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. --Jan
