Re: Understanding I/O behaviour

["Jesper Juhl" <jesper.juhl@gmail.com>]

On 05/07/07, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> On 05/07/07, Martin Knoblauch <spamtrap@knobisoft.de> wrote:
> > Hi,
> >
> >  for a customer we are operating a rackful of HP/DL380/G4 boxes that
> > have given us some problems with system responsiveness under [I/O
> > triggered] system load.
> >
> >  The systems in question have the following HW:
> >
> > 2x Intel/EM64T CPUs
> > 8GB memory
> > CCISS Raid controller with 4x72GB SCSI disks as RAID5
> > 2x BCM5704 NIC (using tg3)
> >
> >  The distribution is RHEL4. We have tested several kernels including
> > the original 2.6.9, 2.6.19.2, 2.6.22-rc7 and 2.6.22-rc7+cfs-v18.
> >
> >  One part of the workload is when several processes try to write 5 GB
> > each to the local filesystem (ext2->LVM->CCISS). When this happens, the
> > load goes up to 12 and responsiveness goes down. This means from one
> > moment to the next things like opening a ssh connection to the host in
> > question, or doing "df" take forever (minutes). Especially bad with the
> > vendor kernel, better (but not perfect) with 2.6.19 and 2.6.22-rc7.
> >
> >  The load basically comes from the writing processes and up to 12
> > "pdflush" threads all being in "D" state.
> >
> >  So, what I would like to understand is how we can maximize the
> > responsiveness of the system, while keeping disk throughput at maximum.
> >
>
> I'd suspect you can't get both at 100%.
>
> I'd guess you are probably using a 100Hz no-preempt kernel.  Have you
> tried a 1000Hz + preempt kernel?   Sure, you'll get a bit lower
> overall throughput, but interactive responsiveness should be better -
> if it is, then you could experiment with various combinations of
> CONFIG_PREEMPT, CONFIG_PREEMPT_VOLUNTARY, CONFIG_PREEMPT_NONE and
> CONFIG_HZ_1000, CONFIG_HZ_300, CONFIG_HZ_250, CONFIG_HZ_100 to see
> what gives you the best balance between throughput and interactive
> responsiveness (you could also throw CONFIG_PREEMPT_BKL and/or
> CONFIG_NO_HZ, but I don't think the impact will be as significant as
> with the other options, so to keep things simple I'd leave those out
> at first) .
>
> I'd guess that something like CONFIG_PREEMPT_VOLUNTARY + CONFIG_HZ_300
> would probably be a good compromise for you, but just to see if
> there's any effect at all, start out with CONFIG_PREEMPT +
> CONFIG_HZ_1000.

I'm currious, did you ever try playing around with CONFIG_PREEMPT* and
CONFIG_HZ* to see if that had any noticable impact on interactive
performance and stuff like logging into the box via ssh etc...?

--
Jesper Juhl <jesper.juhl@gmail.com>
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