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

Yang Shi
2021-10-29T16:56:30Z

On Fri, Oct 29, 2021 at 5:08 AM ning zhang <ningzhang@linux.alibaba.com> wrote:
>
>
> 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.

I'm not sure if this will help the workloads in general or not. The
problem is it doesn't check if the huge page is "hot" or not. It just
picks up the first huge page from the list, which seems like a FIFO
list IIUC. But if the huge page is "hot" even though there is some
internal access imbalance it may be better to keep the huge page since
the performance gain may outperform the memory saving. But if the huge
page is not "hot", then I think the question is why it is a THP in the
first place.

Let's step back to think about whether allocating THP upon first
access for such area or workload is good or not. We should be able to
check the access imbalance in allocation stage instead of reclaim
stage. Currently anonymous THP just supports 3 modes: always, madvise
and none. Both always and madvise tries to allocate THP in page fault
path (assuming anonymous THP) upon first access. I'm wondering if we
could add a "defer" mode or not. It defers THP allocation/collapse to
khugepaged instead of in page fault path. Then all the knobs used by
khugepaged could be applied, particularly max_ptes_none in your case.
You could set a low max_ptes_none if you prefer memory saving. IMHO,
this seems much simpler than scanning list (may be quite long) to find
out suitable candidate then split then replace to zero page.

Of course this may have some potential performance impact since the
THP install is delayed for some time. This could be optimized by
respecting MADV_HUGEPAGE.

Anyway, just some wild idea.

> >> 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.
> >
>