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SubjectRe: follow_page() performance regressions


On Tue, 1 Jul 2008, Paul Mundt wrote:
>
> So yes, the ZERO_PAGE handling in follow_page() is causing the slow-down here.

I suspect it could be any of:

- actual code generation differences in follow_page() (some SH person
needs to check that)

- some cache issue on SH. ZERO_PAGE is a single page at a fixed virtual
address, while it used to populate the page tables with individual
pages. Normally, this should be *better* for caching, but maybe there
is some conflict? What kind of caches does SH have (virtually indexed?)

- hackbench relying on follow_page() to populate the page tables, which
it no longer does for anonymous areas (using ZERO_PAGE directly
instead).

Again, normally this would speed things up (fewer TLB misses etc), but
if it then causes a new page fault that used to have been covered by
follow_page(), who knows?

What does hackbench actually do? Anybody?

Linus


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