Messages in this thread | | | From | "Chen, Kenneth W" <> | Subject | RE: hugetlb demand paging patch part [2/3] | Date | Thu, 15 Apr 2004 21:13:38 -0700 |
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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.
> 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.
> 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.
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