Re: [RFC 1/4] mm, compaction: introduce kcompactd

[Vlastimil Babka]

On 07/23/2015 11:21 PM, David Rientjes wrote:
> On Thu, 23 Jul 2015, Vlastimil Babka wrote:
>
> > > When a khugepaged allocation fails for a node, it could easily kick off
> > > background compaction on that node and revisit the range later, very
> > > similar to how we can kick off background compaction in the page allocator
> > > when async or sync_light compaction fails.
> >
> > The revisiting sounds rather complicated. Page allocator doesn't have to do that.
>
> I'm referring to khugepaged having a hugepage allocation fail, the page
> allocator kicking off background compaction, and khugepaged rescanning the
> same memory for which the allocation failed later.

Ah, OK.

> > > The distinction I'm trying to draw is between "periodic" and "background"
> > > compaction.  I think there're usecases for both and we shouldn't be
> > > limiting ourselves to one or the other.
> >
> > OK, I understand you think we can have both, and the periodic one would be in
> > khugepaged. My main concern is that if we do the periodic one in khugepaged,
> > people might oppose adding yet another one as kcompactd. I hope we agree that
> > khugepaged is not suitable for all the use cases of the background one.
>
> Yes, absolutely.  I agree that we need the ability to do background
> compaction without requiring CONFIG_TRANSPARENT_HUGEPAGE.

Great.

> > My secondary concern/opinion is that I would hope that the background compaction
> > would be good enough to remove the need for the periodic one. So I would try the
> > background one first. But I understand the periodic one is simpler to implement.
> > On the other hand, it's not as urgent if you can simulate it from userspace.
> > With the 15min period you use, there's likely not much overhead saved when
> > invoking it from within the kernel? Sure there wouldn't be the synchronization
> > with khugepaged activity, but I still wonder if wiating for up to 1 minute
> > before khugepaged wakes up can make much difference with the 15min period.
> > Hm, your cron job could also perhaps adjust the khugepaged sleep tunable when
> > compaction is done, which IIRC results in immediate wakeup.
>
> There are certainly ways to do this from userspace, but the premise is
> that this issue, specifically for users of thp, is significant for
> everyone ;)

Well, then the default shouldn't be disabled :)

> The problem that I've encountered with a background-only approach is that
> it doesn't help when you exec a large process that wants to fault most of
> its text and thp immediately cannot be allocated.  This can be a result of
> never having done any compaction at all other than from the page
> allocator, which terminates when a page of the given order is available.
> So on a fragmented machine, all memory faulted is shown in
> thp_fault_fallback and we rely on khugepaged to (slowly) fix this problem
> up for us.  We have shown great improvement in cpu utilization by
> periodically compacting memory today.
>
> Background compaction arguably wouldn't help that situation because it's
> not fast enough to compact memory simultaneous to the large number of page
> faults, and you can't wait for it to complete at exec().  The result is
> the same: large thp_fault_fallback.

You probably presume that background compaction (e.g. kcompactd) is only 
triggered by high-order allocator failures. That would be indeed too 
late. But the plan is that it would be more proactive and e.g. wake up 
periodically to check the fragmentation / availability of high-order pages.

Also in the scenario of suddenly executing a large process, isn't it 
that the most of memory not occupied by other processes is already 
occupied by page cache anyway, so you would have at most high-watermark 
worth of compacted free memory? If the process is larger than that, it 
would still go to direct reclaim/compaction regardless of periodic 
compaction...

> So I can understand the need for both periodic and background compaction
> (and direct compaction for non-thp non-atomic high-order allocations
> today) and I'm perhaps not as convinced as you are that we can eventually
> do without periodic compaction.
>
>
> It seems to me that the vast majority of this discussion has centered
> around the vehicle that performs the compaction.  We certainly require
> kcompactd for background compaction, and we both agree that we need that
> functionality.
>
> Two issues I want to bring up:
>
>   (1) do non-thp configs benefit from periodic compaction?
>
>       In my experience, no, but perhaps there are other use cases where
>       this has been a pain.  The primary candidates, in my opinion,
>       would be the networking stack and slub.  Joonsoo reports having to
>       workaround issues with high-order slub allocations being too
>       expensive.  I'm not sure that would be better served by periodic
>       compaction, but it seems like a candidate for background compaction.

Yes hopefully a proactive background compaction would serve them enough.

>       This is why my rfc tied periodic compaction to khugepaged, and we
>       have strong evidence that this helps thp and cpu utilization.  For
>       periodic compaction to be possible outside of thp, we'd need a use
>       case for it.
>
>   (2) does kcompactd have to be per-node?
>
>       I don't see the immediate benefit since direct compaction can
>       already scan remote memory and migrate it, khugepaged can do the

It can work remotely, but it's slower.

>       same.  Is there evidence that suggests that a per-node kcompactd
>       is significantly better than a single kthread?  I think others
>       would be more receptive of a single kthread addition.

I think it's simpler design wrt waking up the kthread for the desired 
node, and self-tuning any sleeping depending on per-node pressure. It 
also matches the design of kswapd. And IMHO machines with many memory 
nodes should naturally have also many CPU's to cope with the threads, so 
it should all scale well.

> My theory is that periodic compaction is only significantly beneficial for
> thp per my rfc, and I think there's a significant advantage for khugepaged
> to be able to trigger this periodic compaction immediately before scanning
> and allocating to avoid waiting potentially for the lengthy
> alloc_sleep_millisecs.  I don't see a problem with defining the period
> with a khugepaged tunable for that reason.
>
> For background compaction, which is more difficult, it would be simple to
> implement a kcompactd to perform the memory compaction and actually be
> triggered by khugepaged to do the compaction on its behalf and wait to
> scan and allocate until it' complete.  The vehicle will probably end up as
> kcompactd doing the actual compaction is both cases.
>
> But until we have a background compaction implementation, it seems like
> there's no objection to doing and defining periodic compaction in
> khugepaged as the rfc proposes?  It seems like we can easily extend that
> in the future once background compaction is available.

Besides the vehicle details which are possible to change in the future, 
the larger issue IMHO is introducing new tunables and then having to 
commit to them forever. So we should be careful there.

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