Re: [patch 2/14] mm: pte prefetch

Arjan van de Ven
2005-11-06T08:35:19Z

On Sun, 2005-11-06 at 19:20 +1100, Nick Piggin wrote:
> 2/14
>
> plain text document attachment (mm-pte-prefetch.patch)
> Prefetch ptes a line ahead. Worth 25% on ia64 when doing big forks.
>
> Index: linux-2.6/include/asm-generic/pgtable.h
> ===================================================================
> --- linux-2.6.orig/include/asm-generic/pgtable.h
> +++ linux-2.6/include/asm-generic/pgtable.h
> @@ -196,6 +196,33 @@ static inline void ptep_set_wrprotect(st
> })
> #endif
>
> +#ifndef __HAVE_ARCH_PTE_PREFETCH
> +#define PTES_PER_LINE (L1_CACHE_BYTES / sizeof(pte_t))
> +#define PTE_LINE_MASK (~(PTES_PER_LINE - 1))
> +#define ADDR_PER_LINE (PTES_PER_LINE << PAGE_SHIFT)
> +#define ADDR_LINE_MASK (~(ADDR_PER_LINE - 1))
> +
> +#define pte_prefetch(pte, addr, end) \
> +({ \
> + unsigned long __nextline = ((addr) + ADDR_PER_LINE) & ADDR_LINE_MASK; \
> + if (__nextline < (end)) \
> + prefetch(pte + PTES_PER_LINE); \
> +})
> +

are you sure this is right? at least on pc's having a branch predictor
miss is very expensive and might well be more expensive than the gain
you get from a prefetch