Re: [PATCH 44 of 66] skip transhuge pages in ksm for now

[Mel Gorman]

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
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