Description of problem:

We have a SUN/v20z(AMD64/x86). When heavy load IO activities done on local or
nfs disks (almost huge file copy), the kernel shows an abnormal IO's activity
with a state which reaches 100% and freezes totally all current processes. 

Version-Release number of selected component (if applicable):

RHEL3/WS - Linux katy 2.4.21-20.ELsmp #1 SMP Wed Aug 18 20:34:58 EDT 2004 x86_64
x86_64 x86_64 GNU/Linux

How reproducible:
Always

Steps to Reproduce:
1. 2 processes performing in parallel a "dd" read/write more than 1GB data with
bs=512 or bs=2M. test this on local disk and nfs disk.
2. while performing, check kernel activity by running a loop of 2s sleep and
check if the 2s sleep does really 2s or more!
2. after each dd i make a 'sync'
3. check out the iowait state, in average it reaches 98/99% and sometimes 100%
and freezes all current processes 
  
Actual results:
during the heavy io test with dd, the 2s sleep loop does sometimes more than 2s
sleep, it reaches 3s or 5s !! 

Expected results:
a 2s sleep should show 2s sleep even if the kernel is heavily loaded. our server
is running a semi-realtime system and we can't tolerate offset delay >100ms on a
sleep for instance...

Additional info: