On Mon, Jan 29, 2024 at 11:43:08PM +0100, 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. Some IBM clients made use of nohz_full for HPC workloads, and claimed to see significant benefits. (Single-digit percents, if I remember correctly, but still...) Thanx, Paul
