2016-05-25 6:38 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>: > On Tue, May 24, 2016 at 12:57 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote: >> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> >> >> If an emulated lapic timer will fire soon(in the scope of 10us the >> base of dynamic halt-polling, lower-end of message passing workload >> latency TCP_RR's poll time < 10us) we can treat it as a short halt, >> and poll to wait it fire, the fire callback apic_timer_fn() will set >> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll. >> This can avoid context switch overhead and the latency which we wake >> up vCPU. >> >> This feature is slightly different from current advance expiration >> way. Advance expiration rely on the vCPU is running(do polling before >> vmentry). But in some cases, the timer interrupt may be blocked by >> other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to >> run immediately. So even advance the timer early, vCPU may still see >> the latency. But polling is different, it ensures the vCPU to aware >> the timer expiration before schedule out. >> >> echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests. >> >> Context switching - times in microseconds - smaller is better >> ------------------------------------------------------------------------- >> Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K >> ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw >> --------- ------------- ------ ------ ------ ------ ------ ------- ------- >> kernel Linux 4.6.0+ 7.9800 11.0 10.8 14.6 9.4300 13.0 10.2 vanilla >> kernel Linux 4.6.0+ 15.3 13.6 10.7 12.5 9.0000 12.8 7.38000 poll > > These results aren't very compelling. Sometimes polling is faster, > sometimes vanilla is faster, sometimes they are about the same. More processes and bigger cache footprints can get benefit from the result since I open the hrtimer for the precision preemption. Actually I try to emulate Yang's workload, https://lkml.org/lkml/2016/5/22/162. And his real workload can get more benefit as he mentioned, https://lkml.org/lkml/2016/5/19/667. > I imagine there are hyper sensitive workloads which cannot tolerate a > long tail in timer latency (e.g. realtime workloads). I would expect a > patch like this to provide a "smoothing effect", reducing that tail. > But for cloud/server workloads, I would not expect any sensitivity to > jitter in timer latency (especially while the VCPU is halted). Yang's is real cloud workload. > > Note that while halt-polling happens when the CPU is idle, it's still > not free. It constricts the scheduler's cpu load balancer, because the > CPU appears to be busy. In KVM's default configuration, I'd prefer to > only add more polling when the gain is clear. If there are guest > workloads that want this patch, I'd suggest polling for timers be > default-off. At minimum, there should be a module parameter to control > it (like Christian Borntraeger suggested). Yeah, I will add the module parameter in order to enable/disable. Regards, Wanpeng Li -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
