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Message-ID: <459ebad2-3fde-4ee4-84b9-6e1e158632ca@fujitsu.com>
Date: Wed, 9 Jul 2025 01:03:28 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>, "akpm@...ux-foundation.org"
<akpm@...ux-foundation.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "Yasunori Gotou (Fujitsu)"
<y-goto@...itsu.com>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra
<peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
<vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel
Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
"lkp@...el.com" <lkp@...el.com>
Subject: Re: [PATCH RFC v2] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE
accounting
On 08/07/2025 16:56, Huang, Ying wrote:
> "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com> writes:
>
>> On 08/07/2025 10:47, Huang, Ying wrote:
>>> "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com> writes:
>>>
>>>> On 08/07/2025 09:14, Huang, Ying wrote:
>>>>> "Zhijian Li (Fujitsu)" <lizhijian@...itsu.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
>>>>
>>>
>>> Check the usage of PGPROMOTE_CANDIDATE again. It is used not only by
>>> rate limiting, but also promotion threshold adjustment, please take a
>>> look at numa_promotion_adjust_threshold(). Which may have larger
>>> influence on performance.
>>>
>>> After checking the threshold adjustment code, I think the changes in
>>> this patch may confuse threshold adjustment.
>>
>>
>> Indeed, I misunderstood the comment in the previous code:
>> /* workload changed, reset hot threshold */.
>>
>> Originally, this logic only reset the threshold for the current interval.
>> For the next cycle (60 seconds by default), the threshold is
>> re-evaluated based on the historical PGPROMOTE_CANDIDATE counts.
>> Therefore, the current change may affect threshold adjustment in subsequent cycles.
>>
>>
>> Do you think there's still a case to push for this patch?
>>
>> For example, by collecting more data with longer pmbench runs (over two threshold cycles),
>> or explicitly compensating nbp_rl_nr_cand and nbp_th_nr_cand to maintain existing
>> behavior for both the rate limit and threshold logic? something like:
>>
>> if (pgdat_free_space_enough(pgdat)) {
>> /* workload changed, reset hot threshold */
>> pgdat->nbp_threshold = 0;
>>
>> mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE, nr);
>> // compensation for rate limit and threshold
>> pgdat->nbp_rl_nr_cand += nr;
>> pgdat->nbp_th_nr_cand += nr;
>>
>> return true;
>> }
>
> I don't think that it's necessary to make the algorithm harder to be
> understood.
All right,
>
> If you think that the original stat really makes people confusing, I
> guess that we can add a new stat (say PGPROMOTE_CANDIDATE_OTHER).
Actually, I personally don't like to introduce a new stat for this case.
Anyway, we will further discuss this approach internally first.
Thank you !
Thanks
Zhijian
>
> ---
> Best Regards,
> Huang, Ying
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