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

Dave Hansen
2013-08-08T22:58:43Z

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).