On 28 May 2014 10:58, John Baldwin <j...@freebsd.org> wrote:
> On Wednesday, May 28, 2014 1:51:28 pm Adrian Chadd wrote:
>> On 28 May 2014 06:56, John Baldwin <j...@freebsd.org> wrote:
>> > Userland cpusets only default to 128 (CPU_MAXSIZE in <sys/_cpuset.h>).
>> > Changing MAXCPU to even 128 is unfortunately a potential KBI change since 
>> > it
>> > changes the size of 'cpuset_t'.  We can certainly bump these in HEAD for 
>> > 11,
>> > but we might not be able to MFC them without introducing ABI breakage.
>> > (The cpuset APIs do allow the size of cpuset_t to change as the size is
>> > encoded in the API calls, so there is that, it's more that if some public
>> > structure embeds a cpuset_t in the kernel that we would have problems.  I
>> > thought 'struct pcpu' did, but it does not.)
>> >
>> > Hmm, smp_rendezvous() accepts a cpuset_t as its first argument (and is a
>> > public symbol used by kernel modules such as dtrace).  'struct rmlock' also
>> > embeds a cpuset_t.  So, I think we can't bump cpuset_t without breaking
>> > the KBI.  We can bump it in HEAD however.  (Note, if re@ signed off, we 
>> > could
>> > perhaps merge to 10, but we tend to be very hesitant about breaking the 
>> > KBI.)
>> > One thing we could do safely is bump the userland cpuset size to 256 in 10.
>> > It's really only MAXCPU that is problematic.
>> >
>> > In particular, I propose we bump the userland cpuset_t size to 256 now (and
>> > go ahead and merge that to 10).  In HEAD only we can bump MAXCPU for amd64
>> > to 256.
>> Since 11 is going to be around for a few years, can we experiment
>> bumping it up to something compute-cluster-computer-sized just to get
>> it over with? Something stupid, like 4096 or something?
> It costs wired memory to increase it for the kernel.  The userland set size
> can be increased rather arbitrarily, so we don't need to make it but so large
> as it is easy to bump later (even with a branch).

Well, what about making the API/KBI use cpuset_t pointers for things
rather than including it as a bitmask? Do you think there'd be a
noticable performance overhead for the bits where it's indirecting
through a pointer to get to the bitmask data?

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