[RFC] Using large pages for large hash tables

[Hal Finkel via llvm-commits]

----- Original Message -----
> From: "Michael Kruse via llvm-commits" <llvm-commits at lists.llvm.org>
> To: "Rafael Espíndola" <rafael.espindola at gmail.com>
> Cc: "Aaron Ballman" <aaron.ballman at gmail.com>, "llvm-commits" <llvm-commits at lists.llvm.org>
> Sent: Monday, October 17, 2016 10:49:29 AM
> Subject: Re: [RFC] Using large pages for large hash tables
> 
> 2016-10-17 17:38 GMT+02:00 Rafael Espíndola via llvm-commits
> <llvm-commits at lists.llvm.org>:
> > I did a quick and dirty experiment to use large pages when a hash
> > table gets big. The results for lld are pretty impressive (see
> > attached file, but basically 1.04X faster link of files with debug
> > info).
> >
> > I tested disabling madvise and the performance goes back to what it
> > was, so it is really the large pages that improves the performance.
> >
> > The main question is then what the interface should look like. On
> > linux the abstraction could be
> >
> > std::pair<void *, size_t> mallocLarge(size_t Size);
> >
> > which return the allocated memory and how much was actually
> > allocated.
> > The pointer can be passed to free once it is no longer needed.
> >
> > The fallback implementation just calls malloc and returns Size
> > unmodified.
> >
> > On linux x86_64 if size is larger than 2MiB we use posix_memalign
> > and madvise.
> >
> > Would the same interface work on windows?
> 
> Yes, using VirtualAlloc using MEM_LARGE_PAGES flag:
> https://msdn.microsoft.com/fr-fr/library/windows/desktop/aa366887(v=vs.85).aspx
> 
> Size can be determined using GetLargePageMinimum(), or a multiple of
> it.
> 
> Do common implementations of malloc allocate (large) pages
> automatically when the allocated size is large enough?

No :(

 -Hal

> 
> Michael
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory