Messages in this thread | | | Date | Mon, 26 Mar 2018 12:02:50 +0200 | From | Jan Glauber <> | Subject | Re: [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256 |
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On Mon, Mar 26, 2018 at 11:28:28AM +0200, Arnd Bergmann wrote: > On Mon, Mar 26, 2018 at 10:52 AM, Jan Glauber > <jan.glauber@caviumnetworks.com> wrote: > > On Tue, Mar 06, 2018 at 03:02:01PM +0100, Jan Glauber wrote: > >> On Tue, Mar 06, 2018 at 02:12:29PM +0100, Arnd Bergmann wrote: > >> > On Fri, Mar 2, 2018 at 3:37 PM, Jan Glauber <jglauber@cavium.com> wrote: > >> > > ThunderX1 dual socket has 96 CPUs and ThunderX2 has 224 CPUs. > >> > > >> > Are you sure about those numbers? From my counting, I would have expected > >> > twice that number in both cases: 48 cores, 2 chips and 2x SMT for ThunderX > >> > vs 52 Cores, 2 chips and 4x SMT for ThunderX2. > >> > >> That's what I have on those machines. I counted SMT as normal CPUs as it > >> doesn't make a difference for the config. I've not seen SMT on ThunderX. > >> > >> The ThunderX2 number of 224 is already with 4x SMT (and 2 chips) but > >> there may be other versions planned that I'm not aware of. > >> > >> > > Therefore raise the default number of CPUs from 64 to 256 > >> > > by adding an arm64 specific option to override the generic default. > >> > > >> > Regardless of what the correct numbers for your chips are, I'd like > >> > to hear some other opinions on how high we should raise that default > >> > limit, both in arch/arm64/Kconfig and in the defconfig file. > >> > > >> > As I remember it, there is a noticeable cost for taking the limit beyond > >> > BITS_PER_LONG, both in terms of memory consumption and also > >> > runtime performance (copying and comparing CPU masks). > >> > >> OK, that explains the default. My unverified assumption is that > >> increasing the CPU masks wont be a noticable performance hit. > >> > >> Also, I don't think that anyone who wants performance will use > >> defconfig. All server distributions would bump up the NR_CPUS anyway > >> and really small systems will probably need to tune the config > >> anyway. > >> > >> For me defconfig should produce a usable system, not with every last > >> driver configured but with all the basics like CPUs, networking, etc. > >> fully present. > >> > >> > I'm sure someone will keep coming up with even larger configurations > >> > in the future, so we should try to decide how far we can take the > >> > defaults for the moment without impacting users of the smallest > >> > systems. Alternatively, you could add some measurements that > >> > show how much memory and CPU time is used up on a typical > >> > configuration for a small system (4 cores, no SMT, 512 MB RAM). > >> > If that's low enough, we could just do it anyway. > >> > >> OK, I'll take a look. > > > > I've made some measurements on a 4 core board (Cavium 81xx) with > > NR_CPUS set to 64 or 256: > > > > - vmlinux grows by 0.04 % with 256 CPUs > > Ok. Is this both with CONFIG_CPUMASK_OFFSTACK=n?
Yes.
> > - Kernel compile time was a bit faster with 256 CPUS (which does > > not make sense, but at least is seems to not suffer from the change). > > Do you mean compiling the same kernel configuration while running > on a system with less than 64 CPUs on either a CONFIG_NR_CPUS=64 > or CONFIG_NR_PCUS=256 kernel, or do you mean the time to compile > a kernel with either CONFIG_NR_CPUS=64 or CONFIG_NR_CPUS=256, > while running on the same host?
The former, compiling everything on a 4-core system using two different kernels to compile the same thing.
> I assume the former, which is a very interesting result, possibly > pointing to us doing something wrong in the NR_CPUS=64 case > that could be optimized. > > If you ran with CONFIG_CPUMASK_OFFSTACK, that may have made > a significant difference, but I would expect it to be faster without it. > > To get more insight to what is happening, you could rerun the same > test with 'perf record' and then compare the profiles. How significant > is the runtime difference compared to the jitter you get between normal > runs on the same configuration?
I did retry once but the odd case that CONFIG_NR_CPUS=256 was faster was consistent. The difference was very small though so it may be completely due to jitter.
> > Is there a benchmark that will be better suited? Maybe even a > > microbenchmark that will suffer from the longer cpumasks? > > Good question. > > > - Available memory decreased by 0.13% (restricted memory to 512 MB), > > BSS increased 5.3 % > > 0.13% of a few hundred megabytes is still several hundred kb, right? I'd > like to hear some other opinions on that, but it seems to be in the > range of enabling many additional device drivers, which is something > we don't do lightly.
Agreed, available memory was reduced by 128 KB.
--Jan
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