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

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

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在 2021/11/1 下午5:20, Michal Hocko 写道:
> On Sat 30-10-21 00:12:53, ning zhang wrote:
>> 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.
> What do you mean tey can't control this well?

I means they don't know where they should use THP.

And even if they use madvise, memory bloat still exists.
<https://dict.youdao.com/w/still%20exist/#keyfrom=E2Ctranslation>

>
>> 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.
> There will always be some internal fragmentation when huge pages are
> used. The amount will depend on how well the memory is used but huge
> pages give a performance boost in return.
>
> If the memory bloat is a significant problem then overeager THP usage is
> certainly not good and I would argue that applying THP always policy is
> not a proper configuration. No matter how much the MM code can try to
> fix up the situation it will be always a catch up game.
>   
>> 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.
> To my knowledge the most popular reason to disable THP is the runtime
> overhead. A large part of that overhead has been reduced by not doing
> heavy compaction during the page fault allocations by default. Memory
> overhead is certainly an important aspect as well but there is always
> a possibility to reduce that by reducing it to madvised regions for
> page fault (i.e. those where author of the code has considered the
> costs vs. benefits of the huge page) and setting up a conservative
> khugepaged policy. So there are existing tools available. You are trying
> to add quite a lot of code so you should have good arguments to add more
> complexity. I am not sure that popularizing THP is a strong one TBH.

Sorry for relpying late. For the compaction, we can set defrag
of THP to be defer or never, to avoid overhead produced by
direct reclaim. However, there are no way to reduce memory bloat.

If the memory usage reach the limit, and we can't reclaim some
pages, the OOM will be triggered and the process will be killed.
Our patchest is to avoid OOM.

Much code is interface to control ZSR. And we will try to
reduce the complexity.


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