Le Mon, Jan 29, 2024 at 05:53:50PM -0500, Joel Fernandes a écrit : > > > On 1/29/2024 5:43 PM, Frederic Weisbecker wrote: > > Le Mon, Jan 29, 2024 at 05:20:23PM -0500, Joel Fernandes a écrit : > >>> If i do not miss something > >>> the NO_HZ_FULL will disable the timer if there is only one task on CPU > >>> so that running task benefits from not being interrupted thus gets more > >>> CPU time. > >> > >> Yes, that's right. I believe it is well known that HPC-type of workloads benefit > >> from FULL, however it has led to want to try it out for constrained system as > >> well where CPU cycles are a premium, especially if the improvement is like what > >> the report suggests (give or take the concerns/questions Paul raised). > > > > I'll be unable to suggest anything related to that Bogomips calculation but > > I must add something about HPC. > > > > I have long believed that HPC would benefit from nohz_full but I actually never > > heard of any user of that. The current known users of nohz_full are workloads > > that don't use the kernel once the application is launched and do their own > > stack of, for example, networking, talking directly to the device from > > userspace. Using DPDK for example. These usecases are for extremely low latency > > expectations (a single interrupt can make you lose). > > > > HPC looks to me different, making use of syscalls and kernel for I/O. Nohz_full > > may remove timer IRQs but it adds performance loss on kernel entry, making it > > probably unsuitable there. But I might be wrong. > > > > Thanks for the insights! > > The kernel entry/exit overhead bit is an interesting point and also tracking > state for RCU observation purposes. I wonder how much is the overhead in > practical cases and curious to measure (especially the tradeoffs between that > and the tick). Added note to my list ;) And here is a summary of the added overhead with nohz_full: * Use of RCU_NOCB * Kernel entry/exit overhead (context_tracking/rcu/vtime) * Unbound work reaffine (less distributed across CPUs): workqueues, timers, kthreads... * Remote tick every 1Hz on CPU 0 (one day I would love to remove that but that requires too much scheduler knowledge) The difference should be significantly measureable, even as compared to RCU_NOCB alone. Thanks. > > thanks, > > - Joel > > > > >> > >> Thanks, > >> > >> - Joel > >>
