We lately moved to MySQL V5.5 on CentOS release 6.7 with master and 2 slaves. Since then the replication process lags a lot behind the master with the line in the process list of:
invalidating query cache entries (table).
The global query_cache_type & size were set to 0.
I have checked around and I can see it's a known old bug in MySQL but could not find a workaround for this issue.
- query_cache_type OFF in my.cnf before server restart.
- query_cache_wlock_invalidate set to OFF.
- using default % storage_engine as InnoDB.
We ran some more tests and we had an issue with disk IO -
sar -u showed about 3-4 %iowait, disks infra changes help reduce this to about 1.5-2% but it still doesn't help the replication not lagging for 5000(!) seconds behind the master.