2 * See the file LICENSE for redistribution information.
4 * Copyright (c) 1996-2009 Oracle. All rights reserved.
15 * Choose a size for the hash table.
17 * PUBLIC: u_int32_t __db_tablesize __P((u_int32_t));
20 __db_tablesize(n_buckets)
24 * We try to be clever about how big we make the hash tables. Use a
25 * prime number close to the "suggested" number of elements that will
26 * be in the hash table. Use 32 as the minimum hash table size.
28 * Ref: Sedgewick, Algorithms in C, "Hash Functions"
30 * Up to ~250,000 buckets, we use powers of 2. After that, we slow
31 * the rate of increase by half. For each choice, we then use a
32 * nearby prime number as the hash value.
34 * If a terabyte is the maximum cache we'll see, and we assume there
35 * are 10 1K buckets on each hash chain, then 107374182 is the maximum
36 * number of buckets we'll ever need.
38 * We don't use the obvious static data structure because some C
39 * compilers (and I use the term loosely), can't handle them.
41 #define HASH_SIZE(power, prime) { \
42 if ((power) >= n_buckets) \
45 HASH_SIZE(32, 37); /* 2^5 */
46 HASH_SIZE(64, 67); /* 2^6 */
47 HASH_SIZE(128, 131); /* 2^7 */
48 HASH_SIZE(256, 257); /* 2^8 */
49 HASH_SIZE(512, 521); /* 2^9 */
50 HASH_SIZE(1024, 1031); /* 2^10 */
51 HASH_SIZE(2048, 2053); /* 2^11 */
52 HASH_SIZE(4096, 4099); /* 2^12 */
53 HASH_SIZE(8192, 8191); /* 2^13 */
54 HASH_SIZE(16384, 16381); /* 2^14 */
55 HASH_SIZE(32768, 32771); /* 2^15 */
56 HASH_SIZE(65536, 65537); /* 2^16 */
57 HASH_SIZE(131072, 131071); /* 2^17 */
58 HASH_SIZE(262144, 262147); /* 2^18 */
59 HASH_SIZE(393216, 393209); /* 2^18 + 2^18/2 */
60 HASH_SIZE(524288, 524287); /* 2^19 */
61 HASH_SIZE(786432, 786431); /* 2^19 + 2^19/2 */
62 HASH_SIZE(1048576, 1048573); /* 2^20 */
63 HASH_SIZE(1572864, 1572869); /* 2^20 + 2^20/2 */
64 HASH_SIZE(2097152, 2097169); /* 2^21 */
65 HASH_SIZE(3145728, 3145721); /* 2^21 + 2^21/2 */
66 HASH_SIZE(4194304, 4194301); /* 2^22 */
67 HASH_SIZE(6291456, 6291449); /* 2^22 + 2^22/2 */
68 HASH_SIZE(8388608, 8388617); /* 2^23 */
69 HASH_SIZE(12582912, 12582917); /* 2^23 + 2^23/2 */
70 HASH_SIZE(16777216, 16777213); /* 2^24 */
71 HASH_SIZE(25165824, 25165813); /* 2^24 + 2^24/2 */
72 HASH_SIZE(33554432, 33554393); /* 2^25 */
73 HASH_SIZE(50331648, 50331653); /* 2^25 + 2^25/2 */
74 HASH_SIZE(67108864, 67108859); /* 2^26 */
75 HASH_SIZE(100663296, 100663291); /* 2^26 + 2^26/2 */
76 HASH_SIZE(134217728, 134217757); /* 2^27 */
77 HASH_SIZE(201326592, 201326611); /* 2^27 + 2^27/2 */
78 HASH_SIZE(268435456, 268435459); /* 2^28 */
79 HASH_SIZE(402653184, 402653189); /* 2^28 + 2^28/2 */
80 HASH_SIZE(536870912, 536870909); /* 2^29 */
81 HASH_SIZE(805306368, 805306357); /* 2^29 + 2^29/2 */
82 HASH_SIZE(1073741824, 1073741827); /* 2^30 */
88 * Initialize a hash table that resides in shared memory.
90 * PUBLIC: void __db_hashinit __P((void *, u_int32_t));
93 __db_hashinit(begin, nelements)
98 SH_TAILQ_HEAD(hash_head) *headp;
100 headp = (struct hash_head *)begin;
102 for (i = 0; i < nelements; i++, headp++)
103 SH_TAILQ_INIT(headp);