Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings

[David Rientjes]

On Mon, 15 Oct 2018, Andrea Arcangeli wrote:

> > On Mon, 15 Oct 2018 15:30:17 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote:
> > >  Would it be possible to test with my 
> > > patch[*] that does not try reclaim to address the thrashing issue?
> > 
> > Yes please.
> 
> It'd also be great if a testcase reproducing the 40% higher access
> latency (with the one liner original fix) was available.
> 

I never said 40% higher access latency, I said 40% higher fault latency.  

The higher access latency is 13.9% as measured on Haswell.

The test case is rather trivial: fragment all memory with order-4 memory 
to replicate a fragmented local zone, use sched_setaffinity() to bind to 
that node, and fault a reasonable number of hugepages (128MB, 256, 
whatever).  The cost of faulting remotely in this case was measured to be 
40% higher than falling back to local small pages.  This occurs quite 
obviously because you are thrashing the remote node trying to allocate 
thp.

> We don't have a testcase for David's 40% latency increase problem, but
> that's likely to only happen when the system is somewhat low on memory
> globally.

Well, yes, but that's most of our systems.  We can't keep around gigabytes 
of memory free just to work around this patch.  Removing __GFP_THISNODE to 
avoid thrashing the local node obviously will incur a substantial 
performance degradation if you thrash the remote node as well.  This 
should be rather straight forward.

> When there's 75% or more of the RAM free (not even allocated as easily
> reclaimable pagecache) globally, you don't expect to hit heavy
> swapping.
> 

I agree there is no regression introduced by your patch when 75% of memory 
is free.

> The 40% THP allocation latency increase if you use MADV_HUGEPAGE in
> such window where all remote zones are fully fragmented is somehow
> lesser of a concern in my view (plus there's the compact deferred
> logic that should mitigate that scenario). Furthermore it is only a
> concern for page faults in MADV_HUGEPAGE ranges. If MADV_HUGEPAGE is
> set the userland allocation is long lived, so such higher allocation
> latency won't risk to hit short lived allocations that don't set
> MADV_HUGEPAGE (unless madvise=always, but that's not the default
> precisely because not all allocations are long lived).
> 
> If the MADV_HUGEPAGE using library was freely available it'd also be
> nice.
> 

You scan your mappings for .text segments, map a hugepage-aligned region 
sufficient in size, mremap() to that region, and do MADV_HUGEPAGE.