Disk I/O as a bottleneck?

[Gilboa Davara]

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 :)

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

- Gilboa
* At least in my case...