Rusty Russell wrote: > On Monday 08 December 2008 20:16:44 Avi Kivity wrote: >> Rusty Russell wrote: >>>> This isn't on stack, so it isn't buying us anything. >>>> >>> It's the CONFIG_NR_CPUS=4096 but nr_cpu_ids=4 case which we win using >>> dynamic allocation. Gotta love distribution kernels. >>> >>> >> What does it buy? 4096/8 = 512 bytes statically allocated? > > It adds up, and 4096 seems to be only the start of the insanityH^H^Hfun.
The real win though is when cpumask_size represents the actual size of the cpumask (based on # of possible cpus) instead of the pre-configured size of NR_CPUS. So for 99.9% of the systems (having 64 or fewer cpus), the savings will be 504 bytes not allocated. > >>> Not quite. If !CONFIG_CPUMASK_OFFSTACK, cpumask_var_t == cpumask_t[1]. >>> Blame Linus :) >>> >> Hm, is there a C trick which will error out when allocating something on >> the stack, but work when allocating statically? I can think of >> something to do the reverse, but that doesn't help. > > We also need to prevent assignment, eg: > > *foo = *bar; > > Because when we allocate them, we'll cut them to size. > > Cheers, > Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
