On Mon, Aug 07, 2017 at 06:32:16PM +0300, Alexey Budankov wrote: > On 07.08.2017 12:13, Peter Zijlstra wrote: > > On Mon, Aug 07, 2017 at 10:39:13AM +0200, Peter Zijlstra wrote: > >> On Mon, Aug 07, 2017 at 10:17:46AM +0300, Alexey Budankov wrote: > >>> Makes sense. The implementation becomes a bit simpler. The drawbacks > >>> may be several rotations of potentially big tree on the critical path, > >>> instead of updating four pointers in case of the tree of lists. > >> > >> Yes, but like said, it allows implementing a better scheduler than RR, > >> allowing us to fix rotation artifacts where task runtimes are near the > >> rotation window. > > Could you elaborate more on the artifacts or my be share some link to the > theory?
In the extreme, if you construct your program such that you'll never get hit by the tick (this used to be a popular measure to hide yourself from time accounting), you'll never rotate the counters, even though you can rack up quite a lot of runtime. By doing a runtime based scheduler, instead of a tick based RR, we'll still get rotation, and the tick will only function as a forced reprogram point. > >> A slightly more complicated, but also interested scheduling problem is > >> the per-cpu flexible vs the per-task flexible. Ideally we'd rotate them > >> at the same priority based on service, without strictly prioritizing the > >> per-cpu events. > >> > >> Again, that is something that should be possible once we have a more > >> capable event scheduler. > >> > >> > >> So yes, cons and pros.. :-) > > > > Also, I think for AVL tree you could do the erase and (re)insert > > combined and then rebalance in one go, not sure RB allows the same > > thing, but it might be fun looking into. > > Not sure if AVL is more practical here. You get better balancing what gives > you faster average search for the price of longer modifications > so yes, need to measure and compare ... :-) Oh, I wasn't suggesting using AVL (the last thing we need is another balanced tree in the kernel), I was merely wondering if you could do compound/bulk updates on RB as you can with AVL.
