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? 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). 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. [...]
