Re: Prezeroing V2 [0/3]: Why and When it works

From: Paul Mackerras
Date: Thu Dec 23 2004 - 16:02:32 EST

Christoph Lameter writes:

> The most expensive operation in the page fault handler is (apart of SMP
> locking overhead) the zeroing of the page. This zeroing means that all
> cachelines of the faulted page (on Altix that means all 128 cachelines of
> 128 byte each) must be loaded and later written back. This patch allows to
> avoid having to load all cachelines if only a part of the cachelines of
> that page is needed immediately after the fault.

On ppc64 we avoid having to zero newly-allocated page table pages by
using a slab cache for them, with a constructor function that zeroes
them. Page table pages naturally end up being full of zeroes when
they are freed, since ptep_get_and_clear, pmd_clear or pgd_clear has
been used on every non-zero entry by that stage. Thus there is no
extra work required either when allocating them or freeing them.

I don't see any point in your patches for systems which don't have
some magic hardware for zeroing pages. Your patch seems like a lot of
extra code that only benefits a very small number of machines.

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