[PATCH] go back to 256 requests per queue

The request queue was increased from 256 slots to 512 in 2.5.20.  The
throughput of `dbench 128' on Randy's 384 megabyte machine fell 40%.

We do need to understand why that happened, and what we can learn from
it.  But in the meanwhile I'd suggest that we go back to 256 slots so
that this known problem doesn't impact people's evaluation and tuning
of 2.5 performance.

diff --git a/drivers/block/ll_rw_blk.c b/drivers/block/ll_rw_blk.c
index d53122b1ae468e3ad9340b6b35c8f783a32cc2e9..3527afa3cae708e4483927116254e6d0f6d4169f 100644 (file)
--- a/drivers/block/ll_rw_blk.c
+++ b/drivers/block/ll_rw_blk.c
@@ -2002,8 +2002,8 @@ int __init blk_dev_init(void)
        queue_nr_requests = (total_ram >> 8) & ~15;     /* One per quarter-megabyte */
        if (queue_nr_requests < 32)
                queue_nr_requests = 32;
-       if (queue_nr_requests > 512)
-               queue_nr_requests = 512;
+       if (queue_nr_requests > 256)
+               queue_nr_requests = 256;
 
        /*
         * Batch frees according to queue length