mirror of
https://github.com/openvswitch/ovs
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c49d1dd168
murmurhash is faster than Jenkins and slightly higher quality, so switch to it for hashing words. The best timings I got for hashing for data lengths of the following numbers of 32-bit words, in seconds per 1,000,000,000 hashes, were: words murmurhash Jenkins hash ----- ---------- ------------ 1 8.4 10.4 2 10.3 10.3 3 11.2 10.7 4 12.6 18.0 5 13.9 18.3 6 15.2 18.7 In other words, murmurhash outperforms Jenkins for all input lengths other than exactly 3 32-bit words (12 bytes). (It's understandable that Jenkins would have a best case at 12 bytes, because Jenkins works in 12-byte chunks.) Even in the case where Jenkins is faster, it's only by 5%. On average within this data set, murmurhash is 15% faster, and for 4-word input it is 30% faster. We retain Jenkins for flow_hash_symmetric_l4() and flow_hash_fields(), which are cases where the hash value is exposed externally. This commit appears to improve "ovs-benchmark rate" results slightly by a few hundred connections per second (under 1%), when used with an NVP controller. Signed-off-by: Ben Pfaff <blp@nicira.com> Acked-by: Ethan Jackson <ethan@nicira.com>
126 lines
3.7 KiB
C
126 lines
3.7 KiB
C
/*
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* Copyright (c) 2008, 2009, 2010, 2012, 2013 Nicira, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef HASH_H
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#define HASH_H 1
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#include <stdbool.h>
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#include <stddef.h>
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#include <stdint.h>
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#include <string.h>
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#include "util.h"
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#ifdef __cplusplus
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extern "C" {
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#endif
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static inline uint32_t
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hash_rot(uint32_t x, int k)
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{
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return (x << k) | (x >> (32 - k));
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}
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uint32_t hash_words(const uint32_t data[], size_t n_words, uint32_t basis);
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uint32_t hash_bytes(const void *, size_t n_bytes, uint32_t basis);
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static inline uint32_t hash_int(uint32_t x, uint32_t basis);
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static inline uint32_t hash_2words(uint32_t, uint32_t);
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uint32_t hash_3words(uint32_t, uint32_t, uint32_t);
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static inline uint32_t hash_boolean(bool x, uint32_t basis);
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uint32_t hash_double(double, uint32_t basis);
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static inline uint32_t hash_pointer(const void *, uint32_t basis);
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static inline uint32_t hash_string(const char *, uint32_t basis);
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/* Murmurhash by Austin Appleby,
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* from http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp.
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*
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* The upstream license there says:
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*
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* // MurmurHash3 was written by Austin Appleby, and is placed in the public
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* // domain. The author hereby disclaims copyright to this source code.
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*
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* See hash_words() for sample usage. */
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static inline uint32_t mhash_add__(uint32_t hash, uint32_t data)
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{
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data *= 0xcc9e2d51;
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data = hash_rot(data, 15);
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data *= 0x1b873593;
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return hash ^ data;
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}
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static inline uint32_t mhash_add(uint32_t hash, uint32_t data)
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{
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hash = mhash_add__(hash, data);
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hash = hash_rot(hash, 13);
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return hash * 5 + 0xe6546b64;
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}
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static inline uint32_t mhash_finish(uint32_t hash, size_t n_bytes)
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{
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hash ^= n_bytes;
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hash ^= hash >> 16;
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hash *= 0x85ebca6b;
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hash ^= hash >> 13;
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hash *= 0xc2b2ae35;
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hash ^= hash >> 16;
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return hash;
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}
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static inline uint32_t hash_string(const char *s, uint32_t basis)
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{
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return hash_bytes(s, strlen(s), basis);
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}
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static inline uint32_t hash_int(uint32_t x, uint32_t basis)
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{
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return hash_2words(x, basis);
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}
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/* An attempt at a useful 1-bit hash function. Has not been analyzed for
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* quality. */
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static inline uint32_t hash_boolean(bool x, uint32_t basis)
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{
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const uint32_t P0 = 0xc2b73583; /* This is hash_int(1, 0). */
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const uint32_t P1 = 0xe90f1258; /* This is hash_int(2, 0). */
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return (x ? P0 : P1) ^ hash_rot(basis, 1);
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}
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static inline uint32_t hash_pointer(const void *p, uint32_t basis)
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{
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/* Often pointers are hashed simply by casting to integer type, but that
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* has pitfalls since the lower bits of a pointer are often all 0 for
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* alignment reasons. It's hard to guess where the entropy really is, so
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* we give up here and just use a high-quality hash function.
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*
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* The double cast suppresses a warning on 64-bit systems about casting to
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* an integer to different size. That's OK in this case, since most of the
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* entropy in the pointer is almost certainly in the lower 32 bits. */
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return hash_int((uint32_t) (uintptr_t) p, basis);
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}
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static inline uint32_t hash_2words(uint32_t x, uint32_t y)
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{
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return mhash_finish(mhash_add(mhash_add(x, 0), y), 4);
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}
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#ifdef __cplusplus
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}
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#endif
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#endif /* hash.h */
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