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"CPU ballooning" is coming to KVM
Small but interesting bit of news:
https://lwn.net/ml/all/[email protected]/
Typical enterprise workloads are combination of bursty and long running;
their average CPU utilization is low, but they require high core counts
during peak transactions. To accommodate this, customers often
use CPU overcommit strategies i.e. configuring VMs with a large number
of virtual CPUs (vCPUs) while backing them with a smaller, shared pool
of physical CPUs (pCPUs). This achieves a high server consolidation
and excellent cost efficiency.However, when multiple such VMs have high utilization simultaneously,
the shared pCPU pool becomes contended. The hypervisor is forced to preempt
one vCPU to run another to maintain fairness. It maybe schedule vCPU of same
VM or different VM. If a vCPU is preempted while holding a lock or
irq disabled section etc, overall forward progress collapses. There are some
mitigation strategies such as yielding the vCPU to lock-holder, but they
don't cover all the cases. In addition there are hidden costs such as cache,
tlb misses, cost of vCPU preemption, host scheduling overheads etc.Under heavy contention, an effective mitigation strategy for workloads
sensitive to vCPU preemption is for guests/VMs to voluntarily fold their
workloads onto a smaller subset of vCPUs. By demanding fewer pCPUs,
the VMs reduce overall host contention, which decreases vCPU preemption
and improves total throughput for the system.
Just like with memory ballooning, this requires guest cooperation. It could have great benefits when used properly, but it can also cause greedy and dishonest hosts to advertise more vCPUs for a low price than they can deliver.
