On 29 May 2014 11:44, John Baldwin <j...@freebsd.org> wrote:
> On Thursday, May 29, 2014 2:24:45 pm Adrian Chadd wrote:
>> On 29 May 2014 10:18, John Baldwin <j...@freebsd.org> wrote:
>> >> > 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?
>> >
>> > The wired memory is not due to cpuset_t.  The wired memory usage is due to 
>> > things
>> > that do 'struct foo foo_bits[MAXCPU]'.  The KBI issues I mentioned above 
>> > are
>> > 'struct rmlock' (so now you want any rmlock users to malloc space, or you
>> > want rmlock_init() call malloc?  (that seems like a bad idea)).  The other 
>> > one
>> > is smp_rendezvous.  Plus, it's not just a pointer, you really need a 
>> > (pointer,
>> > size_t) tuple similar to what cpuset_getaffinity(), etc. use.
>> Why would calling malloc be a problem? Except for the initial setup of
>> things, anything dynamically allocating structs with embedded things
>> like rmlocks are already dynamically allocating them via malloc or
>> uma.
>> There's a larger fundamental problem with malloc, fragmentation and
>> getting the required larger allocations for things. But even a 4096
>> CPU box would require a 512 byte malloc. That shouldn't be that hard
>> to do. It'd just be from some memory that isn't close to the rest of
>> the lock state.
> Other similar APIs like mtx_init() don't call malloc(), so it would be
> unusual behavior.  However, we have several other problems before we can
> move beyond 256 anyway (like pf).

Maybe behaviour has to change over time. :(

anyway. Besides all of this - I'm thinking of just introducing:

typedef uint32_t cpuid_t;

.. then once we've converted all the users, we can make NOCPU
something other than 255 (which is the other limiting factor here..)

Any objections?

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