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

[Yin Fengwei <fengwei.yin@intel.com>]

On 3/14/23 02:49, Andrew Morton wrote:
> 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).
OK. What about add following in cover letter:
"
  The performance gain of this series can be demonstrated with large
  folio reclaim. In current kernel, vmscan() path will be benefited by
  the changes. But there is no workload/benchmark can show the exact
  performance gain for vmscan() path as far as I am aware.

  Another way to demonstrate the performance benefit is using
  MADV_PAGEOUT which can trigger page reclaim also. The problem is that
  MADV_PAGEOUT always split the large folio because it's not aware of
  large folio for page cache currently. To show the performance benefit,
  MADV_PAGEOUT is updated not to split the large folio.

  For long term with wider adoption of large folio in kernel (like large
  folio for anonymous page), MADV_PAGEOUT needs be updated to handle
  large folio as whole to avoid splitting it always.
"


Regards
Yin, Fengwei

> 
> Thanks.