On Thu, Feb 08, 2024 at 01:53:58PM +0100, Uladzislau Rezki wrote: > On Thu, Feb 08, 2024 at 07:55:37AM +0100, Andrea Righi wrote: > > On Thu, Feb 08, 2024 at 12:54:58AM -0500, Joel Fernandes wrote: > > ... > > > >> Slightly related, but one of the things we are wondering also is how > > > >> much of the overhead for your nohz-full and lazy-RCU test (on top of > > > >> baseline - that is just CONFIG_HZ=1000 without nohz-full or nocbs) is > > > >> because of just using NOCB. Uladsizlau mentioned he might run a test > > > >> for comparing along those lines as well. > > > > > > > > Just to clarify, "lazy rcu on" results are just with rcu_nocb=all and > > > > lazy RCUs enabled (and HZ=1000), so without nohz_full. > > > > > > > > If I enable only nohz_full=all (without rcu_nocb) I see something like > > > > this: > > > > > > Ok. I did want to mention nohz_full implies rcu_nocb on the same CPUs as well. > > > > > > Its also mentioned in the boot param docs on the last line of the description: > > > > > > nohz_full= [KNL,BOOT,SMP,ISOL] > > > The argument is a cpu list, as described above. > > > In kernels built with CONFIG_NO_HZ_FULL=y, set > > > the specified list of CPUs whose tick will be stopped > > > whenever possible. The boot CPU will be forced outside > > > the range to maintain the timekeeping. Any CPUs > > > in this list will have their RCU callbacks offloaded, > > > just as if they had also been called out in the > > > rcu_nocbs= boot parameter. > > > > Ah I didn't realize that, it definitely makes sense, thanks for > > clarifying it. > > > > Then basically in the results that I posted the difference is > > "nohz_full=all+rcu_nocb=all" vs "rcu_nocb=all+lazy_RCU=on". > > > So, you say that a hrtimer_interrupt() handler takes more time in case > of lazy + nocb + rcu_nocb=all and for nohz_full + rcu_nocb=all it faster? > Could you please clarify this? I will try to measure from my side! > > I have done some basic research about hrtimer_interrupt() latency on my > HW with latest Linux kernel. I have compared below cases: > > case a: 1000HZ + lazy + nocb_all_cpus > case b: 1000HZ + nocb_all_cpus > > I used "ftrace" to measure time(in microseconds). Steps: > > echo 0 > tracing_on > echo function_graph > current_tracer > echo funcgraph-proc > trace_options > echo funcgraph-abstime > trace_options > echo hrtimer_interrupt > set_ftrace_filter > > fio --rw=write --bs=1M --size=1G --numjobs=8 --name=worker --time_based --runtime=50& > > echo 1 > tracing_on; sleep 10; echo 0 > tracing_on > > data is based on 10 seconds collection: > > <case a> > 6 2102 ############################################################ > 8 2079 ############################################################ > 10 1464 ########################################## > 7 897 ########################## > 9 625 ################## > 12 490 ############## > 13 479 ############## > 11 289 ######### > 5 249 ######## > 14 124 #### > 15 72 ### > 16 41 ## > 17 24 # > 4 22 # > 18 12 # > 22 2 # > 19 1 # > <case a> > > <case b> > 9 1658 ############################################################ > 13 1308 ################################################ > 12 1224 ############################################# > 10 972 #################################### > 8 703 ########################## > 14 595 ###################### > 15 571 ##################### > 11 525 ################### > 17 350 ############# > 16 235 ######### > 7 214 ######## > 4 73 ### > 5 68 ### > 6 54 ## > 20 9 # > 18 9 # > 19 6 # > 33 1 # > 3 1 # > 28 1 # > 27 1 # > 25 1 # > 22 1 # > 21 1 # > <case b> > > I do not see the difference, there is a nose of 1/2/3 microseconds diff. > Let me further have a look at what we use for lazy in terms on hrtimer though. -- Uladzislau Rezki
