Messages in this thread | | | Date | Mon, 15 Jul 2013 08:22:16 +0800 | From | Zheng Liu <> | Subject | Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting |
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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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