PROBLEM: Heavy iowait on 2.6 kernels

[Guy Van Sanden]

I recently discovered why my new Gentoo server slows to a crawl on a
intermediate load on the 2.6 kernel series.  The reason seems to be an
unusual amount of iowait.
I can reproduce this on two systems, one with a KT400 and and one with
Sis735 chipset.

iostat -c 1 (or vmstat) while running something like bonnie produces
iowaits of 99%.
Any 2.6 kernel responds the same (regardless of the patchset), even
vanilla. (2.6.5-mm, 2.6.6-mm, 2.6.6-vanilla and 2.6.5-gentoo-r1).
Any 2.4-series kernel is fine.

This is a sample iostat output
--
avg-cpu:  %user   %nice    %sys %iowait   %idle
          26.73    0.00   10.89   62.38    0.00
avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.00    0.00   13.00   87.00    0.00
avg-cpu:  %user   %nice    %sys %iowait   %idle
           1.01    0.00   16.16   82.83    0.00
avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.00    0.00   11.88   88.12    0.00
avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.00    0.00    1.01   98.99    0.00
avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.99    0.00    2.97   96.04    0.00
--
-- 
______________________________________________________________________  

  Guy Van Sanden 
  http://unixmafia.port5.com  

  Registered Linux user #249404 - September 1997
______________________________________________________________________

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