Salyzyn, Mark wrote:
This is not a patch to be applied to any release, for discussion only.
We have managed to increase the performance of the I/O to the driver by
pushing back on the scsi_merge layer when we detect that we are issuing
sequential requests (patch enclosed below to demonstrate the technique
used to investigate). In the algorithm used, when we see that we have an
I/O that adjoins the previous request, we reduce the queue depth to a
value of 2 for the device. This allows the incoming I/O to be
scrutinized by the scsi_merge layer for a bit longer permitting them to
be merged together into a larger more efficient request.
By limiting the queue to a depth of two, we also do not delay the system
much since we keep one worker and one outstanding remaining in the
controller. This keeps the I/O's fed without delay.
The net result was instead of receiving, for example, 64 4K sequential
I/O requests to an eager controller more than willing to accept the
commands into it's domain, we instead see two 4K I/O requests, followed
by one 248KB I/O request.
Since you have
./drivers/scsi/aacraid/linit.c: .use_clustering = ENABLE_CLUSTERING,
this smells like a bug or scheduler issue. The block layer should
-already- be coalescing requests.
What happens when you change I/O schedulers? That may be a source of
problems too.
Jeff
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