Messages in this thread | | | Date | Wed, 23 Nov 2005 11:07:48 +1100 | From | Nick Piggin <> | Subject | Re: [patch 12/12] mm: rmap opt |
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Hugh Dickins wrote: > On Mon, 21 Nov 2005, Nick Piggin wrote: > > >>Optimise rmap functions by minimising atomic operations when >>we know there will be no concurrent modifications. > > > It's not quite right yet. A few minor points first: >
Thanks for looking at it.
> You ought to convert the page_add_anon_rmap in fs/exec.c to > page_add_new_anon_rmap: that won't give a huge leap in performance, > but it will save someone coming along later and wondering why that > particular one isn't "new_". >
Yep, you mentioned that before but I must have lost the hunk.
> The mod to page-flags.h at the end: nowhere is __SetPageReferenced > used, just cut the page-flags.h change out of your patch. > > Perhaps that was at one time a half-way house to removing the > SetPageReferenced from do_anonymous_page: I support you in that > removal (I've several times argued that if it's needed there, then > it's also needed in several other like places which lack it; and I > think you concluded that it's just not needed); but you ought at least > to confess to that in the change comments, if it's not a separate patch. >
You're right. I'll split that and fix the page-flags.h.
> I've spent longest staring at page_remove_rmap. Here's how it looks: > > void page_remove_rmap(struct page *page) > { > int fast = (page_mapcount(page) == 1) & > PageAnon(page) & (!PageSwapCache(page)); > > /* fast page may become SwapCache here, but nothing new will map it. */ > if (fast) > reset_page_mapcount(page); > else if (atomic_add_negative(-1, &page->_mapcount)) > BUG_ON(page_mapcount(page) < 0); > if (page_test_and_clear_dirty(page)) > set_page_dirty(page); > else > return; /* non zero mapcount */ > /* [comment snipped for these purposes] */ > __dec_page_state(nr_mapped); > } > > Well, C doesn't yet allow indentation to take the place of braces: > I think you'll find your /proc/meminfo Mapped goes up and up, since > only on s390 will page_test_and_clear_dirty ever say yes. >
Thanks. It is fairly obscure, and possibly has memory ordering problems. Also the conditional jumps and icache usage are increased, so it isn't as clear a win as the add_new_anon_rmap's. I'll drop this part for the moment.
Nick
-- SUSE Labs, Novell Inc.
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