Modern x86-64 servers, even 2 socket ones, typically now have many cores (e.g. 2*8, 2*12, 2*18) so, with Intel hyper-threading 32 or 48 vCPUs is not unusual. Given that Oracle software is generally licensed by core, and out of the box Dom0 will run on any core, I'm investigating whether there are benefits to pinning Dom0 to specific cores.
I think there could be three possible advantages:
- Under very heavy guest vCPU load, when most/all of the vCPUs are allocated, then Dom0 could always have its own CPUs to process device driver interrupts and so on, thus making the overall system more stable
- If Dom0 code always runs on the same small number of CPUs there are fewer scheduling context switches and possibly better L2 caching (?)
- No cycles are used for Dom0 on cores that have DomUs pinned to them, i.e. licensed for other Oracle products
I see that Tuning Xen for Performance - Xen suggests you should not have fewer than 4 vCPUs for Dom0. The key consideration here is going to be how much CPU Dom0 in your own environment needs - that is going to vary on how many VMs you have and whether they are HVM, PVHVM or PVM, plus I suppose how much they are using various devices. Although tuning Dom0 memory is mentioned in the OVM docs I can't see anything on tuning Dom0 CPU.
Has anyone looked into this?
Simon
PS. this question did come up in but I thought I'd start a new discussion to keep it focussed.