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.
> > 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.
> 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 ;)
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.
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.
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
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.
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.
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