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 -- 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
