On Wed, Jun 16, 2010 at 10:39:41AM +0200, Ingo Molnar wrote: > > (Cc:-ed various performance/optimization folks) > > * Avi Kivity <avi@xxxxxxxxxx> wrote: > > > On 06/16/2010 10:32 AM, H. Peter Anvin wrote: > > >On 06/16/2010 12:24 AM, Avi Kivity wrote: > > >>Ingo, Peter, any feedback on this? > > > Conceptually, this makes sense to me. However, I have a concern what > > > happens when a task is scheduled on another CPU, while its FPU state is > > > still in registers in the original CPU. That would seem to require > > > expensive IPIs to spill the state in order for the rescheduling to > > > proceed, and this could really damage performance. > > > > Right, this optimization isn't free. > > > > I think the tradeoff is favourable since task migrations are much > > less frequent than context switches within the same cpu, can the > > scheduler experts comment? > > This cannot be stated categorically without precise measurements of > known-good, known-bad, average FPU usage and average CPU usage scenarios. All > these workloads have different characteristics. > > I can imagine bad effects across all sorts of workloads: tcpbench, AIM7, > various lmbench components, X benchmarks, tiobench - you name it. Combined > with the fact that most micro-benchmarks wont be using the FPU, while in the > long run most processes will be using the FPU due to SIMM instructions. So > even a positive result might be skewed in practice. Has to be measured > carefully IMO - and i havent seen a _single_ performance measurement in the > submission mail. This is really essential. It can be nice to code an absolute worst-case microbenchmark too. Task migration can actually be very important to the point of being almost a fastpath in some workloads where threads are oversubscribed to CPUs and blocking on some contented resource (IO or mutex or whatever). I suspect the main issues in that case is the actual context switching and contention, but it would be nice to see just how much slower it could get. -- 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
