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    SubjectRe: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)
    On 08/09/2013 12:55 AM, Jan Kara wrote:
    > On Thu 08-08-13 15:58:39, Dave Hansen wrote:
    >> 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
    > The reason is that ext2/ext3 do almost nothing in their write fault
    > handler - they are about as fast as it can get. ext4 OTOH needs to reserve
    > blocks for delayed allocation, setup buffers under a page etc. This is
    > necessary if you want to make sure that if data are written via mmap, they
    > also have space available on disk to be written to (ext2 / ext3 do not care
    > and will just drop the data on the floor if you happen to hit ENOSPC during
    > writeback).
    >
    > I'm not saying ext4 write fault path cannot possibly be optimized (noone
    > seriously looked into that AFAIK so there may well be some low hanging
    > fruit) but it will always be slower than ext2/3. A more meaningful
    > comparison would be with filesystems like XFS which make similar guarantees
    > regarding data safety.

    ext4 beats xfs from what I can tell. I ran with fewer steps to make the
    testing faster, which is to blame for the stair-stepping, btw...

    http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&3=xfs&hide=linear,threads,threads_idle,processes_idle





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