Re: [RFC PATCH 00/16] 1GB THP support on x86_64

[Roman Gushchin]

On Thu, Sep 03, 2020 at 05:23:00PM +0300, Kirill A. Shutemov wrote:
> On Wed, Sep 02, 2020 at 02:06:12PM -0400, Zi Yan wrote:
> > From: Zi Yan <ziy@xxxxxxxxxx>
> > 
> > Hi all,
> > 
> > This patchset adds support for 1GB THP on x86_64. It is on top of
> > v5.9-rc2-mmots-2020-08-25-21-13.
> > 
> > 1GB THP is more flexible for reducing translation overhead and increasing the
> > performance of applications with large memory footprint without application
> > changes compared to hugetlb.
> 
> This statement needs a lot of justification. I don't see 1GB THP as viable
> for any workload. Opportunistic 1GB allocation is very questionable
> strategy.

Hello, Kirill!

I share your skepticism about opportunistic 1 GB allocations, however it might be useful
if backed by an madvise() annotations from userspace application. In this case,
1 GB THPs might be an alternative to 1 GB hugetlbfs pages, but with a more convenient
interface.

Thanks!

> 
> > Design
> > =======
> > 
> > 1GB THP implementation looks similar to exiting THP code except some new designs
> > for the additional page table level.
> > 
> > 1. Page table deposit and withdraw using a new pagechain data structure:
> >    instead of one PTE page table page, 1GB THP requires 513 page table pages
> >    (one PMD page table page and 512 PTE page table pages) to be deposited
> >    at the page allocaiton time, so that we can split the page later. Currently,
> >    the page table deposit is using ->lru, thus only one page can be deposited.
> 
> False. Current code can deposit arbitrary number of page tables.
> 
> What can be problem to you is that these page tables tied to struct page
> of PMD page table.
> 
> >    A new pagechain data structure is added to enable multi-page deposit.
> > 
> > 2. Triple mapped 1GB THP : 1GB THP can be mapped by a combination of PUD, PMD,
> >    and PTE entries. Mixing PUD an PTE mapping can be achieved with existing
> >    PageDoubleMap mechanism. To add PMD mapping, PMDPageInPUD and
> >    sub_compound_mapcount are introduced. PMDPageInPUD is the 512-aligned base
> >    page in a 1GB THP and sub_compound_mapcount counts the PMD mapping by using
> >    page[N*512 + 3].compound_mapcount.
> 
> I had hard time reasoning about DoubleMap vs. rmap. Good for you if you
> get it right.
> 
> > 3. Using CMA allocaiton for 1GB THP: instead of bump MAX_ORDER, it is more sane
> >    to use something less intrusive. So all 1GB THPs are allocated from reserved
> >    CMA areas shared with hugetlb. At page splitting time, the bitmap for the 1GB
> >    THP is cleared as the resulting pages can be freed via normal page free path.
> >    We can fall back to alloc_contig_pages for 1GB THP if necessary.
> > 
> 
> -- 
>  Kirill A. Shutemov