Re: [PATCH 0/5] Page demotion for memory reclaim

[Yang Shi]

On Thu, Mar 21, 2019 at 3:36 PM Keith Busch <keith.busch@xxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2019 at 02:20:51PM -0700, Zi Yan wrote:
> > 1. The name of âpage demotionâ seems confusing to me, since I thought it was about large pages
> > demote to small pages as opposite to promoting small pages to THPs. Am I the only
> > one here?
>
> If you have a THP, we'll skip the page migration and fall through to
> split_huge_page_to_list(), then the smaller pages can be considered,
> migrated and reclaimed individually. Not that we couldn't try to migrate
> a THP directly. It was just simpler implementation for this first attempt.
>
> > 2. For the demotion path, a common case would be from high-performance memory, like HBM
> > or Multi-Channel DRAM, to DRAM, then to PMEM, and finally to disks, right? More general
> > case for demotion path would be derived from the memory performance description from HMAT[1],
> > right? Do you have any algorithm to form such a path from HMAT?
>
> Yes, I have a PoC for the kernel setting up a demotion path based on
> HMAT properties here:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/kbusch/linux.git/commit/?h=mm-migrate&id=4d007659e1dd1b0dad49514348be4441fbe7cadb
>
> The above is just from an experimental branch.
>
> > 3. Do you have a plan for promoting pages from lower-level memory to higher-level memory,
> > like from PMEM to DRAM? Will this one-way demotion make all pages sink to PMEM and disk?
>
> Promoting previously demoted pages would require the application do
> something to make that happen if you turn demotion on with this series.
> Kernel auto-promotion is still being investigated, and it's a little
> trickier than reclaim.

Just FYI. I'm currently working on a patchset which tries to promotes
page from second tier memory (i.e. PMEM) to DRAM via NUMA balancing.
But, NUMA balancing can't deal with unmapped page cache, they have to
be promoted from different path, i.e. mark_page_accessed().

And, I do agree with Keith, promotion is definitely trickier than
reclaim since kernel can't recognize "hot" pages accurately. NUMA
balancing is still corse-grained and inaccurate, but it is simple. If
we would like to implement more sophisticated algorithm, in-kernel
implementation might be not a good idea.

Thanks,
Yang

>
> If it sinks to disk, though, the next access behavior is the same as
> before, without this series.
>
> > 4. In your patch 3, you created a new method migrate_demote_mapping() to migrate pages to
> > other memory node, is there any problem of reusing existing migrate_pages() interface?
>
> Yes, we may not want to migrate everything in the shrink_page_list()
> pages. We might want to keep a page, so we have to do those checks first. At
> the point we know we want to attempt migration, the page is already
> locked and not in a list, so it is just easier to directly invoke the
> new __unmap_and_move_locked() that migrate_pages() eventually also calls.
>
> > 5. In addition, you only migrate base pages, is there any performance concern on migrating THPs?
> > Is it too costly to migrate THPs?
>
> It was just easier to consider single pages first, so we let a THP split
> if possible. I'm not sure of the cost in migrating THPs directly.
>