Fix random element selection for large hash tables. (#8133)

When a database on a 64 bit build grows past 2^31 keys, the underlying hash table expands to 2^32 buckets. After this point, the algorithms for selecting random elements only return elements from half of the available buckets because they use random() which has a range of 0 to 2^31 - 1. This causes problems for eviction policies which use dictGetSomeKeys or dictGetRandomKey. Over time they cause the hash table to become unbalanced because, while new keys are spread out evenly across all buckets, evictions come from only half of the available buckets. Eventually this half of the table starts to run out of keys and it takes longer and longer to find candidates for eviction. This continues until no more evictions can happen.

This solution addresses this by using a 64 bit PRNG instead of libc random().

Co-authored-by: Greg Femec <gfemec@google.com>

1 files changed, 87 insertions, 0 deletions

diff --git a/src/mt19937-64.h b/src/mt19937-64.h
new file mode 100644
index 000000000..b98348fd4
--- /dev/null
+++ b/src/mt19937-64.h
@@ -0,0 +1,87 @@
+/*
+   A C-program for MT19937-64 (2004/9/29 version).
+   Coded by Takuji Nishimura and Makoto Matsumoto.
+
+   This is a 64-bit version of Mersenne Twister pseudorandom number
+   generator.
+
+   Before using, initialize the state by using init_genrand64(seed)
+   or init_by_array64(init_key, key_length).
+
+   Copyright (C) 2004, Makoto Matsumoto and Takuji Nishimura,
+   All rights reserved.
+
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions
+   are met:
+
+     1. Redistributions of source code must retain the above copyright
+        notice, this list of conditions and the following disclaimer.
+
+     2. Redistributions in binary form must reproduce the above copyright
+        notice, this list of conditions and the following disclaimer in the
+        documentation and/or other materials provided with the distribution.
+
+     3. The names of its contributors may not be used to endorse or promote
+        products derived from this software without specific prior written
+        permission.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+   References:
+   T. Nishimura, ``Tables of 64-bit Mersenne Twisters''
+     ACM Transactions on Modeling and
+     Computer Simulation 10. (2000) 348--357.
+   M. Matsumoto and T. Nishimura,
+     ``Mersenne Twister: a 623-dimensionally equidistributed
+       uniform pseudorandom number generator''
+     ACM Transactions on Modeling and
+     Computer Simulation 8. (Jan. 1998) 3--30.
+
+   Any feedback is very welcome.
+   http://www.math.hiroshima-u.ac.jp/~m-mat/MT/emt.html
+   email: m-mat @ math.sci.hiroshima-u.ac.jp (remove spaces)
+*/
+
+#ifndef __MT19937_64_H
+#define __MT19937_64_H
+
+/* initializes mt[NN] with a seed */
+void init_genrand64(unsigned long long seed);
+
+/* initialize by an array with array-length */
+/* init_key is the array for initializing keys */
+/* key_length is its length */
+void init_by_array64(unsigned long long init_key[],
+                     unsigned long long key_length);
+
+/* generates a random number on [0, 2^64-1]-interval */
+unsigned long long genrand64_int64(void);
+
+
+/* generates a random number on [0, 2^63-1]-interval */
+long long genrand64_int63(void);
+
+/* generates a random number on [0,1]-real-interval */
+double genrand64_real1(void);
+
+/* generates a random number on [0,1)-real-interval */
+double genrand64_real2(void);
+
+/* generates a random number on (0,1)-real-interval */
+double genrand64_real3(void);
+
+/* generates a random number on (0,1]-real-interval */
+double genrand64_real4(void);
+
+#endif