Re: Understanding I/O behaviour

[Martin Knoblauch <knobi@knobisoft.de>]

Martin Knoblauch wrote:
>--- Robert Hancock <hancockr@xxxxxxx> wrote:
>
>>
>> Try playing with reducing /proc/sys/vm/dirty_ratio and see how that
>> helps. This workload will fill up memory with dirty data very
>> quickly,
>> and it seems like system responsiveness often goes down the toilet
>> when
>> this happens and the system is going crazy trying to write it all
>> out.
>>
>
>Definitely the "going crazy" part is the worst problem I see with 2.6
>based kernels (late 2.4 was really better in this corner case).
>
>I am just now playing with dirty_ratio. Anybody knows what the lower
>limit is? "0" seems acceptabel, but does it actually imply "write out
>immediatelly"?
>
>Another problem, the VM parameters are not really well documented in
>their behaviour and interdependence.

 Lowering dirty_ration just leads to more imbalanced write-speed for
the three dd's. Even when lowering the number to 0, the hich load
stays.

 Now, on another experiment I mounted the FS with "sync". And now the
load stays below/around 3. No more "pdflush" daemons going wild. And
the responsiveness is good, with no drops.

 My question is now: is there a parameter that one can use to force
immediate writeout for every process. This may hurt overall performance
of the system, but might really help my situation. Setting dirty_ratio
to 0 does not seem to do it.

Cheers
Martin

------------------------------------------------------
Martin Knoblauch
email: k n o b i AT knobisoft DOT de
www:   http://www.knobisoft.de