Re: [RFC] mm: support multi_freearea to the reduction of external fragmentation

David Hildenbrand
2021-04-26T08:37:31Z

On 26.04.21 05:19, lipeifeng@oppo.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