Re: ext3 writeback mode slower than ordered mode?

[Zlatko Calusic <zlatko.calusic@iskon.hr>]

Andrew Morton <akpm@zip.com.au> writes:

> Zlatko Calusic wrote:
> > 
> > Hi!
> > 
> > My apologies if this is an FAQ, and I'm still catching up with
> > the linux-kernel list.
> > 
> > Today I decided to convert my /tmp partition to be mounted in
> > writeback mode, as I noticed that ext3 in ordered mode syncs every 5
> > seconds and that is something defenitely not needed for /tmp, IMHO.
> > 
> > Then I did some tests in order to prove my theory. :)
> > 
> > But, alas, writeback is slower.
> > 
> 
> I cannot reproduce this.  Using http://www.zip.com.au/~akpm/writer.c
> 
> ext2:            0.03s user 1.43s system 97% cpu 1.501 total
> ext3 writeback:  0.02s user 2.33s system 96% cpu 2.431 total
> ext3 ordered:    0.02s user 2.52s system 98% cpu 2.574 total
> 

Hm, at first I got exactly the same results for writeback/ordered
cases, as you did above, so my theory fell on the ground.
Later, bloody thing resurected again. Something really fishy is goin'
on here...


 {atlas} [/mnt]# time ~zcalusic/try/awriter
 ~zcalusic/try/awriter  0.07s user 3.50s system 99% cpu 3.594 total
 {atlas} [/mnt]# cd /tmp
 {atlas} [/tmp]# time ~zcalusic/try/awriter
 ~zcalusic/try/awriter  0.00s user 6.05s system 98% cpu 6.129 total
 {atlas} [/tmp]# mount | egrep '/tmp|/mnt'
 /dev/hde2 on /tmp type ext3 (rw,data=writeback)
 /dev/hde3 on /mnt type ext3 (rw)


So /tmp is writeback and /mnt is ordered (doublechecked!). See for
yourself how ext3 is slower in writeback mode. awriter is your
small program, of course.

Just for the record, I mke2fs-ed /dev/hde3 again and made it pure
ext2.


 {atlas} [~]# mount | grep '/mnt'      
 /dev/hde3 on /mnt type ext2 (rw)
 {atlas} [~]# cd /mnt
 {atlas} [/mnt]# time ~zcalusic/try/awriter
 ~zcalusic/try/awriter  0.01s user 1.86s system 98% cpu 1.893 total


To sumarize:

ext2            0.01s user 1.86s system 98% cpu 1.893 total
ext3/ordered    0.07s user 3.50s system 99% cpu 3.594 total
ext3/writeback  0.00s user 6.05s system 98% cpu 6.129 total

What is strange is that not always I've been able to get different
results for writeback case (comparing to ordered), but when I get it,
it is repeatable.

This is a SMP machine, if that makes any difference.

Regards,
-- 
Zlatko