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

[David Hildenbrand <david@redhat.com>]

On 20.03.23 14:47, Yin, Fengwei wrote:
> Hi Andrew, David,
> 
> On 3/14/2023 11:09 AM, Yin Fengwei wrote:
>> 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.
>> "
> I just want to check how I can move this work forward. Is it enough
> by adding above message? Or still need some other work be done first? Thanks.

I think Andrew can add that, no need to resend. But we should see more 
review (I'm fairly busy ...).

-- 
Thanks,

David / dhildenb