powerpc kernel flavour

[Tim Gardner]

On 12/06/2010 07:20 PM, Jeremy Kerr wrote:
> Hi Tim,
>
>> I'm all for dropping flavours, but I have no idea how many G4s there are
>> still running, and if 3.5% has a noticeable impact.
>
> Ben, any idea about general UP ppc32 usage? I'm guessing we're only looking at
> the pmac platforms here.
>
>> Doesn't x86 rewrite locks at early boot time depending on UP or SMP? Is
>> that something we could do for powerpc ?
>
> There's no spinlock-patching on powerpc currently. There has been some
> interest in doing this, but I don't think this is a high priority for the ppc
> folks; most of the current development is on SMP machines.
>
>> Though its likely the dependency on CONFIG_PPC32=y goes deeper then just
>> spinlocks.
>
>  From a quick look, these seem to be the major runtime differences between
> running a single-CPU machine with CONFIG_SMP and !CONFIG_SMP:
>
>   * accessing the paca (ie, per-cpu info) is slightly more expensive
>
>   * tlb teardown is more complex
>
>   * mm code locking required - eg mmu_hash_lock avoided on !CONFIG_SMP
>
>   * no lazy FP/VMX switching - on !SMP, reg state is only saved if another
>     process uses FP/VMX instructions (although we still only restore state
>     when required).
>
>   * higher memory usage for per-cpu vars (but NR_CPUS=4, so this is
>     pretty small)
>
>   * larger code size (7.3M vs 7.6M), larger bss (484k vs 751k).
>
> [if there's anything I've missed, please add]
>
> Cheers,
>
>
> Jeremy

Andy and I think we should just leave the existing powerpc flavours as 
they are for now. The cost of the 32 bit flavour is relatively 
inconsequential.

rtg
-- 
Tim Gardner tim.gardner at canonical.com