Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

[Dave Hansen <dave.hansen@intel.com>]

I was coincidentally tracking down what I thought was a scalability
problem (turned out to be full disks :).  I noticed, though, that ext4
is about 20% slower than ext2/3 at doing write page faults (x-axis is
number of tasks):

http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&hide=linear,threads,threads_idle,processes_idle&rollPeriod=5

The test case is:

	https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c

A 'perf diff' shows some of the same suspects that you've been talking
about, Andy:

	http://www.sr71.net/~dave/intel/page-fault-exts/diffprofile.txt

>      2.39%   +2.34%  [kernel.kallsyms]      [k] __set_page_dirty_buffers               
>              +2.50%  [kernel.kallsyms]      [k] __block_write_begin                    
>              +2.16%  [kernel.kallsyms]      [k] __block_commit_write                   

The same test on ext4 but doing MAP_PRIVATE instead of MAP_SHARED goes
at the same speed as ext2/3:

	https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault2.c

This is looking to me more like an ext4-specific problem that needs to
get solved rather than through some interfaces (like MADV_WILLWRITE).

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