Re: [PATCH -next v2] mm, proc: collect percpu free pages into the free pages

[Liu Shixin <liushixin2@huawei.com>]

On 2022/8/23 21:37, Michal Hocko wrote:
> On Tue 23-08-22 20:46:43, Liu Shixin wrote:
>> On 2022/8/23 15:50, Michal Hocko wrote:
>>> On Mon 22-08-22 14:12:07, Andrew Morton wrote:
>>>> On Mon, 22 Aug 2022 11:33:54 +0800 Liu Shixin <liushixin2@huawei.com> wrote:
>>>>
>>>>> The page on pcplist could be used, but not counted into memory free or
>>>>> avaliable, and pcp_free is only showed by show_mem() for now. Since commit
>>>>> d8a759b57035 ("mm, page_alloc: double zone's batchsize"), there is a
>>>>> significant decrease in the display of free memory, with a large number
>>>>> of cpus and zones, the number of pages in the percpu list can be very
>>>>> large, so it is better to let user to know the pcp count.
>>>>>
>>>>> On a machine with 3 zones and 72 CPUs. Before commit d8a759b57035, the
>>>>> maximum amount of pages in the pcp lists was theoretically 162MB(3*72*768KB).
>>>>> After the patch, the lists can hold 324MB. It has been observed to be 114MB
>>>>> in the idle state after system startup in practice(increased 80 MB).
>>>>>
>>>> Seems reasonable.
>>> I have asked in the previous incarnation of the patch but haven't really
>>> received any answer[1]. Is this a _real_ problem? The absolute amount of
>>> memory could be perceived as a lot but is this really noticeable wrt
>>> overall memory on those systems?
>> This may not obvious when the memory is sufficient. However, as products monitor the
>> memory to plan it. The change has caused warning.
> Is it possible that the said monitor is over sensitive and looking at
> wrong numbers? Overall free memory doesn't really tell much TBH.
> MemAvailable is a very rough estimation as well.
>
> In reality what really matters much more is whether the memory is
> readily available when it is required and none of MemFree/MemAvailable
> gives you that information in general case.
>
>> We have also considered using /proc/zoneinfo to calculate the total
>> number of pcplists. However, we think it is more appropriate to add
>> the total number of pcplists to free and available pages. After all,
>> this part is also free pages.
> Those free pages are not generally available as exaplained. They are
> available to a specific CPU, drained under memory pressure and other
> events but still there is no guarantee a specific process can harvest
> that memory because the pcp caches are replenished all the time.
> So in a sense it is a semi-hidden memory.
>
> That being said, I am still not convinced this is actually going to help
> all that much. You will see a slightly different numbers which do not
> tell much one way or another and if the sole reason for tweaking these
> numbers is that some monitor is complaining because X became X-epsilon
> then this sounds like a weak justification to me. That epsilon happens
> all the time because there are quite some hidden caches that are
> released under memory pressure. I am not sure it is maintainable to
> consider each one of them and pretend that MemFree/MemAvailable is
> somehow precise. It has never been and likely never will be.
Thanks for your explanation. As you said, it seems that merge these memory into
MemFree/MemAvailable directly may affect the performance under memory pressure.
That sounds reasonable.
 
But since these memory is also free memory that can be uesd and is large, I think we
should still provide a statistic for the user. Perhaps add a new statistic is better?

Thanks,