linux-kernel - Re: [RFC PATCH V4 1/1] mm: add last level page table numa info to /proc/pid/numa

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Date:   Thu, 4 Aug 2022 10:12:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     haoxin <xhao@...ux.alibaba.com>, willy@...radead.org
Cc:     akpm@...ux-foundation.org, adobriyan@...il.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH V4 1/1] mm: add last level page table numa info to
 /proc/pid/numa_pgtable

On 04.08.22 10:04, haoxin wrote:
> 
> 在 2022/8/1 下午9:28, David Hildenbrand 写道:
>> On 01.08.22 14:17, Xin Hao wrote:
>>> In many data center servers, the shared memory architectures is
>>> Non-Uniform Memory Access (NUMA), remote numa node data access
>>> often brings a high latency problem, but what we are easy to ignore
>>> is that the page table remote numa access, It can also leads to a
>>> performance degradation.
>> Let me try rewriting:
>>
>> "
>> Many data center servers employ Non-Uniform Memory Access (NUMA)
>> architectures. Remote numa memory access results in high latency. While
>> memory placement is one issue, sub-optimal page table placement can also
>> result in surprise performance degradation.
>> "
> Thanks,  it reads more clearly.
> 
>>> So there add a new interface in /proc, This will help developers to
>>> get more info about performance issues if they are caused by cross-NUMA.
>>
>> Why do we only care about "last level page table", why not about the others?
>>
>> IMHO, we could emit something like "0, 1, 3, 0" instead for a given user
>> space address, showing the NUMA node the page table belongs to from
>> highest to lowest page table level.
> 
> I have planned to implement the PTE page table in this version first,  
> and then support other page tables in the next patch later.

If there are plans, let's do it all at once, to get a good and single
interface to expose that information.

-- 
Thanks,

David / dhildenb