powerpc kernel flavour
Jeremy Kerr
jeremy.kerr at canonical.com
Tue Dec 7 02:20:40 UTC 2010
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
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