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