On Fri 09-08-13 10:36:41, Andy Lutomirski wrote: > On Fri, Aug 9, 2013 at 12:55 AM, Jan Kara <jack@xxxxxxx> 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). > > Out of curiosity, why does ext4 need to set up buffers? That is, as > long as the fs can guarantee that there is reserved space to write out > the page, why isn't it sufficient to just mark the page dirty and let > the writeback code set up the buffers? Well, because we track the fact that the space is reserved in the buffer itself. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>