Re: [RFC 0/6] Reclaim zero subpages of thp to avoid memory bloat

[ning zhang <ningzhang@linux.alibaba.com>]

在 2021/10/28 下午10:13, Kirill A. Shutemov 写道:
> On Thu, Oct 28, 2021 at 07:56:49PM +0800, Ning Zhang wrote:
>> As we know, thp may lead to memory bloat which may cause OOM.
>> Through testing with some apps, we found that the reason of
>> memory bloat is a huge page may contain some zero subpages
>> (may accessed or not). And we found that most zero subpages
>> are centralized in a few huge pages.
>>
>> Following is a text_classification_rnn case for tensorflow:
>>
>>    zero_subpages   huge_pages  waste
>>    [     0,     1) 186         0.00%
>>    [     1,     2) 23          0.01%
>>    [     2,     4) 36          0.02%
>>    [     4,     8) 67          0.08%
>>    [     8,    16) 80          0.23%
>>    [    16,    32) 109         0.61%
>>    [    32,    64) 44          0.49%
>>    [    64,   128) 12          0.30%
>>    [   128,   256) 28          1.54%
>>    [   256,   513) 159        18.03%
>>
>> In the case, there are 187 huge pages (25% of the total huge pages)
>> which contain more then 128 zero subpages. And these huge pages
>> lead to 19.57% waste of the total rss. It means we can reclaim
>> 19.57% memory by splitting the 187 huge pages and reclaiming the
>> zero subpages.
>>
>> This patchset introduce a new mechanism to split the huge page
>> which has zero subpages and reclaim these zero subpages.
>>
>> We add the anonymous huge page to a list to reduce the cost of
>> finding the huge page. When the memory reclaim is triggering,
>> the list will be walked and the huge page contains enough zero
>> subpages may be reclaimed. Meanwhile, replace the zero subpages
>> by ZERO_PAGE(0).
> Does it actually help your workload?
>
> I mean this will only be triggered via vmscan that was going to split
> pages and free anyway.
>
> You prioritize splitting THP and freeing zero subpages over reclaiming
> other pages. It may or may not be right thing to do, depending on
> workload.
>
> Maybe it makes more sense to check for all-zero pages just after
> split_huge_page_to_list() in vmscan and free such pages immediately rather
> then add all this complexity?
>
The purpose of zero subpages reclaim(ZSR) is to pick out the huge pages 
which
have waste and reclaim them.

We do this for two reasons:
1. If swap is off, anonymous pages will not be scanned, and we don't 
have the
    opportunity  to split the huge page. ZSR can be helpful for this.
2. If swap is on, splitting first will not only split the huge page, but 
also
    swap out the nonzero subpages, while ZSR will only split the huge page.
    Splitting first will result to more performance degradation. If ZSR 
can't
    reclaim enough pages, swap can still work.

Why use a seperate ZSR list instead of the default LRU list?

Because it may cause high CPU overhead to scan for target huge pages if 
there
both exist a lot of regular and huge pages. And it maybe especially 
terrible
when swap is off, we may scan the whole LRU list many times. A huge page 
will
be deleted from ZSR list when it was scanned, so the page will be 
scanned only
once. It's hard to use LRU list, because it may add new pages into LRU list
continuously when scanning.

Also, we can decrease the priority to prioritize reclaiming file-backed 
page.
For example, only triggerring ZSR when the priority is less than 4.
>> Yu Zhao has done some similar work when the huge page is swap out
>> or migrated to accelerate[1]. While we do this in the normal memory
>> shrink path for the swapoff scene to avoid OOM.
>>
>> In the future, we will do the proactive reclaim to reclaim the "cold"
>> huge page proactively. This is for keeping the performance of thp as
>> for as possible. In addition to that, some users want the memory usage
>> using thp is equal to the usage using 4K.
> Proactive reclaim can be harmful if your max_ptes_none allows to recreate
> THP back.
Thanks! We will consider it.
>