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

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

在 2021/10/29 下午9:38, Michal Hocko 写道:
> On Thu 28-10-21 19:56:49, 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.
> What is the THP policy configuration in your testing? I assume you are
> using defaults right? That would be always for THP and madvise for
> defrag. Would it make more sense to use madvise mode for THP for your
> workload? The THP code is rather complex and just by looking at the
> diffstat this add quite a lot on top. Is this really worth it?

The THP configuration is always.

Madvise needs users to set MADV_HUGEPAGE by themselves if they want use 
huge page, while many users don't do set this, and they can't control 
this well.

Such as java, users can set heap and metaspace to use huge pages with 
madvise, but there is also memory bloat. Users still need to test 
whether their app can accept the waste.

For the case above, if we set THP configuration to be madvise, all the 
pages it uses will be 4K-page.

Memory bloat is one of the most important reasons that users disable 
THP.  We do this to popularize THP to be default enabled.