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

[Sam Ben]

On 07/02/2013 10:37 AM, Zheng Liu wrote:
> 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.

How do you pre-fault the mapping manually in your product system? By 
walking through the file touching each page?

> Regards,
>                                                  - Zheng
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