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

[Zheng Liu]

On Sun, Jul 14, 2013 at 11:12:58AM +0800, Sam Ben wrote:
> 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?

Yes, in our product system most applications do like this.

Regards,
                                                - Zheng
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