On Thu, Dec 09, 2010 at 07:13:54PM +0100, Andrea Arcangeli wrote: > On Thu, Nov 18, 2010 at 04:06:13PM +0000, Mel Gorman wrote: > > On Wed, Nov 03, 2010 at 04:28:19PM +0100, Andrea Arcangeli wrote: > > > From: Andrea Arcangeli <aarcange@xxxxxxxxxx> > > > > > > Skip transhuge pages in ksm for now. > > > > > > Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > > > Reviewed-by: Rik van Riel <riel@xxxxxxxxxx> > > > > Acked-by: Mel Gorman <mel@xxxxxxxxx> > > > > This is an idle concern that I haven't looked into but is there any conflict > > between khugepaged scanning the KSM scanning? > > > > Specifically, I *think* the impact of this patch is that KSM will not > > accidentally split a huge page. Is that right? If so, it could do with > > being included in the changelog. > > KSM wasn't aware about hugepages and in turn it'd never split them > anyway. We want KSM to split hugepages only when if finds two equal > subpages. That will happen later. > Ok. > Right now there is no collision of ksmd and khugepaged, regular pages, > hugepages and ksm pages will co-exist fine in the same vma. The only > problem is that the system has now to start swapping before KSM has a > chance to find equal pages and we'll fix it in the future so KSM can > scan inside hugepages too and split them and merge the subpages as > needed before the memory pressure starts. > Ok. So it's not a perfect mesh but it's not broken either. > > On the other hand, can khugepaged be prevented from promoting a hugepage > > because of KSM? > > Sure, khugepaged won't promote if there's any ksm page in the > range. That's not going to change. When KSM is started, the priority > remains in saving memory. If people uses enabled=madvise and > MADV_HUGEPAGE+MADV_MERGEABLE there is actually zero memory loss > because of THP and there is a speed improvement for all pages that > aren't equal. So it's an ideal setup even for embedded. Regular cloud > setup would be enabled=always + MADV_MERGEABLE (with enabled=always > MADV_HUGEPAGE becomes a noop). > That's a reasonable compromise. Thanks for clarifying. > On a related note I'm also going to introduce a MADV_NO_HUGEPAGE, is > that a good name for it? cloud management wants to be able to disable > THP per-VM basis (when the VM are totally idle, and low priority, this > currently also helps to maximize the power of KSM that would otherwise > be activated only after initial sawpping, but the KSM part will be > fixed). It could be achieved also with enabled=madvise and > MADV_HUGEPAGE but we don't want to change the system wide default in > order to disable THP on a per-VM basis: it's much nicer if the default > behavior of the host remains the same in case it's not a pure > hypervisor usage but there are other loads running in parallel to the > virt load. In theory a prctl(PR_NO_HUGEPAGE) could also do it and it'd > be possible to use from a wrapper (madvise can't be wrapped), but I > think MADV_NO_HUGEPAGE is cleaner and it won't require brand new > per-process info. > I see no problem with the proposal. The name seems as good as any other name. I guess the only other sensible alternative might be MADV_BASEPAGE. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>