linux-kernel - Re: [RFC][PATCH] mm: madvise: MADV

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Message-ID: <51E2173A.8080003@gmail.com>
Date:	Sun, 14 Jul 2013 11:12:58 +0800
From:	Sam Ben <sam.bennn@...il.com>
To:	Dave Hansen <dave@...1.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

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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