Re: [Question] About memory.c: process_huge_page

[Zhu Haoran <zhr1502@sjtu.edu.cn>]

Dev Jain <dev.jain@arm.com> writes:
>On 25/09/25 7:02 am, Huang, Ying wrote:
>> Hi, Haoran,
>>
>> Zhu Haoran <zhr1502@sjtu.edu.cn> writes:
>>
>>> Hi!
>>>
>>> I recently noticed the process_huge_page function in memory.c, which was
>>> intended to keep the cache hotness of target page after processing. I compared
>>> the vm-scalability anon-cow-seq-hugetlb microbench using the default
>>> process_huge_page and sequential processing (code posted below).
>>>
>>> I ran test on epyc-7T83 with 36vCPUs and 64GB memory. Using default
>>> process_huge_page, the avg bandwidth is 1148 mb/s. However sequential
>>> processing yielded a better bandwidth of about 1255 mb/s and only
>>> one-third cache-miss rate compared with default one.
>>>
>>> The same test was run on epyc-9654 with 36vCPU and 64GB mem. The
>>> bandwidth result was similar but the difference was smaller: 1170mb/s
>>> for default and 1230 mb/s for sequential. Although we did find the cache
>>> miss rate here did the reverse, since the sequential processing seen 3
>>> times miss more than the default.
>>>
>>> These result seem really inconsitent with the what described in your
>>> patchset [1]. What factors might explain these behaviors?
>> One possible difference is cache topology.  Can you try to bind the test
>> process to the CPUs in one CCX (that is, share one LLC).  This make it
>> possible to hit the local cache.
>
>Hi, I just had a different question, why is the function sprinkled with
>cond_resched() in each loop, especially the last one in which we are calling
>it every iteration? I suppose one reason for slowdown may be this too.

However, whether it is process_huge_page or sequential processing, the
implementation always performs cond_resched before each process_subpage. Seems
no difference.

>> Thanks for your time.
>>
>> [1] https://lkml.org/lkml/2018/5/23/1072
>>
>> ---
>> Sincere,
>> Zhu Haoran
>>
>> ---
>>
>> static int process_huge_page(
>>          unsigned long addr_hint, unsigned int nr_pages,
>>          int (*process_subpage)(unsigned long addr, int idx, void *arg),
>>          void *arg)
>> {
>>      int i, ret;
>>      unsigned long addr = addr_hint &
>>          ~(((unsigned long)nr_pages << PAGE_SHIFT) - 1);
>>
>>      might_sleep();
>>      for (i = 0; i < nr_pages; i++) {
>>              cond_resched();
>>              ret = process_subpage(addr + i * PAGE_SIZE, i, arg);
>>              if (ret)
>>                      return ret;
>>      }
>>
>>      return 0;
>> }
> ---
> Best Regards,
> Huang, Ying

---
Sincere,
Zhu Haoran