On Fri, Aug 9, 2013 at 10:42 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > 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 did try throwing a fallocate() in there to see if it helped. It > didn't appear to help. Should it have? Try reading all the pages after mmap (and keep the fallocate). In theory, MAP_POPULATE should help some, but until Linux 3.9 MAP_POPULATE was a disaster, and I'm still a bit afraid of it. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- 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
