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 <jglau...@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 - 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). Is there a benchmark that will be better suited? Maybe even a microbenchmark that will suffer from the longer cpumasks? - Available memory decreased by 0.13% (restricted memory to 512 MB), BSS increased 5.3 % Cheers, Jan
