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). -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
