On Tue, Feb 06, 2024 at 06:51:02PM +0100, Andrea Righi wrote: > On Tue, Jan 30, 2024 at 05:39:37AM -0800, Paul E. McKenney wrote: > > On Tue, Jan 30, 2024 at 12:27:43PM +0100, Andrea Righi wrote: > > > On Tue, Jan 30, 2024 at 12:06:49PM +0100, Uladzislau Rezki wrote: > > > > On Tue, Jan 30, 2024 at 02:17:22AM -0800, Paul E. McKenney wrote: > > > > > On Tue, Jan 30, 2024 at 07:58:18AM +0100, Andrea Righi wrote: > > > > > > Hi Joel and Paul, > > > > > > > > > > > > comments below. > > > > > > > > > > > > On Mon, Jan 29, 2024 at 05:16:38PM -0500, Joel Fernandes wrote: > > > > > > > Hi Paul, > > > > > > > > > > > > > > On 1/29/2024 3:41 PM, Paul E. McKenney wrote: > > > > > > > > On Mon, Jan 29, 2024 at 05:47:39PM +0000, Joel Fernandes wrote: > > > > > > > >> Hi Guys, > > > > > > > >> Something caught my eye in [1] which a colleague pointed me to > > > > > > > >> - CONFIG_HZ=1000 : 14866.05 bogo ops/s > > > > > > > >> - CONFIG_HZ=1000+nohz_full : 18505.52 bogo ops/s > > > > > > > >> > > > > > > > >> The test in concern is: > > > > > > > >> stress-ng --matrix $(getconf _NPROCESSORS_ONLN) --timeout 5m --metrics-brief > > > > > > > >> > > > > > > > >> which is a CPU intensive test. > > > > > > > >> > > > > > > > >> Any thoughts on what else can attribute a 30% performance increase > > > > > > > >> versus non-nohz_full ? (Confession: No idea if the baseline is > > > > > > > >> nohz_idle or no nohz at all). If it is 30%, I may want to evaluate > > > > > > > >> nohz_full on some of our limited-CPU devices :) > > > > > > > > > > > > > > > > The usual questions. ;-) > > > > > > > > > > > > > > > > Is this repeatable? Is it under the same conditions of temperature, > > > > > > > > load, and so on? Was it running on bare metal or on a guest OS? If on a > > > > > > > > guest OS, what was the load from other guest OSes on the same hypervisor > > > > > > > > or on the hypervisor itself? > > > > > > > > > > > > That was the result of a quick test, so I expect it has some fuzzyness > > > > > > in there. > > > > > > > > > > > > It's an average of 10 runs, it was bare metal (my laptop, 8 cores 11th > > > > > > Gen Intel(R) Core(TM) i7-1195G7 @ 2.90GHz), *but* I wanted to run the > > > > > > test with the default Ubuntu settings, that means having "power mode: > > > > > > balanced" enabled. I don't know exactly what it's doing (I'll check how > > > > > > it works in details), I think it's using intel p-states IIRC. > > > > > > > > > > > > Also, the system was not completely isolated (my email client was > > > > > > running) but the system was mostly idle in general. > > > > > > > > > > > > I was already planning to repeat the tests in a more "isolated" > > > > > > environment and add details to the bug tracker. > > > > > > > > > > > > > > > > > > > > > > The bug report ad "CONFIG_HZ=250 : 17415.60 bogo ops/s", which makes > > > > > > > > me wonder if someone enabled some heavy debug that is greatly > > > > > > > > increasing the overhead of the scheduling-clock interrupt. > > > > > > > > > > > > > > > > Now, if that was the case, I would expect the 250HZ number to have > > > > > > > > three-quarters of the improvement of the nohz_full number compared > > > > > > > > to the 1000HZ number: > > > > > > > >> 17415.60-14866.05=2549.55 > > > > > > > > 18505.52-14866.05=3639.47 > > > > > > > > > > > > > > > > 2549.55/3639.47=0.70 > > > > > > > > > > > > > > I wonder if the difference here could possibly also be because of CPU idle > > > > > > > governor. It may behave differently at differently clock rates so perhaps has > > > > > > > different overhead. > > > > > > > > > > > > Could be, but, again, the balanced power mode could play a major role > > > > > > here. > > > > > > > > > > > > > > > > > > > > I have added trying nohz full to my list as well to evaluate. FWIW, when we > > > > > > > moved from 250HZ to 1000HZ, it actually improved power because the CPUidle > > > > > > > governor could put the CPUs in deeper idle states more quickly! > > > > > > > > > > > > Interesting, another benefit to add to my proposal. :) > > > > > > > > > > > > > > > > > > > > > OK, 0.70 is not *that* far off of 0.75. So what debugging does that > > > > > > > > test have enabled? Also, if you use tracing (or whatever) to measure > > > > > > > > the typical duration of the scheduling-clock interrupt and related things > > > > > > > > like softirq handlers, does it fit with these numbers? Such a measurment > > > > > > > > would look at how long it took to get back into userspace. > > > > > > > > > > Just to emphasize... > > > > > > > > > > The above calculations show that your measurements are close to what you > > > > > would expect if scheduling-clock interrupts took longer than one would > > > > > expect. Here "scheduling-clock interrupts" includes softirq processing > > > > > (timers, networking, RCU, ...) that piggybacks on each such interrupt. > > > > > > > > > > Although softirq makes the most sense given the amount of time that must > > > > > be consumed, for the most part softirq work is conserved. which suggests > > > > > that you should also at the rest of the system to check whether the > > > > > reported speedup is instead due to this work simply being moved to some > > > > > other CPU. > > > > > > > > > > But maybe the fat softirqs are due to some debugging option that Ubuntu > > > > > enabled. In which case checking up on the actual duration (perhaps > > > > > using some form of tracing) would provide useful information. ;-) > > > > > > > > > As a first step i would have a look at perf figures what is going on > > > > during a test run. For such purpose the "perf" tool can be used. As a > > > > basic step it can be run in a "top" mode: > > > > > > > > perf top -a -g -e cycles:k > > > > > > > > Sorry for the noise :) > > > > > > Yep, I'm planning to do better tests and collect more info (perf, > > > bpftrace). Also making sure that we don't have some crazy debugging > > > config enabled in the Ubuntu kernel, as correctly pointed by Paul. But > > > first of all I need to repeat the tests in a more isolated environment, > > > just to make sure we're looking at reasonable numbers here. > > > > Very good, and looking forward to seeing what you come up with! > > > > Thanx, Paul > > I repeated some tests in a more isolated environment and posted the > results here: > https://discourse.ubuntu.com/t/enable-low-latency-features-in-the-generic-ubuntu-kernel-for-24-04/42255 > > Highlights: > > - stress-ng --matrix seems quite unpredictable to be used a benchmarks > in this scenario (the bogo-ops/s are very susceptible to any kind of > interference, so even if in the long runs NO_HZ_FULL still seems to > provide some benefits looking at the average, we also need to > consider that there might be a significant error in the measurements, > standard deviation was pretty high) > > - fio doing short writes (in page cache) seems to perform like 2x > better in terms of iops with nohz_full, respect to the other cases > and it performs 2x slower with large IO writes (not sure why... need > to investigate more) > > - with lazy RCU enabled hrtimer_interrupt() takes like 2x more to > return, respect to the other cases (is this expected?) This last is surprising at first glance, but I could be missing something. Joel, Uladzislau, thoughts? Thanx, Paul > Let me know if you notice anything odd, if you need more information, > etc. > > Thanks, > -Andrea
