cas_event_mutex has contention for random seed

[jty2]

In [1] and [2], it was identified that the ut_rnd_ulint_counter global variable is updated by every thread for a random backoff spinlock, "leading to cache coherency and performance degradation" under heavy mutex contention. A patch to make the variable thread-local first appeared in mysql-5.7.14, but was then reverted [3], even though the MySQL 5.7.15 Release Notes indicated it as fixed [4]. The patch remained reverted for the rest of the mysql-5.7 series, even in the final 5.7.44 release [5]. The patch finally appeared without reversion in mysql-8.0.0 [6].

Although a fix was implemented, it remained reverted, so this is a performance issue representative of the entire mysql-5.7.x series. Lockhammer portrays the software from external projects as-is, so this will not be fixed except through a whole refresh of code from a newer release of MySQL.

References:

[1] Bug #22733635 CACHE LINE CONTENTION ON UT_RND_ULINT_COUNTER
mysql/mysql-server@32b184c

[2] Cache line contention on ut_rnd_ulint_counter
https://bugs.mysql.com/bug.php?id=80354

[3] Revert "Bug #22733635 CACHE LINE CONTENTION ON UT_RND_ULINT_COUNTER"
mysql/mysql-server@1f906de

[4] Changes in MySQL 5.7.15 (2016-09-06, General Availability)
https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-15.html

InnoDB: A global counter (ut_rnd_ulint_counter) was changed to a thread-local counter to make it scalable on multi-core systems. (Bug #22733635, Bug #80354)

[5]
https://github.com/mysql/mysql-server/blame/mysql-5.7.44/storage/innobase/include/ut0rnd.ic

[6]
https://github.com/mysql/mysql-server/blame/mysql-8.0.0/storage/innobase/include/ut0rnd.ic