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

[Dave Hansen]

On 06/27/2013 10:47 PM, Zheng Liu wrote:
>> I've been doing some testing involving large amounts of
>> page cache.  It's quite painful to get hundreds of GB
>> of page cache mapped in, especially when I am trying to
>> do it in parallel threads.  This is true even when the
>> page cache is already allocated and I only need to map
>> it in.  The test:
>>
>> 1. take 160 16MB files
>> 2. clone 160 threads, mmap the 16MB files, and either
>>   a. walk through the file touching each page
> 
> Why not change MAP_POPULATE flag in mmap(2)?  Now it is only for private
> mappings.  But maybe we could let it support shared mapping.

Adding that support to mmap() will certainly _help_ some folks.  But,
anything that mmap()s something is taking mmap_sem for write.  That
means that threaded apps doing mmap()/munmap() frequently are _not_
scalable.

IOW, a process needing to do a bunch of MAP_POPULATEs isn't
parallelizable, but one using this mechanism would be.
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