Messages in this thread | | | Date | Fri, 16 Apr 2004 14:49:17 +1000 | From | 'David Gibson' <> | Subject | Re: hugetlb demand paging patch part [2/3] |
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On Thu, Apr 15, 2004 at 09:13:38PM -0700, Chen, Kenneth W wrote: > David Gibson wrote on Thursday, April 15, 2004 8:27 PM > > Ah! So it's just an optimiziation - it makes a bit more sense to me > > now. I had assumed that this case (hugepage get_user_pages()) would > > be sufficiently rare that it would not require optimization. > > Apparently not. > > It's a huge deal because for *every* I/O, kernel has to do get_user_pages() > to lock the page, it's really gets in the way with the spin_lock as well. > > spin_lock(&mm->page_table_lock); > do { > struct page *map; > int lookup_write = write; > while (!(map = follow_page(mm, start, lookup_write))) { > > With current state of art platform, I/O requirement pushes into 200K > per second, this become quite significant.
Ok. This makes sense now that you explain it.
> > Do you know where the cycles are going without this optimization? In > > particular, could it be just the find_vma() in hugepage_vma() called > > before follow_huge_addr()? I note that IA64 is the only arch to have > > a non-trivial hugepage_vma()/follow_huge_addr() and that its > > follow_huge_addr() doesn't actually use the vma passed in. > > That's one, plus the spin lock mentioned above.
And akpm has just explained why it can be avoided in the hugepage case.
> > If we could get rid of follow_hugetlb_pages() it would remove an ugly > > function from every arch, which would be nice. > > I hope the goal here is not to trim code for existing prefaulting scheme. > That function has to go for demand paging, and demand paging comes with > a performance price most people don't realize. If the goal here is to > make the code prettier, I vote against that.
Well, I'm attempting to understand the hugepage code across all the archs, so that I can try to implement copy-on-write with a minimum of arch specific gunk. Simplifying and consolidating the existing code across archs would be a helpful first step, if possible.
-- David Gibson | For every complex problem there is a david AT gibson.dropbear.id.au | solution which is simple, neat and | wrong. http://www.ozlabs.org/people/dgibson - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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