Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page | Date | Wed, 15 Apr 2009 00:26:34 +1000 |
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On Wednesday 15 April 2009 00:12:09 Andrea Arcangeli wrote: > On Tue, Apr 14, 2009 at 10:39:54PM +0900, KOSAKI Motohiro wrote: > > I guess you dislike get_user_page_fast() grab pte_lock too, right? > > If get_user_page_fast is vetoed to run a set_bit on the already cache > hot and exclusive struct page, I doubt taking a potentially cache > cold, mm-wide or pmd-wide pte_lock is ok.
Yes, I'd *really* rather not. I actually implemented gup_fast in response to problem reported with DB2 workload hitting the ptl (and not the more obvious mmap_sem, although certainly they had some gain from removing that cacheline as well).
gup_fast iirc is worth nearly 10% on a 4 socket x86 system with DB2. That's the same order of magnitude as the speedups quoted to justify the addition of hugepages, or O_DIRECT itself.
Andrea: I didn't veto that set_bit change of yours as such. I just noted there could be more atomic operations. Actually I would welcome more comparison between our two approaches, but they seem to be stuck with Linus refusing (I think) to copy the page at fork() time :(
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