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

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Message-ID: <51CDB056.5090308@sr71.net>
Date:	Fri, 28 Jun 2013 08:48:38 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

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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