January 26, 2012 at 5:31 pm
Looking for some thoughts and opinions regarding using processor affinity. We've got a large cluster that hosts, spread over the nodes, 8 different instances supporting (mostly) OLTP loads. The guy before me did some slicing up of the instances for cores using the affinity mask. Most of the isntances are MAXDOP 1, with a couple MAXDOP 4, so we pretty much don't have parallelization. The cluster itself is 4 nodes, with each node having 24 logical CPU cores (DL380s with a beefy processor), so we'll be spreading the instances around that sort of firepower.
While we're not seeing any obvious CPU pressure, I feel like we're not taking full advantage of the resources and I'm looking at changing the affinity settings to open things up, basically setting all instances at cores-2 (leaving 2 cores always reserved for the OS). The only caveat is we're looking to add 3-4 additional instances on to this cluster, so there should be additional load.
Just trying to sniff check my thought process and get some opinions. I appreciate the input!
January 26, 2012 at 6:25 pm
I have looked at this is the past and decided against it.
If we are having trouble with CPU pressure I might consider it but having to take into consideration all other possible combinations of instances and nodes would definitely cause me to not do it without a REALLY good reason.
I had one case and it wasn't clustered where I changed the processor affinity. Two instances, each locked into 4 cores. 2 processor machine with 4 cores per processor. The one instance, if allowed, could monopolize the server, so limiting it to 4 cores made sure it wouldn't happen..
Short answer, don't use processor affinity without a VERY well considered plan taking into account ALL possible combinations of instances and nodes.
CEWII
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