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:

quoted

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.

quoted

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.