Disk I/O as a bottleneck?

[Omer Zak]

[This E-mail message is bottom-posting contrary to my usual custom.]

On Sun, 2011-05-08 at 17:26 +0300, Gilboa Davara wrote:
> On Sat, 2011-05-07 at 15:29 +0300, Omer Zak wrote:
> > I have a PC with powerful processor, lots of RAM and SATA hard disk.
> > Nevertheless I noticed that sometimes applications (evolution E-mail
> > software and Firefox[iceweasel] Web browser) have the sluggish feel of a
> > busy system (command line response time remains crisp, however, because
> > the processor is 4x2 core one [4 cores, each multithreads as 2]).
> > 
> > I run the gnome-system-monitor all the time.
> > 
> > I notice that even when those applications feel sluggish, only one or at
> > most two CPUs have high utilization, and there is plenty of free RAM (no
> > swap space is used at all).
> > 
> > Disk I/O is not monitored by gnome-system-monitor.
> > So I suspect that the system is slowed down by disk I/O.  I would like
> > to eliminate it as a possible cause for the applications' sluggish feel.
> > 
> > I ran smartctl tests on the hard disk, and they gave it clean bill of
> > health.  Therefore I/O error recovery should not be the reason for
> > performance degradation.
> > 
> > I am asking Collective Wisdom for advice about how to do:
> > 1. Monitoring disk I/O load (counting I/O requests is not sufficient, as
> > each request takes different time to complete due for example to disk
> > head seeks or platter rotation time).
> > 2. Disk scheduler fine-tuning possibilities to optimize disk I/O
> > handling.
> > 3. If smartctl is not sufficient to ensure that no I/O error overhead is
> > incurred, how to better assess the hard disk's health?
> > 
> > Thanks,
> > --- Omer
> > 
> 
> Hello Omer,
> 
> Two questions:
> 
> 1. Kernel version?
> 2.6.38 added a very small patch that that done wonders to eliminate
> foreground process scheduling issues that plagued desktop setups since
> the introduction of the CFS scheduler*. (Google for +2.6.38 +group
> +scheduling)
> On my Fedora 14 + 2.6.38 / dual 6C Xeon workstation I can easily
> watching a DVD movies while compiling the kernel and running a couple of
> VM's (using qemu-kvm) in the background. Doing the same using the stock
> Fedora 14 2.6.35 kernel is... err... interesting experience :)

Standard Debian Squeeze kernel:
$ uname -a
Linux c4 2.6.32-5-vserver-amd64 #1 SMP Mon Mar 7 23:14:47 UTC 2011
x86_64 GNU/Linux

> 2. Are you sure your SATA is in AHCI mode?
> (Simply type: $ find /sys/devices | grep ahci)

My SATA indeed runs in non-AHCI mode.

The question now is how to configure it to run in AHCI mode?
According to the Wikipedia article
http://en.wikipedia.org/wiki/Advanced_Host_Controller_Interface, I need
to modify a BIOS setting and maybe create a new initrd image.
Another source:
http://www.techmetica.com/howto/sata-ahci-mode-bios-setting.-what-does-it-do/

I can handle the BIOS setting but how to check whether a new initrd
image is needed, and if necessary create it?

--- Omer


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