Re: [PATCH v4 0/5] batched remove rmap in try_to_unmap_one()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 13 Mar 2023 20:45:21 +0800 Yin Fengwei <fengwei.yin@intel.com> wrote:

> This series is trying to bring the batched rmap removing to
> try_to_unmap_one(). It's expected that the batched rmap
> removing bring performance gain than remove rmap per page.
> 
> This series reconstruct the try_to_unmap_one() from:
>   loop:
>      clear and update PTE
>      unmap one page
>      goto loop
> to:
>   loop:
>      clear and update PTE
>      goto loop
>   unmap the range of folio in one call
> It is one step to always map/unmap the entire folio in one call.
> Which can simplify the folio mapcount handling by avoid dealing
> with each page map/unmap.
>
> ...
> 
> For performance gain demonstration, changed the MADV_PAGEOUT not
> to split the large folio for page cache and created a micro
> benchmark mainly as following:

Please remind me why it's necessary to patch the kernel to actually
performance test this?  And why it's proving so hard to demonstrate
benefits in real-world workloads?

(Yes, this was touched on in earlier discussion, but I do think these
considerations should be spelled out in the [0/N] changelog).

Thanks.