Re: [PATCH v3] mm: add thp_utilization metrics to /proc/thp_utilization

[Yang Shi <shy828301@gmail.com>]

On Thu, Aug 11, 2022 at 2:55 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Thu, Aug 11, 2022 at 1:20 PM Alex Zhu (Kernel) <alexlzhu@fb.com> wrote:
> >
> > Hi Yu,
> >
> > I’ve updated your patch set from last year to work with folio and am testing it now. The functionality in split_huge_page() is the same as what I have. Was there any follow up work done later?
>
> Yes, but it won't change the landscape any time soon (see below). So
> please feel free to continue along your current direction.
>
> > If not, I would like to incorporate this into what I have, and then resubmit. Will reference the original patchset. We need this functionality for the shrinker, but even the changes to split_huge_page() by itself it should show some performance improvement when used by the existing deferred_split_huge_page().
>
> SGTM. Thanks!
>
> A side note:
>
> I'm working on a new mode: THP=auto, meaning the kernel will detect
> internal fragmentation of 2MB compound pages to decide whether to map
> them by PMDs or split them under memory pressure. The general workflow
> of this new mode is as follows.

I tend to agree that avoiding allocating THP in the first place is the
preferred way to avoid internal fragmentation. But I got some
questions about your design/implementation:

>
> In the page fault path:
> 1. Compound pages are allocated as usual.
> 2. Each is mapped by 512 consecutive PTEs rather than a PMD.
> 3. There will be more TLB misses but the same number of page faults.
> 4. TLB coalescing can mitigate the performance degradation.

Why not just allocate base pages in the first place? Khugepaged has
max_pte_none tunable to detect internal fragmentation. If you worry
about zero page, you could add max_pte_zero tunable.

Or did you investigate whether the new MADV_COLLAPSE may be helpful or
not? It leaves the decision to the userspace.

>
> In khugepaged:
> 1. Check the dirty bit in the PTEs mapping a compound page, to
> determine its utilization.
> 2. Remap compound pages that meet a certain utilization threshold by
> PMDs in place, i.e., no migrations.
>
> In the reclaim path, e.g., MGLRU page table scanning:
> 1. Decide whether compound pages mapped by PTEs should be split based
> on their utilizations and memory pressure, e.g., reclaim priority.
> 2. Clean subpages should be freed directly after split, rather than swapped out.
>
> N.B.
> 1. This workflow relies on the dirty bit rather examining the content of a page.
> 2. Sampling can be done by periodically switching between a PMD and
> 512 consecutive PTEs.
> 3. It only needs to hold mmap_lock for read because this special mode
> (512 consecutive PTEs) is not considered the split mode.
> 4. Don't hold your breath :)
>
> Other references:
> 1. https://www.usenix.org/system/files/atc20-zhu-weixi_0.pdf
> 2. https://www.usenix.org/system/files/osdi21-hunter.pdf