Re: [Lsf-pc] [LSF/MM ATTEND] Memory management -- THP, hugetlb, scalability

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Jan 10, 2014 at 05:51:16PM -0500, Matthew Wilcox wrote:
> On Fri, Jan 10, 2014 at 07:42:04PM +0200, Kirill A. Shutemov wrote:
> > On Wed, Jan 08, 2014 at 03:13:21PM +0000, Mel Gorman wrote:
> > > I think transparent huge pagecache is likely to crop up for more than one
> > > reason. There is the TLB issue and the motivation that i-TLB pressure is
> > > a problem in some specialised cases. Whatever the merits of that case,
> > > transparent hugepage cache has been raised as a potential solution for
> > > some VM scalability problems. I recognise that dealing with large numbers
> > > of struct pages is now a problem on larger machines (although I have not
> > > seen quantified data on the problem nor do I have access to a machine large
> > > enough to measure it myself) but I'm wary of transparent hugepage cache
> > > being treated as a primary solution for VM scalability problems. Lacking
> > > performance data I have no suggestions on what these alternative solutions
> > > might look like.
> 
> Something I'd like to see discussed (but don't have the MM chops to
> lead a discussion on myself) is the PAGE_CACHE_SIZE vs PAGE_SIZE split.
> This needs to be either fixed or removed, IMO.  It's been in the tree
> since before git history began (ie before 2005), it imposes a reasonably
> large cognitive burden on programmers ("what kind of page size do I want
> here?"), it's not intuitively obvious (to a non-mm person) which page
> size is which, and it's never actually bought us anything because it's
> always been the same!
> 
> Also, it bitrots.  Look at this:
> 
>         pgoff_t pgoff = (((address & PAGE_MASK)
>                         - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
>         vmf.pgoff = pgoff;
>         pgoff_t offset = vmf->pgoff;
>         size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
>         if (offset >= size)
>                 return VM_FAULT_SIGBUS;
> 
> That's spread over three functions, but that goes to illustrate my point;
> getting this stuff right is Hard; core mm developers get it wrong, we
> don't have the right types to document whether a variable is in PAGE_SIZE
> or PAGE_CACHE_SIZE units, and we're not getting any benefit from it today.

I also want to drop PAGE_CACHE_*. It's on my todo list almost a year now ;)

> > Sibling topic is THP for XIP (see Matthew's patchset). Guys want to manage
> > persistent memory in 2M chunks where it's possible. And THP (but without
> > struct page in this case) is the obvious solution.
> 
> Not just 2MB, we also want 1GB pages for some special cases.  It looks
> doable (XFS can allocate aligned 1GB blocks).  I've written some
> supporting code that will at least get us to the point where we can
> insert a 1GB page.  I haven't been able to test anything yet.

It's probably doable from fs point of view, but adding PUD-level THP page
is not trivial at all. I think it's more productive better to concentrate
on 2M for now.

-- 
 Kirill A. Shutemov

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