Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Chen Ridong]

On 2025/12/5 10:57, zhongjinji wrote:
>> From: Chen Ridong <chenridong@xxxxxxxxxx>
>>
>> The memcg LRU was originally introduced for global reclaim to enhance
>> scalability. However, its implementation complexity has led to performance
>> regressions when dealing with a large number of memory cgroups [1].
>>
>> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
>> cookie-based iteration for global reclaim, aligning with the approach
>> already used in shrink_node_memcgs. This simplification removes the
>> dedicated memcg LRU tracking while maintaining the core functionality.
>>
>> It performed a stress test based on Zhao Yu's methodology [2] on a
>> 1 TB, 4-node NUMA system. The results are summarized below:
>>
>> 					memcg LRU    memcg iter
>> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
>> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> Are there more data available? For example, the load of kswapd or the refault values.
> 
> I am concerned about these two data points because Yu Zhao's implementation controls
> the fairness of aging through memcg gen (get_memcg_gen). This helps reduce excessive
> aging for certain cgroups, which is beneficial for kswapd's power consumption.
> 
> At the same time, pages that age earlier can be considered colder pages (in the entire system),
> so reclaiming them should also help with the refault values.
> 

I re-ran the test and observed a 3.2% increase in refaults. Is this enough for what you were
concerned about?

The complete data set is offered in my earlier email:

https://lore.kernel.org/all/e657d5ac-6f92-4dbb-bf32-76084988d024@xxxxxxxxxxxxxxx/

>> The new implementation demonstrates a significant improvement in
>> fairness, reducing the standard deviation relative to the mean by
>> 15.5 percentage points. While the reclaim accuracy shows a slight
>> increase in overscan (from 85086871 to 90633890, 6.5%).
>>
>> The primary benefits of this change are:
>> 1. Simplified codebase by removing custom memcg LRU infrastructure
>> 2. Improved fairness in memory reclaim across multiple cgroups
>> 3. Better performance when creating many memory cgroups
>>
>> [1] https://lore.kernel.org/r/20251126171513.GC135004@xxxxxxxxxxx
>> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@xxxxxxxxxx
>> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>

-- 
Best regards,
Ridong