Messages in this thread | | | From | Yury Norov <> | Date | Thu, 22 Sep 2022 18:45:27 -0700 | Subject | Re: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist nodes |
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On Thu, Sep 22, 2022 at 5:38 PM Phil Auld <pauld@redhat.com> wrote: > > Hi Andy, > > On Thu, Sep 22, 2022 at 10:49:54PM +0300 Andy Shevchenko wrote: > > Currently the approximation is used which wastes the more memory > > the more CPUs are present on the system. Proposed change calculates > > the exact maximum needed in the worst case: > > > > NR_CPUS old new > > ------- --- --- > > 1 .. 1170 4096 4096 > > 1171 .. 1860 4098 .. 6510 4096 > > ... ... ... > > 2*4096 28672 19925 > > 4*4096 57344 43597 > > 8*4096 114688 92749 > > 16*4096 229376 191053 > > 32*4096 458752 403197 > > 64*4096 917504 861949 > > 128*4096 1835008 1779453 > > 256*4096 3670016 3670016 > > > > Under the hood the reccurent formula is being used: > > (5 - 0) * 2 + > > (50 - 5) * 3 + > > (500 - 50) * 4 + > > (5000 - 500) * 5 + > > ... > > (X[i] - X[i-1]) * i > > > > which allows to count the exact maximum length in the worst case, > > i.e. when each second CPU is being listed. For backward compatibility > > for more than 1170 and less than 1861 CPUs the page size is preserved. > > > > For less than 1171 and more than 1 million CPUs the old is being used. > > The memory is not really wasted since it's probably temporary in userspace > and in the kernel it _is_ temporary and is only the length of the kasprintf > string, which is most of the time much less. > > But that said, it is more accurate than the previous estimate. > > I was wondering if you were going to try to come up with a suitable > compile time macro :) > > I tested 2, 8192 and 16k since the kernel does not want to build for other > reasons with NR_CPUS at 32k. > > Reviewed-by: Phil Auld <pauld@redhat.com> > Tested-by: Phil Auld <pauld@redhat.com>
By the way, why don't we use nr_cpu_ids? It's set at boot-time, or even at compile-time in some cases and never changed at runtime.
nr_cpu_ids is set very early, when ACPI tables are parsed. I don't think it's possible for a userspace to observe it uninitialized. Am I wrong?
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