On Thursday, February 02, 2017 03:47:55 PM Marcelo Tosatti wrote: > Implement KVM hypercalls for the guest > to issue frequency changes. > > Current situation with DPDK and frequency changes is as follows: > An algorithm in the guest decides when to increase/decrease > frequency based on the queue length of the device. > > On the host, a power manager daemon is used to listen for > frequency change requests (on another core) and issue these > requests. > > However frequency changes are performance sensitive events because: > On a change from low load condition to max load condition, > the frequency should be raised as soon as possible. > Sending a virtio-serial notification to another pCPU, > waiting for that pCPU to initiate an IPI to the requestor pCPU > to change frequency, is slower and more cache costly than > a direct hypercall to host to switch the frequency. > > If the pCPU where the power manager daemon is running > is not busy spinning on requests from the isolated DPDK vcpus, > there is also the cost of HLT wakeup for that pCPU. > > Moreover, the daemon serves multiple VMs, meaning that > the scheme is subject to additional delays from > queueing of power change requests from VMs. > > A direct hypercall from userspace is the fastest most direct > method for the guest to change frequency and does not suffer > from the issues above. > > The usage scenario for this hypercalls is for pinned vCPUs <-> pCPUs. Any chance to CC this to linux-pm in the future? That would help the review quite a bit. Thanks, Rafael
