b = JHASH_GOLDEN_RATIO;
- c = initval;
-
- while (len >= 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));
-
- __jhash_mix(a,b,c);
-
- k += 12;
- len -= 12;
- }
-
- c += length;
- switch (len) {
- case 11: c += ((u32)k[10]<<24);
- case 10: c += ((u32)k[9]<<16);
- case 9 : c += ((u32)k[8]<<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];
- };
-
- __jhash_mix(a,b,c);
-
- return c;
+/* Best hash sizes are of power of two */
+#define jhash_size(n) ((u32)1<<(n))
+/* Mask the hash value, i.e (value & jhash_mask(n)) instead of (value % n) */
+#define jhash_mask(n) (jhash_size(n)-1)
+
+/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
+#define __jhash_final(a, b, c) \
+{ \
+ c ^= b; c -= rol32(b, 14); \
+ a ^= c; a -= rol32(c, 11); \
+ b ^= a; b -= rol32(a, 25); \
+ c ^= b; c -= rol32(b, 16); \
+ a ^= c; a -= rol32(c, 4); \
+ b ^= a; b -= rol32(a, 14); \
+ c ^= b; c -= rol32(b, 24); \
}

-/* 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 jhash2(const u32 *k, u32 length, u32 initval)
-{
- u32 a, b, c, len;
-
- a = b = JHASH_GOLDEN_RATIO;
- c = initval;
- len = length;
-
- while (len >= 3) {
- a += k[0];
- b += k[1];
- c += k[2];
- __jhash_mix(a, b, c);
- k += 3; len -= 3;
- }
-
- c += length * 4;
-
- switch (len) {
- case 2 : b += k[1];
- case 1 : a += k[0];
- };
-
- __jhash_mix(a,b,c);
-
- return c;
-}
+/* An arbitrary initial parameter */
+#define JHASH_INITVAL 0xdeadbeef

+extern u32 jhash(const void *key, u32 length, u32 initval);
+extern u32 jhash2(const u32 *k, u32 length, u32 initval);

-/* A special ultra-optimized versions that knows they are hashing exactly
- * 3, 2 or 1 word(s).
- *
- * NOTE: In particular the "c += length; __jhash_mix(a,b,c);" normally
- * done at the end is not done here.
- */
+/* jhash_3words - hash exactly 3, 2 or 1 word(s) */
static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
{
- a += JHASH_GOLDEN_RATIO;
- b += JHASH_GOLDEN_RATIO;
+ a += JHASH_INITVAL;
+ b += JHASH_INITVAL;
c += initval;

- __jhash_mix(a, b, c);
+ __jhash_final(a, b, c);

return c;
}
diff --git a/lib/Makefile b/lib/Makefile
index e6a3763..a1a4932 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -10,7 +10,7 @@ endif
lib-y := ctype.o string.o vsprintf.o cmdline.o \
rbtree.o radix-tree.o dump_stack.o \
idr.o int_sqrt.o extable.o prio_tree.o \
- sha1.o irq_regs.o reciprocal_div.o argv_split.o \
+ jhash.o sha1.o irq_regs.o reciprocal_div.o argv_split.o \
proportions.o prio_heap.o ratelimit.o show_mem.o \
is_single_threaded.o plist.o decompress.o flex_array.o

diff --git a/lib/jhash.c b/lib/jhash.c
new file mode 100644
index 0000000..e0c8d11
--- /dev/null
+++ b/lib/jhash.c
@@ -0,0 +1,127 @@
+/* jhash.c: Jenkins hash support.
+ *
+ * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.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) 2009-2010 Jozsef Kadlecsik (kadlec@blackhole.kfki.hu)
+ *
+ * I've modified Bob's hash to be useful in the Linux kernel, and
+ * any bugs present are my fault.
+ * Jozsef
+ */
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/unaligned/packed_struct.h>
+#include <linux/jhash.h>
+
+/* __jhash_mix -- mix 3 32-bit values reversibly. */
+#define __jhash_mix(a, b, c) \
+{ \
+ a -= c; a ^= rol32(c, 4); c += b; \
+ b -= a; b ^= rol32(a, 6); a += c; \
+ c -= b; c ^= rol32(b, 8); b += a; \
+ a -= c; a ^= rol32(c, 16); c += b; \
+ b -= a; b ^= rol32(a, 19); a += c; \
+ c -= b; c ^= rol32(b, 4); b += a; \
+}
+
+/* jhash - hash an arbitrary key
+ * @k: sequence of bytes as key
+ * @length: the length of the key
+ * @initval: the previous hash, or an arbitray value
+ *
+ * The generic version, hashes an arbitrary sequence of bytes.
+ * No alignment or length assumptions are made about the input key.
+ *
+ * Returns the hash value of the key. The result depends on endianness.
+ */
+u32 jhash(const void *key, u32 length, u32 initval)
+{
+ u32 a, b, c;
+ const u8 *k = key;
+
+ /* Set up the internal state */
+ a = b = c = JHASH_INITVAL + length + initval;
+
+ /* All but the last block: affect some 32 bits of (a,b,c) */
+ while (length > 12) {
+ a += __get_unaligned_cpu32(k);
+ b += __get_unaligned_cpu32(k + 4);
+ c += __get_unaligned_cpu32(k + 8);
+ __jhash_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];
+ __jhash_final(a, b, c);
+ case 0: /* Nothing left to add */
+ break;
+ }
+
+ return c;
+}
+EXPORT_SYMBOL(jhash);
+
+/* jhash2 - hash an array of u32's
+ * @k: the key which must be an array of u32's
+ * @length: the number of u32's in the key
+ * @initval: the previous hash, or an arbitray value
+ *
+ * Returns the hash value of the key.
+ */
+u32 jhash2(const u32 *k, u32 length, u32 initval)
+{
+ u32 a, b, c;
+
+ /* Set up the internal state */
+ a = b = c = JHASH_INITVAL + (length<<2) + initval;
+
+ /* Handle most of the key */
+ while (length > 3) {
+ a += k[0];
+ b += k[1];
+ c += k[2];
+ __jhash_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];
+ __jhash_final(a, b, c);
+ case 0: /* Nothing left to add */
+ break;
+ }
+
+ return c;
+}
+EXPORT_SYMBOL(jhash2);
+
--
1.7.0.4
Best regards,
Jozsef
-
E-mail : kadlec@blackhole.kfki.hu, kadlec@mail.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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