Date:   Mon, 26 Apr 2021 10:37:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "lipeifeng@...o.com" <lipeifeng@...o.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        peifengl55 <peifengl55@...il.com>,
        schwidefsky <schwidefsky@...ibm.com>,
        "heiko.carstens" <heiko.carstens@...ibm.com>,
        zhangshiming <zhangshiming@...o.com>,
        zhouhuacai <zhouhuacai@...o.com>,
        guoweichao <guoweichao@...o.com>, guojian <guojian@...o.com>
Cc:     linux-s390 <linux-s390@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] mm: support multi_freearea to the reduction of external
 fragmentation

On 26.04.21 05:19, lipeifeng@...o.com wrote:
> 
>  >> Let's consider part 3 only and ignore the 1) multi freearea (which might
>  >> be problematic with sparcity) and 2) the modified allocation scheme
>  >> (which doesn't yet quite sense to me yet, e.g., because we group by
>  >> mobility and have compaction in place; I assume this really only helps
>  >> in some special cases -- like the test case you are giving; I might be
>  >> wrong)
>  >> Right now, we decide whether to but to head or tail based on how likely
>  >> it is that we might merge to a higher-order page (buddy_merge_likely())
>  >> in the future. So we only consider the current "neighborhood" of the
>  >> page we're freeing. As we restrict our neighborhood to MAX_ORDER - 1
>  >> pages (what we can actually merge). Of course, we can easily be wrong
>  >> here. Grouping by movability and compaction only helps to some degree I
>  >> guess.
>  >> AFAIK, what you propose is basing the decisions where to place a page
>  >> (in addition?) on a median_pfn. Without 1) and 2) I cannot completely
>  >> understand if 3) itself would help at all (and how to set the
>  >> median_pfn). But it would certainly be interesting if we can tweak the
>  >> current logic to better identify merge targets simply by tweaking
>  >> buddy_merge_likely() or the assumptions it is making.
> 
> 
> 
> Hi David Hildenbrand，Vlastimil Babka:
>      Thank you very much indeed for advices.
> 
>>> 2) the modified allocation scheme
>  >> (which doesn't yet quite sense to me yet, e.g., because we group by
>  >> mobility and have compaction in place; I assume this really only helps
>  >> in some special cases -- like the test case you are giving;
>   ---------------------------------------------------------------------------------
> 1) Divide memory into several segments by pages-PFN
> 2) Select the corresponding freearea to alloc-pages
>      These two parts art for the same purpose:
> low-order-pages allocation will be concentrated in the front area of 
> physical memory
> so that few memory-pollution in the back area of memory, the sussessful 
> probablity
> of high-order allocation would be improved.
> 
>      I think that it would help in almost all cases of high-oder-pages 
> allocation, instead
>      of special case, because it can let more high-order free-pages in 
> buddy, example:

See, and I am not convinced that this is the case, because you really 
only report one example (Monkey) and I have to assume it is a special 
case then.

> 
>   * when user alloc 64K bytes, if the unit is page(4K bytes) and it
>     needs to 16 times. 
> 
> if the unit is 64Kbytes, it only takes once.
> 
>   * if there are more free-high-order-pages in buddy that few
>     compact-stall in
> 
> alloction-process, the allocstall-time would be shortened.
> 
>      We tested the speed of the high-orders-pages(order=4 and order = 8) 
> allocation
> after monkey and found that it increased by more than 18%.
> 

And you don't mention what the baseline configuration was. For example, 
how was compaction configured?

Just to clarify, what is monkey?

Monkey HTTP server? MonkeyTest disk benchmark? UI/Application Exerciser 
Monkey?

> 3) Adjust the location of free-pages in the free_list
>>>Without 1) and 2) I cannot completely
>  >>understand if 3) itself would help at all (and how to set the median_pfn)
> -----------------------------------------------------------------------------------------------------
>      Median_pfn is set by the range of pages-PFN of free_area. if part 
> 3) would be tried separately
>      without 1) and 2), the simple setting is the median of the entire 
> memory. But i think it will play the
> better role in optimization based on the 1) and 2).
> 
> 
> 
>  >> Last but not least, there have to be more benchmarks and test cases that
>  >> proof that other workload won't be degraded to a degree that people
>  >> care; as one example, this includes runtime overhead when
>>> allocating/freeing pages.
> ---------------------------------------------
> 1. For modification of buddy: the modified allocation scheme 1)+2)
>      Is thers any standard detailed test-list  of the modified 
> allocation in the community? like benchmarks
> or any other tests? if  i pass the test required by communiry that can 
> proof the patch will not degraded
> to a degree that people care and can merge it in the baseline?

IIRC, there are plenty. One example is will-it-scale.

Have a look at https://github.com/intel/lkp-tests.git


-- 
Thanks,

David / dhildenb

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