I have tried the vanilla kernel (2.6.22-14) on Gutsy. Now I get better disc 
performance, (ext3 / bs=10 ~4MB/s / bs=100 7MB/s), but desktop responsiveness 
becomes worse.
I have tried Hardy too for a while and recognise, that the responsiveness of 
the desktop is sometimes worse than under gutsy, even when there is no or light 
disc access only.
Sometimes the gnome menu needs about 2 seconds before it appears. And there are 
only logos and metadata to load. This are the problems as in gutsy and 
especially in gusty with 2.6.22 kernel.
Hdparm reposts, that UDMA is on. And there is only iowait cpu consume on lowest 
cpu frequency and a great disc performace, when coping a normal blocksize (>= 
4k) with dd.
Compiz is disabled on every of my installation. I use a 64-bit version of  
gutsy and hardy. 
I don't know the internal design of the linux kernel, so it's only a guess of 
mine. There must be a bottleneck, which is especially caused by high disc 
access, but also occurs on other activities. Perhaps interrupt handling. 
Powertop reports ~400 awaking of the keyboard while copying to files from one 
partition to another partition with a block size of 4k and writing this text. 
And I am writing only maximal two or three letters per second. And the count is 
only as high, when coping the files. Without disc access there are only about 
200 awaking, while writing in the same speed.

With two writing and two reading disc access
  38,9% (445,7)       <interrupt> : PS/2 keyboard/mouse/touchpad 
  17,5% (200,7)       <interrupt> : extra timer interrupt 
  14,5% (165,8)       <interrupt> : libata 
   8,8% (100,5)       <interrupt> : uhci_hcd:usb1, eth0 

Without high io access
  31,2% (213,6)       <interrupt> : extra timer interrupt 
  25,6% (175,2)       <interrupt> : PS/2 keyboard/mouse/touchpad 
  14,7% (100,5)       <interrupt> : uhci_hcd:usb1, eth0 
   8,9% ( 60,8)       <interrupt> : uhci_hcd:usb3, ahci, yenta, nvidia

-- 
Heavy Disk I/O harms desktop responsiveness
https://bugs.launchpad.net/bugs/131094
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