Re: [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256

[Jan Glauber <jan.glauber@caviumnetworks.com>]

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

- 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