From: Nick Piggin <piggin@cyberone.com.au>

The following patch gets batching working how it should be.

After a process is woken up, it is allowed to allocate up to 32 requests
for 20ms.  It does not stop other processes submitting requests if it isn't
submitting though.  This should allow less context switches, and allow
batches of requests from each process to be sent to the io scheduler
instead of 1 request from each process.

tiobench sequential writes are more than tripled, random writes are nearly
doubled over mm1.  In earlier tests I generally saw better CPU efficiency
but it doesn't show here.  There is still debug to be taken out.  Its also
only on UP.

                                Avg     Maximum     Lat%   Lat%   CPU
 Identifier    Rate  (CPU%)  Latency   Latency     >2s    >10s   Eff
 ------------------- ------ --------- ---------- ------- ------ ----
 -2.5.71-mm1   11.13 3.783%    46.10    24668.01   0.84   0.02   294
 +2.5.71-mm1   13.21 4.489%    37.37     5691.66   0.76   0.00   294

 Random Reads
 ------------------- ------ --------- ---------- ------- ------ ----
 -2.5.71-mm1    0.97 0.582%   519.86     6444.66  11.93   0.00   167
 +2.5.71-mm1    1.01 0.604%   484.59     6604.93  10.73   0.00   167

 Sequential Writes
 ------------------- ------ --------- ---------- ------- ------ ----
 -2.5.71-mm1    4.85 4.456%    77.80    99359.39   0.18   0.13   109
 +2.5.71-mm1   14.11 14.19%    10.07    22805.47   0.09   0.04    99

 Random Writes
 ------------------- ------ --------- ---------- ------- ------ ----
 -2.5.71-mm1    0.46 0.371%    14.48     6173.90   0.23   0.00   125
 +2.5.71-mm1    0.86 0.744%    24.08     8753.66   0.31   0.00   115

It decreases context switch rate on IBM's 8-way on ext2 tiobench 64 threads
from ~2500/s to ~140/s on their regression tests.