Hi Dietmar, On Thu, Apr 14, 2016 at 01:52:43PM +0100, Dietmar Eggemann wrote: > On 13/04/16 19:44, Yuyang Du wrote: > > On Wed, Apr 13, 2016 at 05:28:18PM +0200, Vincent Guittot wrote: > > [...] > > > By "bailing out", you mean return without update because the delta is less > > than 1ms? > > yes. > > > > >>> Examples of 1 periodic task pinned to a cpu on an ARM64 system, HZ=250 > >>> in steady state: > >>> > >>> (1) task runtime = 100us period = 200us > >>> > >>> pelt load/util signal > >>> > >>> 1us: 488-491 > >>> > >>> 1ms: 483-534 > > > > 100us/200us = 50%, so the util should center around 512, it seems in this > > regard, it is better, but the variance is undesirable. > > I see. You mentioned the over-decay thing in the patch header. Is this > also why you change the contribution of the most recent period from 1002 > (1024*y) to 1024?
Yes, it is because that (most recent) period is the "current" period. > This variance gets worse if the ratio runtime/period is further reduced > (e.g. 25us/1000us). > > You can even create tasks which go stealth mode (e.g. 25us/1048us). En... this is a good case to beat it. > It shows periods of 0 load/util (~1.55s) and than massive spikes (~700 for > ~300ms). The short runtime and the task period synced to 1024*1024ns > allow that we hit consecutive enqueues or dequeues for a long time even > the task might drift relative to the pelt window. But whenever we pass 1ms, we will update. And I am curious, how does the current 1us works in this case? Anyway, I will reproduce it myself.
