On Sun, 9 May 2010 20:21:45 +0100 Mel Gorman <mel@xxxxxxxxx> wrote: > On Thu, May 06, 2010 at 07:12:59PM -0700, Linus Torvalds wrote: > > > > > > On Fri, 7 May 2010, KAMEZAWA Hiroyuki wrote: > > > > > > IIUC, move_page_tables() may call "page table allocation" and it cannot be > > > done under spinlock. > > > > Bah. It only does a "alloc_new_pmd()", and we could easily move that out > > of the loop and pre-allocate the pmd's. > > > > If that's the only reason, then it's a really weak one, methinks. > > > > It turns out not to be easy to the preallocating of PUDs, PMDs and PTEs > move_page_tables() needs. To avoid overallocating, it has to follow the same > logic as move_page_tables duplicating some code in the process. The ugliest > aspect of all is passing those pre-allocated pages back into move_page_tables > where they need to be passed down to such functions as __pte_alloc. It turns > extremely messy. > > I stopped working on it about half way through as it was already too ugly > to live and would have similar cost to Kamezawa's much more straight-forward > approach of using move_vma(). > > While using move_vma is straight-forward and solves the problem, it's > not as cheap as Andrea's solution. Andrea allocates a temporary VMA and > puts it on a list and very little else. It didn't show up any problems > in microbenchmarks. Calling move_vma does a lot more work particularly in > copy_vma and this slows down exec. > > With Kamezawa's patch, kernbench was fine on wall time but in System Time, > it slowed by up 1.48% in comparison to Andrea's slowing up by 0.64%[1]. > > aim9 was slowed as well. Kamezawa's slowed by 2.77% where Andrea's reported > faster by 2.58%. While AIM9 is flaky and these figures are barely outside > the noise, calling move_vma() is obviously more expensive. > Thank you for testing. > While my solution at http://lkml.org/lkml/2010/4/30/198 is cheapest as it > does not touch exec() at all, is_vma_temporary_stack() could be broken in > the future if any of the assumptions it makes change. > > So what you have is an inverse relationship between magic and > performance. Mine has the most magic and is fastest. Kamezawa's has the > least magic but slowest and Andrea has the goldilocks factor. Which do > you prefer? > I like the fastest one ;) Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>