Re: [RFC PATCH] mm: thp: Add per-mm_struct flag to control THP

From: Alex Thorlton
Date: Wed Jan 22 2014 - 12:53:17 EST

On Wed, Jan 22, 2014 at 10:26:21AM +0000, Mel Gorman wrote:
> On Tue, Jan 14, 2014 at 01:38:01PM -0600, Alex Thorlton wrote:
> > On Tue, Jan 14, 2014 at 03:44:57PM +0000, Mel Gorman wrote:
> > > On Fri, Jan 10, 2014 at 04:39:09PM -0600, Alex Thorlton wrote:
> > > > On Fri, Jan 10, 2014 at 11:10:10PM +0100, Peter Zijlstra wrote:
> > > > > We already have the information to determine if a page is shared across
> > > > > nodes, Mel even had some prototype code to do splits under those
> > > > > conditions.
> > > >
> > > > I'm aware that we can determine if pages are shared across nodes, but I
> > > > thought that Mel's code to split pages under these conditions had some
> > > > performance issues. I know I've seen the code that Mel wrote to do
> > > > this, but I can't seem to dig it up right now. Could you point me to
> > > > it?
> > > >
> > >
> > > It was a lot of revisions ago! The git branches no longer exist but the
> > > diff from the monolithic patches is below. The baseline was v3.10 and
> > > this will no longer apply but you'll see the two places where I added a
> > > split_huge_page and prevented khugepaged collapsing them again.
> >
> > Thanks, Mel. I remember seeing this code a while back when we were
> > discussing THP/locality issues.
> >
> > > At the time, the performance with it applied was much worse but it was a
> > > 10 minute patch as a distraction. There was a range of basic problems that
> > > had to be tackled before there was any point looking at splitting THP due
> > > to locality. I did not pursue it further and have not revisited it since.
> >
> > So, in your opinion, is this something we should look into further
> > before moving towards the per-mm switch that I propose here?
> No because they have different purposes. Any potential split of THP from
> automatic NUMA balancing context is due to it detecting that threads running
> on CPUs on different nodes are accessing a THP. You are proposing to have
> a per-mm flag that prevents THP being allocated in the first place. They
> are two separate problems with decisions that are made at completely
> different times.

Makes sense. That discussion doesn't really fit here.

> > I
> > personally think that it will be tough to get this to perform as well as
> > a method that totally disables THP when requested, or a method that
> > tries to prevent THPs from being handed out in certain situations, since
> > we'll be doing the work of both making and splitting a THP in the case
> > where remote accesses are made to the page.
> >
> I would expect that the alternative solution to a per-mm switch is to
> reserve the naturally aligned pages for a THP promotion. Have a threshold
> of pages pages that must be faulted before the full THP's worth of pages
> is allocated, zero'd and a huge pmd established. That would defer the
> THP setup costs until it was detected that it was necessary.

I have some half-finished patches that I was working on a month or so
ago, to do exactly this (I think you were involved with some of the
discussion, maybe? I'd have to dig up the e-mails to be sure). After
cycling through numerous other methods of handling this problem, I still
like that idea, but I think it's going to require a decent amount of
effort to get finished.

> The per-mm THP switch is a massive hammer but not necessarily a bad one.

I agree that it's a big hammer, but I suppose it's a bit more of a
surgical strike than using the system-wide switch, and can be especially
useful in some cases I've discussed, where madvise isn't really an

> > I also think there could be some issues with over-zealously splitting
> > pages, since it sounds like we can only determine if an access is from a
> > remote node. We don't have a good way of determining how many accesses
> > are remote vs. local, or how many separate nodes are accessing a page.
> >
> Indeed not, but it's a different problem. We also do not know if the
> remote accesses are to a single page in which case splitting it would
> have zero benefit anyway.

Agreed. There are several possible problems with that approach, but, as
you've stated, that's a different problem, so no reason to discuss
here :)

Thanks for the input, Mel!

- Alex
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