Messages in this thread | | | Date | Thu, 21 May 2015 07:14:09 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set |
| |
On Thu, May 21, 2015 at 06:59:05PM +0530, Afzal Mohammed wrote: > Hi, > > On Thu, May 21, 2015 at 03:06:23PM +0200, Frederic Weisbecker wrote: > > On Thu, May 21, 2015 at 05:57:59AM -0700, Paul E. McKenney wrote: > > > > Indeed, NO_HZ_FULL is special purpose. You normally would select > > > NO_HZ_FULL_ALL only on a system intended for heavy compute without > > > normal-workload distractions or for some real-time systems. For mixed > > > workloads, you would build with NO_HZ_FULL (but not NO_HZ_FULL_ALL) and > > > use the boot parameters to select which CPUs are to be running the > > > specialized portion of the workload. > > > > > > And you would of course need to lead enough CPUs running normally to > > > handle the non-specialized portion of the workload. > > > > > > This sort of thing has traditionally required specialized kernels, > > > so the cool thing here is that we can make Linux do it. Though, as > > > you noticed, careful configuration is still required. > > > > > > Seem reasonable? > > Yes, thanks, some dots got connected :) > > > That said if he saw a big performance regression after applying these patches, > > then there is likely a problem in the patchset. Well it could be due to that mode > > which loops on full dynticks before resuming to userspace. Indeed when that is > > enabled, I expect real throughput issues on workloads doing lots of kernel <-> > > userspace roundtrips. We just need to make sure this thing only works when requested. > > With this change (& having NO_HZ_FULL_ALL), hackbench was being served > only by the boot cpu, while w/o this change, all 8 (this is a quad > core HT processor) was being used - observation based on 'top'.
Good to know! And it will be interesting to see what Frederic decides based on his review of the patchset.
Thanx, Paul
| |