Re: IO performance problems in 2.4.19-pre5 when writing to DVD-RAM/ZIP/MO

[Moritz Franosch <jfranosc@physik.tu-muenchen.de>]

> > The problem is that writing to a DVD-RAM, ZIP or MO device almost
> > totally blocks reading from a _different_ device. Here is some data.
> > 
> > nr bench read       write      2.4.18  2.4.19-rc5  expected factor
> > 1  dd    30GB HDD   DVD-RAM    278     490         60       8.2
> > 2  dd    120GB HDD  DVD-RAM    197     438         32       14
> > 3  dd    30GB HDD   ZIP        158     239         60       4.0
> > 4  dd    120GB HDD  ZIP        142     249         32       7.8
> > 5  dd    30GB HDD   120GB HDD   87      89         60       1.5
> > 6  dd    120GB HDD  30GB HDD    66      69         32       2.2
> > 7  cp    30GB HDD   120GB HDD   97      77         60       1.3
> > 8  cp    120GB HDD  30GB HDD    78      65         50       1.3

Should be -pre5, sorry.

> The reason hd is faster is because new algorithm is much better than the
> previous mainline code. Now the reason the DVDRAM hangs the machine
> more, that's probably because more ram can be marked dirty with those
> new changes (beneficial for some workload, but it stalls much more the
> fast hd, if there's one very slow blkdev in the system). You can try
> decrasing the percent of vm dirty in the system with:
> 
> 	echo 2 500 0 0 500 3000 3 1 0 >/proc/sys/vm/bdflush

With the bdflush-parameters above, I get

nr bench read       write      2.4.19-pre5  expected factor
9  dd    30GB HDD   DVD-RAM    208/0/6      60       3.5
10 dd    120GB HDD  DVD-RAM    39/0/6       32       1.2
11 dd    30GB HDD   ZIP        66/0/10      60       1.1
12 dd    120GB HDD  ZIP        85/0/7       32       2.7

Numbers in the column 2.4.19-pre5 are total time / user time / system
time in seconds.

Performance is much better with the new parameters. Also, with the new
parameters, the system can read from HDD almost steadily while writing
to DVD. This should much increase responsiveness.

In cases 9 and 12 where performance is bad, both tested drives are on
the same IDE controller. Should that matter?

> Right fix is different but not suitable for 2.4.

I'm looking forward to the definitive solution.

Thank you very much,

Moritz


-- 
Dipl.-Phys. Moritz Franosch
http://Franosch.org