> From: Vivek Goyal <vgoyal@xxxxxxxxxx> > Date: Fri, May 29, 2009 12:06:10PM -0400 > > On Thu, May 28, 2009 at 12:41:27PM -0700, Nauman Rafique wrote: ... > > I have some concerns about the new preemption logic. > > Actually we need a more proper definition of in-class preemption. Across > class preemption means that RT class always gets to run first. > > What does in-class preemption mean? If I look at the current CFQ code, > it does look like that preempting process will gain share. It is always > added to the front of the tree with "rb_key=0" and that means, this new > queue will get fresh time slice (even if it got time slice very recently). > > Currently I have just tried to make the behavior same as CFQ to reduce > the possiblility of regressions. That's a different thing that we can > discuss what should be the exact behavior in case of in-class preemption > and first it needs to be fixed in CFQ, if current behavior is an issue. > > On the other hand, I am not sure if previous bfq preemption logic was > working. We were checking if the new request belonged to the queue which > will be served next, then preempt the existing queue. While looking > for the next queue, I think we did not consider the current active > entity (as it was not on the tree). So after expiry of the current > queue, it might get selected next if it has not got its share. So there > was no point in preempting the queue. If queue already got its share, then > anyway the next queue will be selected next and there is no point in > preempting the current queue. > BFQ had no preemption logic, as far as I know; it simply was not preemptive, and the guarantees it provided took that into account. I don't know what is the best way to introduce a CFQ-like preemption logic into the wf2q+ code; for sure anything that does not schedule according to the algorithm's timestamps is a good candidate to break the guarantees the scheduler can provide, making it an extremely complex way to get the same worst-case delays of a (much simpler) round-robin scheduler. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
