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SubjectRe: [RFC PATCH v1 13/13] mm: splice local lists onto the front of the LRU
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On 02/02/2018 12:21 AM, Aaron Lu wrote:
> On Wed, Jan 31, 2018 at 06:04:13PM -0500, daniel.m.jordan@oracle.com wrote:
>> Now that release_pages is scaling better with concurrent removals from
>> the LRU, the performance results (included below) showed increased
>> contention on lru_lock in the add-to-LRU path.
>>
>> To alleviate some of this contention, do more work outside the LRU lock.
>> Prepare a local list of pages to be spliced onto the front of the LRU,
>> including setting PageLRU in each page, before taking lru_lock. Since
>> other threads use this page flag in certain checks outside lru_lock,
>> ensure each page's LRU links have been properly initialized before
>> setting the flag, and use memory barriers accordingly.
>>
>> Performance Results
>>
>> This is a will-it-scale run of page_fault1 using 4 different kernels.
>>
>> kernel kern #
>>
>> 4.15-rc2 1
>> large-zone-batch 2
>> lru-lock-base 3
>> lru-lock-splice 4
>>
>> Each kernel builds on the last. The first is a baseline, the second
>> makes zone->lock more scalable by increasing an order-0 per-cpu
>> pagelist's 'batch' and 'high' values to 310 and 1860 respectively
>
> Since the purpose of the patchset is to optimize lru_lock, you may
> consider adjusting pcp->high to be >= 32768(page_fault1's test size is
> 128M = 32768 pages). That should eliminate zone->lock contention
> entirely.

Interesting, hadn't thought about taking zone->lock completely out of
the equation. Will try this next time I test this series.


While we're on this topic, it does seem from the performance of kernel
#2, and the numbers Aaron posted in a previous thread[*], that the
default 'batch' and 'high' values should be bigger on large systems.

The code to control these two values last changed in 2005[**], so we hit
the largest values with just a 512M zone:

zone 4k_pages batch high high/4k_pages
64M 16,384 3 18 0.10986%
128M 32,768 7 42 0.12817%
256M 65,536 15 90 0.13733%
512M 131,072 31 186 0.14191%
1G 262,144 31 186 0.07095%
2G 524,288 31 186 0.03548%
4G 1,048,576 31 186 0.01774%
8G 2,097,152 31 186 0.00887%
16G 4,194,304 31 186 0.00443%
32G 8,388,608 31 186 0.00222%
64G 16,777,216 31 186 0.00111%
128G 33,554,432 31 186 0.00055%
256G 67,108,864 31 186 0.00028%
512G 134,217,728 31 186 0.00014%
1024G 268,435,456 31 186 0.00007%


[*] https://marc.info/?l=linux-netdev&m=150572010919327
[**] ba56e91c9401 ("[PATCH] mm: page_alloc: increase size of per-cpu-pages")

>
>> (courtesy of Aaron Lu's patch), the third scales lru_lock without
>> splicing pages (the previous patch in this series), and the fourth adds
>> page splicing (this patch).
>>
>> N tasks mmap, fault, and munmap anonymous pages in a loop until the test
>> time has elapsed.

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