rwatson at FreeBSD.org
Mon Sep 27 21:21:32 UTC 2010
On Mon, 27 Sep 2010, Sean Bruno wrote:
>> wouldn't it be better to do a sysctlbyname() and use the real value for the
libmemstat contains some useful sample code showing how this might be done.
> That was my initial thought (as prodded by scottl and peter).
> If it is made dynamic, could this be opening a race condition where the call
> to sysctlbyname() returns a count of CPUS that is in turn changed by the
> offlining of a CPU? Or am I thinking to much about this?
Yes, you are. MAXCPU is a compile-time constant for kernel builds, so (at
least a the world is today), that can't happen.
I think there's a reasonable argument that MEMSTAT_MAXCPU should be phased out
and all internal structures in libmemstat should be dynamically sized.
However, core counts aren't growing that fast, and it's quite a bit of work,
and probably not worth it just yet.
I'm somewhat averse to using MAXCPU in libmemstat, however, because MAXCPU is
actually not a constant in the general case: FreeBSD/i386, for example,
regularly uses two different values: 1 for !SMP kernels, and 32 for SMP
kernels. That's why libmemstat encodes its own value, for better or worse.
A reasonable alternative would be to replace 32 with MAXCPU * 2, or if we're
feeling particularly optimistic, MAXCPU * 4. Or just another big number, like
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