berkeley db 内存池hash table的大小

BDB中 内存池是用一个 hash table来管理 buffers. 这个hash table的大小首先是和 内存池的大小成正比. 

其次, bdb code会经常遍历hash chain, 它的长度 不能太大. 计算公式为:

  cache 大小/region数目 = 每个region的大小
  bucket数目= __db_tablesize(region大小 / 2.5个page size)))

 

代码:

mp_region.c, __memp_region_size():

cache_size = (roff_t)dbenv->mp_gbytes * GIGABYTE + dbenv->mp_bytes; // mpool的总大小
reg_size = cache_size / dbenv->mp_ncache; // 计算出每一个mpool region的大小
if (reg_sizep != NULL)
    *reg_sizep = reg_size;

if (htab_bucketsp != NULL) {
    if (dbenv->mp_tablesize != 0)
        *htab_bucketsp = __db_tablesize(dbenv->mp_tablesize);
    else {
        if ((pgsize = dbenv->mp_pagesize) == 0)
            pgsize = MPOOL_DEFAULT_PAGESIZE; // 默认为4k
        *htab_bucketsp = __db_tablesize(
            (u_int32_t)(reg_size / (2.5 * pgsize)));
    }
}

db_shash.c, __db_tablesize():

  /*
     * We try to be clever about how big we make the hash tables.  Use a
     * prime number close to the "suggested" number of elements that will
     * be in the hash table.  Use 16 as the minimum hash table size.
     *
     * Ref: Sedgewick, Algorithms in C, "Hash Functions"
     *
     * Up to ~250,000 buckets, we use powers of 2.  After that, we slow
     * the rate of increase by half.  For each choice, we then use a
     * nearby prime number as the hash value.
     *
     * If a terabyte is the maximum cache we'll see, and we assume there
     * are 10 1K buckets on each hash chain, then 107374182 is the maximum
     * number of buckets we'll ever need.
     *
     * We don't use the obvious static data structure because some C
     * compilers (and I use the term loosely), can't handle them.
     */
#define    HASH_SIZE(power, prime) {                    \
    if ((power) >= n_buckets)                    \
        return (prime);                        \
}
    HASH_SIZE(16, 17);            /* 2^4 */
    HASH_SIZE(32, 37);            /* 2^5 */
    HASH_SIZE(64, 67);            /* 2^6 */
    HASH_SIZE(128, 131);            /* 2^7 */
    HASH_SIZE(256, 257);            /* 2^8 */
    HASH_SIZE(512, 521);            /* 2^9 */
    HASH_SIZE(1024, 1031);            /* 2^10 */
    HASH_SIZE(2048, 2053);            /* 2^11 */
    HASH_SIZE(4096, 4099);            /* 2^12 */
    HASH_SIZE(8192, 8191);            /* 2^13 */
    HASH_SIZE(16384, 16381);        /* 2^14 */
    HASH_SIZE(32768, 32771);        /* 2^15 */
    HASH_SIZE(65536, 65537);        /* 2^16 */
    HASH_SIZE(131072, 131071);        /* 2^17 */
    HASH_SIZE(262144, 262147);        /* 2^18 */
    HASH_SIZE(393216, 393209);        /* 2^18 + 2^18/2 */
    HASH_SIZE(524288, 524287);        /* 2^19 */
    HASH_SIZE(786432, 786431);        /* 2^19 + 2^19/2 */
    HASH_SIZE(1048576, 1048573);        /* 2^20 */
    HASH_SIZE(1572864, 1572869);        /* 2^20 + 2^20/2 */
    HASH_SIZE(2097152, 2097169);        /* 2^21 */
    HASH_SIZE(3145728, 3145721);        /* 2^21 + 2^21/2 */
    HASH_SIZE(4194304, 4194301);        /* 2^22 */
    HASH_SIZE(6291456, 6291449);        /* 2^22 + 2^22/2 */
    HASH_SIZE(8388608, 8388617);        /* 2^23 */
    HASH_SIZE(12582912, 12582917);        /* 2^23 + 2^23/2 */
    HASH_SIZE(16777216, 16777213);        /* 2^24 */
    HASH_SIZE(25165824, 25165813);        /* 2^24 + 2^24/2 */
    HASH_SIZE(33554432, 33554393);        /* 2^25 */
    HASH_SIZE(50331648, 50331653);        /* 2^25 + 2^25/2 */
    HASH_SIZE(67108864, 67108859);        /* 2^26 */
    HASH_SIZE(100663296, 100663291);    /* 2^26 + 2^26/2 */
    HASH_SIZE(134217728, 134217757);    /* 2^27 */
    HASH_SIZE(201326592, 201326611);    /* 2^27 + 2^27/2 */
    HASH_SIZE(268435456, 268435459);    /* 2^28 */
    HASH_SIZE(402653184, 402653189);    /* 2^28 + 2^28/2 */
    HASH_SIZE(536870912, 536870909);    /* 2^29 */
    HASH_SIZE(805306368, 805306357);    /* 2^29 + 2^29/2 */
    HASH_SIZE(1073741824, 1073741827);    /* 2^30 */
    return (1073741827);
}