RE: mutex contention for the query cache

[Rick James]

> A hint for me has been high idle time reported by vmstat for workloads
> that should be CPU bound on multi-core servers. That has been the case
> for Linux 2.6 on 8 core servers.

I continue to run into anecdotal evidence saying:
  * HyperThreading slows down MySQL; turn it off.
  * 8-core Linux box runs slower than 4-core; turn off 4 cores.
  * "My db is melting down".  But the stats show low Disk usage, cpu
never above 12%.  This is on 8-core box.  (12% ~= 1/8, as if ONLY ONE
cpu is being used)
  * "MySQL does a poor job on inter-cpu contention"

Sorry, I don't yet have enough evidence to distinguish these possible
variables:
  * 4.1 or 5.1  (rather current versions of either)
  * RHEL 4u4 or 4u6
  * 32-bit or 64-bit
  * MyISAM or InnoDB

I see that Venu has talked to you:
http://venublog.com/2008/06/04/innodb-performance-on-4-and-8-core-cpu/

Do you have any further comments?  We stress MySQL a lot here.  And some
groups abandon MySQL for the big O.  I would like to make use of 8
cores, or help them otherwise get the most out of their MySQL box.