Re: [RFC PATCH 0/3] Change how we determine when to hand out THPs

[Andrea Arcangeli]

On Mon, Dec 16, 2013 at 11:12:15AM -0600, Alex Thorlton wrote:
> As it stands right now, yes, since we're pulling a 512 page contiguous
> chunk off the free list, everything from that chunk will reside on the
> same node, but as I (stupidly) forgot to mention in my original e-mail,
> one piece I have yet to add is the functionality to put the remaining
> unfaulted pages from our chunk *back* on the free list after we give up
> on handing out a THP.  Once this is in there, things will behave more
> like they do when THP is turned completely off, i.e. pages will get
> faulted in closer to the CPU that first referenced them once we give up
> on handing out the THP.

The only problem is the additional complexity and the slowdown to the
common cases that benefit from THP immediately.

> Yes.  Due to the time it takes to search for the temporary THP, I'm sure
> we won't get down to 76 secs, but hopefully we'll get close.  I'm also

Did you consider using MADV_NOHUGEPAGE?

Clearly this will disable it not just on NUMA, but NUMA vs non-NUMA
the problem is pretty much the same. You may want to verify if your
runs faster on non-NUMA too, with MADV_NOHUGEPAGE.

If every thread only touches 1 subpage of every hugepage mapped, the
number of TLB misses will be lower with many 4k d-TLB than with fewer
2M d-TLB entries. The only benefit that remains from THP in such case
is that the TLB miss is faster with THP and that's not always enough
to offset the cost of the increased number of TLB misses.

But MADV_NOHUGEPAGE is made specifically to tune for those non common
cases and if you know what the app is doing and you know the faster
TLB miss is a win despite the fewer 2M TLB entries on non-NUMA, you
could do:

      if (numnodes() > 1)
      	 madvise(MADV_NOHUGEPAGE, ...);

> considering switching the linked list that stores the temporary THPs
> over to an rbtree to make that search faster, just fyi.

Problem is that it'll be still slower than no change.

I'm certainly not against trying to make the kernel smarter to
optimize for non common workloads, but if the default behavior shall
not change and this is a tweak the admin should tune manually, I'd
like an explanation of why the non privileged MADV_NOHUGEPAGE madvise
is worse solution for this than a privileged tweak in sysfs that the
root user may also forget if not careful.

The problem in all this, is that this is a tradeoff and depending on
the app anything in between the settings "never" and "always" could be
optimal.

The idea was just to map THP whenever possible by default to keep the
kernel simpler and to gain the maximum performance from the faster TLB
miss immediately (and hopefully offset those cases were the number of
TLB misses doesn't decrease with THP enabled, like probably your app,
and at the same time avoiding the need later for a THP collapsing
event that requires TLB flushes and will add additional costs). For
what is extreme and just wants THP off, I thought MADV_NOHUGEPAGE
would be fine solution.

I doubt we can change the default behavior, at the very least it would
require lots of benchmarking and the only benchmarking I've seen here
is for the corner case app which may actually run the fastest with
MADV_NOHUGEPAGE than with a intermediate threshold.

If we instead we need an intermediate threshold less aggressive than
MADV_NOHUGEPAGE, I think the tweaks should be per-process and not
privileged, like MADV_NOHUGEPAGE. Because by definition of the
tradeoff, every app could have its own preferred threshold. And like
your app wants THP off, the majority still wants it on without
intermediate steps. So with a system wide setting you can't make
everyone happy unless you're like #osv in a VM running a single app.
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