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.
Thank you!
--
Uladzislau Rezki
