Re: [PATCH 2/7] memcg: Scale down MEMCG_CHARGE_BATCH with increase in PAGE_SIZE

[Waiman Long <longman@redhat.com>]

On 3/20/26 7:26 AM, Li Wang wrote:
> Waiman Long wrote:
>
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -328,8 +328,14 @@ struct mem_cgroup {
>>    * size of first charge trial.
>>    * TODO: maybe necessary to use big numbers in big irons or dynamic based of the
>>    * workload.
>> + *
>> + * There are 3 common base page sizes - 4k, 16k & 64k. In order to limit the
>> + * amount of memory that can be hidden in each percpu memcg_stock for a given
>> + * memcg, we scale down MEMCG_CHARGE_BATCH by 2 for 16k and 4 for 64k.
>>    */
>> -#define MEMCG_CHARGE_BATCH 64U
>> +#define MEMCG_CHARGE_BATCH_BASE  64U
>> +#define MEMCG_CHARGE_BATCH_SHIFT ((PAGE_SHIFT <= 16) ? (PAGE_SHIFT - 12)/2 : 2)
>> +#define MEMCG_CHARGE_BATCH	 (MEMCG_CHARGE_BATCH_BASE >> MEMCG_CHARGE_BATCH_SHIFT)
> This is a good complement to the first patch. With this change,
> I got a chart to compare the three methods (linear, log2, sqrt)
> in the count threshold:
>
> 4k page size (BATCH=64):
>    
>    CPUs    linear    log2     sqrt
>    --------------------------------
>    1       256KB     256KB    256KB
>    8       2MB       1MB      512KB
>    128     32MB      2MB      2.75MB
>    1024    256MB     2.75MB   8MB
> 	
> 64k page size (BATCH=16):
>
>    CPUs    linear    log2     sqrt
>    -------------------------------
>    1       1MB       1MB      1MB
>    8       8MB       4MB      2MB
>    128     128MB     8MB      11MB
>    1024    1GB       11MB     32MB
>
>
> Both are huge improvements.
>
> log2 flushes more aggressively on large systems, which gives more accurate
> stats but at the cost of more frequent synchronous flushes.
>
> sqrt is more conservative, still a massive reduction from linear but gives
> more breathing room on large systems, which may be better for performance.
>
> I would leave this choice to you, Waiman, and the data is for reference.
>
I think it is a good idea to use the int_sqrt() function and I will use 
it in the next version.

Cheers,
Longman