Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On Mon, Jul 01, 2013 at 09:16:46AM -0700, Dave Hansen wrote:
> On 06/28/2013 07:20 PM, Zheng Liu wrote:
> >> > IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> >> > parallelizable, but one using this mechanism would be.
> > I look at the code, and it seems that we will handle MAP_POPULATE flag
> > after we release mmap_sem locking in vm_mmap_pgoff():
> > 
> >                 down_write(&mm->mmap_sem);
> >                 ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
> >                                     &populate);
> >                 up_write(&mm->mmap_sem);
> >                 if (populate)
> >                         mm_populate(ret, populate);
> > 
> > Am I missing something?
> 
> I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
> versus using MADV_POPULATE in 160 threads in parallel.
> 
> MADV_POPULATE was about 10x faster in the threaded configuration.
> 
> With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
> around so that we can write the reader count update in to it.  With
> mmap(), there is a lot of _contention_ on that lock which is much, much
> more expensive than simply bouncing a cacheline around.

Thanks for your explanation.

FWIW, it would be great if we can let MAP_POPULATE flag support shared
mappings because in our product system there has a lot of applications
that uses mmap(2) and then pre-faults this mapping.  Currently these
applications need to pre-fault the mapping manually.

Regards,
                                                - Zheng
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