2.5.70-mm2 causes performance drop of random read O_DIRECT

[Steven Pratt]

Starting in 2.5.70-mm2 and continuing in the mm tree, there is a 
significant degrade in random read for block devices using O_DIRECT.   
 The drop occurs for all block sizes and ranges from 30%-40.  CPU usage 
is also lower although it may already be so low as to be irrelavent.

                                 tolerance = 0.00 + 3.00% of 2.5.70-mm1
             2.5.70-mm1   2.5.70-mm2
 Blocksize      KBs/sec      KBs/sec    %diff         diff    tolerance
---------- ------------ ------------ -------- ------------ ------------
      4096         1567          924   -41.03      -643.00        47.01  * 
      8192         3057         1815   -40.63     -1242.00        91.71  * 
     16384         5745         3509   -38.92     -2236.00       172.35  * 
     65536        17357        11283   -34.99     -6074.00       520.71  * 
    262144        37537        27302   -27.27    -10235.00      1126.11  * 

Full results can be found at:
http://www-124.ibm.com/developerworks/oss/linuxperf/regression/2.5.70-mm2/2.5.70-mm1-vs-2.5.70-mm2/

Steve