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

[Zi Yan]

On 10 Sep 2020, at 10:34, David Hildenbrand wrote:

>>> As long as we stay in safe zone boundaries you get a benefit in most
>>> scenarios. As soon as we would have a (temporary) workload that would
>>> require more unmovable allocations we would fallback to polluting some
>>> pageblocks only.
>>
>> The idea would work well until unmoveable pages begin to overflow into
>> ZONE_PREFER_MOVABLE or we move the boundary of ZONE_PREFER_MOVABLE to
>> avoid unmoveable page overflow. The issue comes from the lifetime of
>> the unmoveable pages. Since some long-live ones can be around the boundary,
>> there is no guarantee that ZONE_PREFER_MOVABLE cannot grow back
>> even if other unmoveable pages are deallocated. Ultimately,
>> ZONE_PREFER_MOVABLE would be shrink to a small size and the situation is
>> back to what we have now.
>
> As discussed this would not happen in the usual case in case we size it
> reasonable. Of course, if you push it to the extreme (which was never
> suggested!), you would create mess. There is always a way to create a
> mess if you abuse such mechanism. Also see Rik's reply regarding reclaim.
>
>>
>> OK. I have a stupid question here. Why not just grow pageblock to a larger
>> size, like 1GB? So the fragmentation of unmoveable pages will be at larger
>> granularity. But it is less likely unmoveable pages will be allocated at
>> a movable pageblock, since the kernel has 1GB pageblock for them after
>> a pageblock stealing. If other kinds of pageblocks run out, moveable and
>> reclaimable pages can fall back to unmoveable pageblocks.
>> What am I missing here?
>
> Oh no. For example pageblocks have to completely fit into a single
> section (that's where metadata is maintained). Please refrain from
> suggesting to increase the section size ;)

Thank you for the explanation. I have no idea about the restrictions on
pageblock and section. Out of curiosity, what prevents the growth of
the section size?

—
Best Regards,
Yan Zi

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