On Tue, Dec 14, 2021 at 12:42:58PM +0100, Christoph Lameter wrote: > On Tue, 14 Dec 2021, Marcelo Tosatti wrote: > > > > downside is that there will be a performance penalty if an application > > > running on a NOHZ_FULL CPU is page allocator intensive for whatever > > > reason. However, I guess this is unlikely because if there was a lot > > > of kernel activity for a NOHZ_FULL CPU, the vmstat shepherd would also > > > cause interference. > > > > Yes, it does, and its being fixed: > > > > https://lkml.org/lkml/2021/12/8/663 > > > > Honestly i am not sure whether the association between a nohz_full CPU > > and "should be mostly in userspace" is desired. The RCU solution > > would be more generic. As Nicolas mentioned, for the usecases in > > questions, either solution is OK. > > > > Thomas, Frederic, Christoph, do you have any opinion on this ? > > Applications running would ideally have no performance penalty and there > is no issue with kernel activity unless the application is in its special > low latency loop. NOHZ is currently only activated after spinning in that > loop for 2 seconds or so. Would be best to be able to trigger that > manually somehow. Can add a task isolation feature to do that. > And I would prefer to be able to run the whole system as > NOHZ and have the ability to selectively enable the quiet mode if a > process requires it for its processing. IIRC Frederic has been working on that. Thanks.
