On Fri 09-08-13 10:36:41, Andy Lutomirski wrote:

On Fri, Aug 9, 2013 at 12:55 AM, Jan Kara [off-list ref] wrote:

quoted

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

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).

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 [off-list ref]
SUSE Labs, CR

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