On Fri, Aug 9, 2013 at 10:42 AM, Dave Hansen [off-list ref] wrote:

On 08/09/2013 12:55 AM, Jan Kara wrote:

quoted

On Thu 08-08-13 15:58:39, Dave Hansen wrote:

quoted

quoted

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

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