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SubjectRe: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node
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On 27 Mar 2019, at 6:08, Keith Busch wrote:

> On Tue, Mar 26, 2019 at 08:41:15PM -0700, Yang Shi wrote:
>> On 3/26/19 5:35 PM, Keith Busch wrote:
>>> migration nodes have higher free capacity than source nodes. And since
>>> your attempting THP's without ever splitting them, that also requires
>>> lower fragmentation for a successful migration.
>>
>> Yes, it is possible. However, migrate_pages() already has logic to
>> handle such case. If the target node has not enough space for migrating
>> THP in a whole, it would split THP then retry with base pages.
>
> Oh, you're right, my mistake on splitting. So you have a good best effort
> migrate, but I still think it can fail for legitimate reasons that should
> have a swap fallback.

Does this mean we might want to factor out the page reclaim code in shrink_page_list()
and call it for each page, which fails to migrate to PMEM. Or do you still prefer
to migrate one page at a time, like what you did in your patch?

I ask this because I observe that migrating a list of pages can achieve higher
throughput compared to migrating individual page. For example, migrating 512 4KB
pages can achieve ~750MB/s throughput, whereas migrating one 4KB page might only
achieve ~40MB/s throughput. The experiments were done on a two-socket machine
with two Xeon E5-2650 v3 @ 2.30GHz across the QPI link.


--
Best Regards,
Yan Zi
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