Re: A scrub daemon (prezeroing)

[Paul Mackerras]

Rik van Riel writes:

> I'm not convinced.  Zeroing a page takes 2000-4000 CPU
> cycles, while faulting the page from RAM into cache takes
> 200-400 CPU cycles per cache line, or 6000-12000 CPU
> cycles.

On my G5 it takes ~200 cycles to zero a whole page.  In other words it
takes about the same time to zero a page as to bring in a single cache
line from memory.  (PPC has an instruction to establish a whole cache
line of zeroes in modified state without reading anything from
memory.)

Thus I can't see how prezeroing can ever be a win on ppc64.

Regards,
Paul.
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