On 12/ 3/16 03:43 AM, Nahum Shalman wrote:
I like to think about CPU as a resource either under contention (more
processes need CPU time than CPU cores to go around) or not:
CPU caps limit how much you can use when there's no contention. In the
Joyent Could it ensures people don't get more CPU time than they pay
for. (example 1 below)
CPU shares ensure fairness when there is contention. If CPU shares are
doled out in proportion to memory doled out (Like in the Joyent
Cloud), when everyone is fighting for CPU, the more memory you're
paying for, the more CPU cycles you'll get. (example 2 below)
If all the zones on a box belong to the same customer (as might be the
case on your machine), you could set the cpu caps much higher, and
depend on the shares for fairness.
Examples for clarification:
1. I fire up a g4-highcpu-2G zone in the Joyent cloud. It gets a CPU
cap of 100. It happens to land on a freshly deployed compute node with
no other zones on it.
I then run 2 CPU intensive processes. There is no contention so the
machine is totally capable of scheduling each of them on a separate
CPU and running them full throttle.
However, because I'm only paying for 1 1vCPU I hit that cap of 100 and
my two processes have to take turns running. If my workload is really
that CPU intensive I should scale the zone up to g4-highcpu-4G which
gets 2vCPUs and a cap of 200.
Hello Nahum,
The problem with the current scheme is when you hit a capped zone with
something like a parallel compile. The build tool will see all of the
cores and happily spin up a compile or two on each. So on a decent
machine like the one Len describes, there will be 64 CPU intensive jobs
running on a machine with a cap one CPU... The result is the load
average goes through the roof and the the box falls over. If you do
this in a KVM, the KVM will curl up and die.
The only way I know to properly confine a guest is to use a KVM with
matching CPU cap and count.
Cheers,
--
Ian.
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