Re: [PATCH v2 00/16] Multigenerational LRU Framework

[Yu Zhao]

On Wed, Apr 14, 2021 at 9:00 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
> > We fall back to the rmap when it's obviously not smart to do so. There
> > is still a lot of room for improvement in this function though, i.e.,
> > it should be per VMA and NUMA aware.
>
> Okay so it's more a question to tune the cross over heuristic. That
> sounds much easier than replacing everything.
>
> Of course long term it might be a problem to maintain too many
> different ways to do things, but I suppose short term it's a reasonable
> strategy.

Hi Rik, Ying,

Sorry for being persistent. I want to make sure we are on the same page:

Page table scanning doesn't replace the existing rmap walk. It is
complementary and only happens when it is likely that most of the
pages on a system under pressure have been referenced, i.e., out of
*inactive* pages, by definition of the existing implementation. Under
such a condition, scanning *active* pages one by one with the rmap is
likely to cost more than scanning them all at once via page tables.
When we evict *inactive* pages, we still use the rmap and share a
common path with the existing code.

Page table scanning falls back to the rmap walk if the page tables of
a process are apparently sparse, i.e., rss < size of the page tables.

I should have clarified this at the very beginning of the discussion.
But it has become so natural to me and I assumed we'd all see it this
way.

Your concern regarding the NUMA optimization is still valid, and it's
a high priority.

Thanks.