On Fri, Sep 29, 2017 at 07:05:41PM +0200, Paolo Bonzini wrote: > On 29/09/2017 18:40, Marcelo Tosatti wrote: > >> If you know you have this kind disk workload, you must use virtio-blk or > >> virtio-scsi with iothreads and place the iothreads on their own physical > >> CPUs. > >> > >> Among "run arbitrary workloads", "run real-time workloads", "pack stuff > >> into as few physical CPUs as possible", you can only pick two. > > > > Thats not the state of things (userspace in vcpu-0 is not specially tailored > > to not violate latencies in vcpu-1): that is not all user triggered > > actions can be verified. > > > > Think "updatedb", and so on... > > _Which_ spinlock is it that can cause unwanted latency while running > updatedb on VCPU0 and a real-time workload on VCPU1, and only so on virt > because of the emulator thread? Hundreds of them (the one being hit is in timer_interrupt), but i went to check and there are hundreds of raw spinlocks shared between the kernel threads that run on isolated CPUs and vcpu-0. > Is this still broken if you set up > priorities for the emulator thread correctly and use PI mutexes in QEMU? I don't see why it would not, if you have to schedule the emulator thread to process and inject I/O interrupts for example. > And if so, what is the cause of interruptions in the emulator thread > and how are these interruptions causing the jitter? Interrupt injections. > Priorities and priority inheritance (or lack of them) is a _known_ > issue. Jan was doing his KVM-RT things in 2009 and he was talking about > priorities[1] back then. The effect of correct priorities is to _lower_ > jitter, not to make it worse, and anyway certainly not worse than > SCHED_NORMAL I/O thread. Once that's fixed, we can look at other problems. > > Paolo > > [1] http://static.lwn.net/images/conf/rtlws11/papers/proc/p18.pdf which > also mentions pv scheduling
