Re: [PATCH RFC v2] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE accounting

["Zhijian Li (Fujitsu)" <lizhijian@fujitsu.com>]

On 08/07/2025 09:14, Huang, Ying wrote:
> "Zhijian Li (Fujitsu)" <lizhijian@fujitsu.com> writes:
> 
>> Hi,
>>
>>
>> On 25/06/2025 10:13, Li Zhijian wrote:
>>> V2:
>>> Fix compiling error # Reported by LKP
>>>
>>> As Ying suggested, we need to assess whether this change causes regression.
>>> However, considering the stringent conditions this patch involves,
>>> properly evaluating it may be challenging, as the outcomes depend on your
>>> perspective. Much like in a zero-sum game, if someone benefits, another
>>> might lose.
>>>
>>> If there are subsequent results, I will update them here.
>>
>> I ran memhog + pmbench to evaluate the impact of the patch(3 runs [1] for each kernel).
>>
>> The results show an approximate 4% performance increase in pmbench after applying this patch.
>>
>> Average     pmbench-access            max-promotion-rate
>> Before:     7956805 pages/sec                168301 pages/sec
>> After:      8313666 pages/sec (+4.4%)        207149 pages/sec
> 
> It's hard for me to understand why performance increases because of
> higher promotion rate, while the expected behavior is more promotion
> rate limiting.

Good question.

Above max-promotion-rate means the maximum rate during the WHOLE pmbench period which
can not indicate the total promoted pages.

Allow me to present each sample [0] recorded per second during the pmbench duration, as exemplified below:


             |       AFTER             |VS |           BEFORE       |
------------+-------------------------+++++------------------------|
| Timestamp |  pgprom/s   |  pgdem/s  |   |  pgprom/s  |  pgdem/s  |
|-----------|-------------|-----------|---|------------|-----------|
|     1     |   122977    |     0     |   |   123051   |     0     |
|     2     |   50171     |     0     |   |   50159    |     0     |
|     3     |     18      |     0     |   |     28     |     0     |
|     4     |   16647     |     0     |   |     0      |     0     |
|     5     | 207149.5    |     0     |   |   78895    |     0     |
|     6     | 193411      | 161521    |   |  168301    |   8702    |
|     7     |  52464      |  53989    |   |   42294    |  39108    |
|     8     |   5133      |   2627    |   |     0      |     0     |
|     9     |     24      |     8     |   |   3875     |   6213    |
|    10     |     0       |     0     |   |  45513     |  43260    |
|    11     |     0       |     0     |   |  36600     |  44982    |
|    12     |     0       |     0     |   |  21091     |  11631    |
|    13     |     0       |     0     |   |  12276     |  10719    |
|    14     |     0       |     0     |   | 149699     | 149400    |
|    15     |     0       |     0     |   |   4026     |   4933    |
|    16     |     0       |     0     |   |   3780     |     0     |
|    17     |     0       |     0     |   |     2      |     0     |
|    18     |     0       |     0     |   |     0      |     0     |
|    19     |     0       |     0     |   |     0      |     0     |
|    20     |     0       |     0     |   |     0      |     0     |
|    21     |     0       |     0     |   |    62      |     0     |
|    22     |     0       |     0     |   |   2016     |     0     |
|    23     |     0       |     0     |   |     0      |     0     |
|    24     |     0       |     0     |   |    62      |     0     |
|    25     |   8308      |     0     |   |     1      |     0     |
|    26     |   220       |     0     |   |     0      |     0     |
|    27     |     0       |     0     |   |  1995.05   |     0     |
|    28     |     0       |     0     |   |     1      |     0     |
|    29     |   5791      |     0     |   |     0      |     0     |
|    30     |     0       |     0     |   |    62      |     0     |
------------+-------------------------+++++------------------------|
|   total   | 662313.5    | 218145    |   | 743789.05  | 318948    |
|    max    | 207149.5    | 161521    |   |  168301    | 149400    |
------------+-------------------------+++++------------------------|
|   pmbench |        8416250          |VS |        8079500         |


As far as I can tell, the higher pmbench scores applied-patch may be attributed to
a reduction in the total number of promoted pages in the entire pmbench execution period.
(Similar circumstances were observed in the results of other tests conducted)



[0]
before:
https://github.com/zhijianli88/misc/blob/main/20250627/promotion-evaluation/without-patch/pmbench-1750988862.log
https://github.com/zhijianli88/misc/blob/main/20250627/promotion-evaluation/without-patch/sar-1750988862.log
after:
https://github.com/zhijianli88/misc/blob/main/20250627/promotion-evaluation/with-patch/pmbench-1750988291.log
https://github.com/zhijianli88/misc/blob/main/20250627/promotion-evaluation/with-patch/sar-1750988291.log


Thanks
Zhijian

> 
>> The detailed logs are available at [2].
>>
>> [1] https://github.com/zhijianli88/misc/blob/main/20250627/promotion-evaluation/reproduce.sh
>> [2] https://github.com/zhijianli88/misc/tree/main/20250627/promotion-evaluation
> 
> [snip]
> 
> ---
> Best Regards,
> Huang, Ying