Three cheers for the Fair Share Scheduler, 22/05/07, Secret Plans and Clever Tricks

May 22, 2007

Three cheers for the Fair Share Scheduler

Writing about web page http://www.sun.com/software/solaris/utilization.jsp

The more I use my Solaris Zones boxes, the more (mostly) I like them. Yes, there are some niggles about how you cope with container failures, how you clone zones between boxes, the odd un-killable process, and so on, but for the most part, they just do exactly what you’d expect them to, all the time.

Take the FSS for instance. This little widget takes care of allocating CPU between your zones. A big problem in server consolidation, at least in web-land, is the “spiky” nature of CPU usage; Web-apps tend to be relatively low consumers of CPU most of the time, but occasionally will want a much larger amount.
If you’re consolidating apps, you don’t want one busy app to steal CPU off all the others, but if all the others are idle, then you might as well let the busy app take whatever it needs.

The FSS solves this problem elegantly. Each zone is allocated “shares” representing the minimum proportion of CPU that it is able to allocate if needed. So if I have 3 zones, and give them each 1 share, then if each zone is working flat out, they’ll get 1/3 of the CPU time allocated. But if one zone goes idle, the other two will get 50% each. If only one zone is busy, it’ll get 100%. Better still, if one zone has 100% of CPU, and another zone becomes busy, the first is reined in instantly to give the other one the CPU it’s entitled to.

And does it work in real life? Oh yes…here’s one of our apps getting a bit overloaded. You can see the box load go up to 20 (on an 8-way box; in this case it was about 99% busy for 15-20 minutes), and the zone that’s causing all the trouble gets pretty unresponsive. But the other zone doesn’t even register the extra CPU load. Awesome.