> We have a linux guest that got really busy for about 10-15 minutes > nightly. Oracle process. Guest has 2 virtual CPU's defined. > > 1) Our Oracle DBA (consultant and I believe he is coming from an intel > world) says we need more CPU's. I say no. Who's right and why?
This is not a problem unique to VM -- VMWare and Xen suffer the same issue. You're both partially right. The workload may need more REAL CPUs (in that there may not be enough real cycles available to meet the demand at a point in time), but defining more virtual CPUs will probably make the problem worse (your dispatch timeslice for the whole virtual machine is divided as equally as possible between the # of virtual CPUs defined, so defining more virtual CPUs actually DECREASES the amount of processing time available to each virtual CPU per timeslice). It also depends a lot on what the Oracle instance is being asked to do - some activities in Oracle aren't really very MP-friendly, so even if you DID add the virtual CPUs, it wouldn't make any difference because the code won't care (the task is scheduled on a virtual CPU and just runs until the timeslice is exhausted). If you have lots of tasks like that, the number of CPUs is irrelevant; the code is only going to use one at a time. Monitor data on the VM side will tell you more about how the real CPUs are being used in total; the performance data inside the VM will tell you how Linux is allocating workload to the virtual CPUs it sees, but that data alone is totally unreliable for capacity planning. It can only reliably see the division of labor, not the overall available machine usage. > 2) on another guest on the same LPAR, we have 4 CPU's defined just to run > Oracle (for PeopleSoft). I've never seen the CPU's 250% (out of 400%). > Should we drop it down to 3 (The oracle DBA says no and wants more). See above. If he's just looking at data from inside the virtual machine, more virtual CPUs make the problem worse. Ask him what the problem workload is. If it's single long-running queries (Peoplesoft does a lot of those, and they're often stupidly constructed), more CPUs won't help. He'll likely get more bang for the buck optimizing the queries or adding indexes, but that's more work for him. ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
