On Fri, May 1, 2009 at 3:30 AM, Leland Lucius <[email protected]> wrote:
> But, if there's more VCPUs lying about, doesn't z/VM have to schedule > them? If they're "offline" in the guest, doesn't z/VM see that and > ignore 'em when figuring out how to schedule stuff? (I'm trying to talk > WAY over my expertise here, so ignore me if you like. ;-)) Can't ignore you when you ask interesting questions... Not really. CP scheduler and dispatcher work a bit different than Linux. The virtual CPU that does not require service is not involved in scheduling. There's probably some minimal work involved with the plain number of virtual CPUs in the system, so the base cost for 10 servers with 10 virtual CPUs each is more than for 10 virtual-UP servers. But "unplugging" them would not reduce that (in fact, the act of plugging and unplugging does create some work). > Also, if load averages increase sufficiently, doesn't adding another > VCPU help smooth out the load (mainly during spikes)? If you can manage to distribute resources in an unfair manner so that you get more than the rest, then you may be able to take advantage of that. That's the kind of performance measurements that I don't care for. In real life there is little value in tuning the system to make one server run faster at the expense of others (unless that is justified by the business, for example production and development servers). If we think that 2 virtual CPUs is better, then it is better for all of them and all get 2 virtual CPUs. And in that case the contention gets more and chances of getting both your virtual CPUs dispatched at the same time are not better than when everyone was having just one car in the race. I know one case with Domino where we assume 2 virtual CPUs could be better than one. Domino does its own dispatching of tasks, and the idea was that by (bad) design it sometimes would only run the background replication task and not service the end-users. By giving it two half CPUs rather than one full CPU, it could not spend more than half a CPU on the single threaded background process, and thus was forced to also service the end-users. So I stick to my motto: "When you don't know, one will do. When you have measured, probably too." Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ ---------------------------------------------------------------------- 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
