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MDEV-21256: Replace the 64-bit LCG with a 32-bit Galois LFSR

We should not need anywhere near 32 bits of entropy, so we might
just limit ourselves to a 32-bit random number generator.

Also, it might be cheaper to use exclusive-or, bit shifting and
conditional jumps, instead of multiplication and addition.

We use relaxed atomic operations on the global random number generator
state in order in an attempt to silence any warnings about race conditions.
There is an obvious race condition between the load and store in
ut_rnd_gen(), but we do not think that it matters much that the
state of the random number generator could 'stutter'.

This change seems makes the 'uncompress_ops' nondeterministic
in innodb_zip.cmp_per_index after the restart. It looks like
there is an inherent race condition in the test, because the
table could be opened for InnoDB statistics recalculation
already before innodb_cmp_per_index_enabled was set. We might
end up having uncompress_ops anywhere between 0 and 9, or perhaps
even more. Let us remove that part of the test.
This commit is contained in:
Marko Mäkelä
2019-12-10 11:20:26 +02:00
parent d146e3dcfe
commit b1f2d3a8c8
4 changed files with 32 additions and 72 deletions

View File

@ -1,3 +1,4 @@
SET @save_enabled= @@GLOBAL.innodb_cmp_per_index_enabled;
SET GLOBAL innodb_cmp_per_index_enabled=ON;
SELECT * FROM information_schema.innodb_cmp_per_index;
CREATE TABLE t (
@ -70,33 +71,5 @@ index_name PRIMARY
compress_ops 65
compress_ops_ok 65
uncompress_ops 0
SHOW CREATE TABLE t;
Table t
Create Table CREATE TABLE `t` (
`a` int(11) NOT NULL,
`b` varchar(512) DEFAULT NULL,
`c` varchar(16) DEFAULT NULL,
PRIMARY KEY (`a`),
KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 KEY_BLOCK_SIZE=2
SET GLOBAL innodb_cmp_per_index_enabled=ON;
SELECT COUNT(*) FROM t IGNORE INDEX(b);
COUNT(*) 128
SELECT
database_name,
table_name,
index_name,
compress_ops,
compress_ops_ok,
CASE WHEN uncompress_ops=6 and @@innodb_compression_level IN (4,8,9) THEN 9
ELSE uncompress_ops END as uncompress_ops
FROM information_schema.innodb_cmp_per_index
ORDER BY 1, 2, 3;
database_name test
table_name t
index_name PRIMARY
compress_ops 0
compress_ops_ok 0
uncompress_ops 4
DROP TABLE t;
SET GLOBAL innodb_cmp_per_index_enabled=default;
SET GLOBAL innodb_cmp_per_index_enabled=@save_enabled;