> We are occasionally seeing heavy CPU contention with hundreds of processes active but waiting on a lightweight lock - usually lock manager or buffer mapping it seems. This is happening with VMs configured with about 64 CPUs, 350GBs ram, and while we would typically only have 30-100 concurrent processes, there will suddenly be ~300 and many show active with LwLock and they take much longer than usual. Any suggested options to monitor for such issues or logging to setup so the next issue can be debugged properly? > > It has seemed to me that this occurs when there are more than the usual number of a particular process type and also something that is a bit heavy in usage of memory/disk. It has happened on various tenant instances and different application processes as well. > > Would/how might the use of huge pages (or transparent huge pages, or OFF) play into this scenario? I've also been contending with a good bit of lightweight lock contention that causes performance issues. Most often we see this with the WAL write lock, but when we get too many parallel queries running we end up in a "thundering herd" type of issue were the contention for the lock manager lock consumes significant CPU resources causing the number of parallel queries to increase as more clients back up behind the lock contention leading to even more lock contention. When this happens we have to pause our background workers long enough to allow the lock contention to reduce and then we can resume our background workers. When we hit the lock contention it's not a gradual degredation, it goes immediately from nothing more than 100% CPU usage. The same is true when reducing the lock contention - it goes from 100% to nothing. I've been working under the assumption that this has to do with our native partitioning scheme and the fact that some queries cannot take advantage of partition pruning because they don't contain the partition column. My understanding is that when this happens ACCESS SHARED locks have to be taken on all tables as well as all associated resources (indexes, sequences, etc.) and the act of taking and releasing all of those locks will increase the lock contention significantly. We're working to update our application so that we can take advantage of the pruning. Are you also using native partitioning? - Chris
