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SubjectRe: [PATCH 0/5] Page demotion for memory reclaim
Date
On 21 Mar 2019, at 13:01, Keith Busch wrote:

> The kernel has recently added support for using persistent memory as
> normal RAM:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c221c0b0308fd01d9fb33a16f64d2fd95f8830a4
>
> The persistent memory is hot added to nodes separate from other memory
> types, which makes it convenient to make node based memory policies.
>
> When persistent memory provides a larger and cheaper address space, but
> with slower access characteristics than system RAM, we'd like the kernel
> to make use of these memory-only nodes as a migration tier for pages
> that would normally be discared during memory reclaim. This is faster
> than doing IO for swap or page cache, and makes better utilization of
> available physical address space.
>
> The feature is not enabled by default. The user must opt-in to kernel
> managed page migration by defining the demotion path. In the future,
> we may want to have the kernel automatically create this based on
> heterogeneous memory attributes and CPU locality.
>

Cc more people here.

Thank you for the patchset. This is definitely useful when we have larger PMEM
backing existing DRAM. I have several questions:

1. The name of “page demotion” 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?

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?

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?

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?

5. In addition, you only migrate base pages, is there any performance concern on migrating THPs?
Is it too costly to migrate THPs?

Thanks.


[1] https://lwn.net/Articles/724562/

--
Best Regards,
Yan Zi
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