On Thu, 2016-06-16 at 04:19 -0700, Eric Dumazet wrote: > On Thu, Jun 16, 2016 at 3:39 AM, Paolo Abeni <pab...@redhat.com> wrote: > > We used a different setup to explicitly avoid the (guest) userspace > > starvation issue. Using a guest with 2vCPUs (or more) and a single queue > > avoids the starvation issue, because the scheduler moves the user space > > processes on a different vCPU in respect to the ksoftirqd thread. > > > > In the hypervisor, with a vanilla kernel, the qemu process receives a > > fair share of the cpu time, but considerably less 100%, and his > > performances are bounded to a considerable lower throughput than the > > theoretical one. > > > > Completely different setup than last time. I am kind of lost. > > Are you trying to find the optimal way to demonstrate your patch can be > useful ? > > In a case with 2 vcpus, then the _standard_ kernel will migrate the > user thread on the cpu not used by the IRQ, > once process scheduler can see two threads competing on one cpu > (ksoftirqd and the user thread), and the other cpu being idle. > > Trying to shift the IRQ 'thread' is not nice, since the hardware IRQ > will be delivered on the wrong cpu. > > Unless user space forces cpu pinning ? Then tell the user it should not. > > The natural choice is to put both producer and consumer on same cpu > for cache locality reasons (wake affine), > but in stress mode allow to run the consumer on another cpu if available. > > If the process scheduler fails to migrate the producer, then there is > a bug needing to be fixed.
I guess you means 'consumer' here. The scheduler doesn't fail to migrate it: the consumer is actually migrated a lot of times, but on each cpu a competing and running ksoftirqd thread is found. The general problem is that under significant network load (not necessary udp flood, similar behavior is observed even with TCP_RR tests), with enough rx queue available and enough flows running, no single thread/process can use 100% of any cpu, even if the overall capacity would allow it. Paolo
