Re: Understanding I/O behaviour

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

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.

Hope that helps.

(PS. please don't do crap like using that spamtrap@ address and have
people manually replace it with the one from your .signature when
posting on LKML - it's annoying as hell)

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