Re: [PATCH] thp: reduce usage of huge zero page's atomic counter

[Aaron Lu]

On 08/30/2016 12:44 PM, Anshuman Khandual wrote:
> On 08/30/2016 09:09 AM, Andrew Morton wrote:
>> On Tue, 30 Aug 2016 11:09:15 +0800 Aaron Lu <aaron.lu@xxxxxxxxx> wrote:
>>
>>>>> Case used for test on Haswell EP:
>>>>> usemem -n 72 --readonly -j 0x200000 100G
>>>>> Which spawns 72 processes and each will mmap 100G anonymous space and
>>>>> then do read only access to that space sequentially with a step of 2MB.
>>>>>
>>>>> perf report for base commit:
>>>>>     54.03%  usemem   [kernel.kallsyms]   [k] get_huge_zero_page
>>>>> perf report for this commit:
>>>>>      0.11%  usemem   [kernel.kallsyms]   [k] mm_get_huge_zero_page
>>>>
>>>> Does this mean that overall usemem runtime halved?
>>>
>>> Sorry for the confusion, the above line is extracted from perf report.
>>> It shows the percent of CPU cycles executed in a specific function.
>>>
>>> The above two perf lines are used to show get_huge_zero_page doesn't
>>> consume that much CPU cycles after applying the patch.
>>>
>>>>
>>>> Do we have any numbers for something which is more real-wordly?
>>>
>>> Unfortunately, no real world numbers.
>>>
>>> We think the global atomic counter could be an issue for performance
>>> so I'm trying to solve the problem.
>>
>> So, umm, we don't actually know if the patch is useful to anyone?
> 
> On a POWER system it improves the CPU consumption of the above mentioned
> function a little bit. Dont think its going to improve actual throughput
> of the workload substantially.
> 
> 0.07%  usemem  [kernel.vmlinux]  [k] mm_get_huge_zero_page

I guess this is the base commit? But there shouldn't be the new
mm_get_huge_zero_page symbol before this patch. A typo perhaps?

Regards,
Aaron

> to
> 
> 0.01%  usemem  [kernel.vmlinux]  [k] mm_get_huge_zero_page
>