Reducing I/O latency

[Bart Van Assche <bvanassche@xxxxxxx>]

Hello,

Recently I have been looking for opportunities to reduce command
processing latency in the Linux SCSI initiator stack. I've noticed the
following:
- In drivers/scsi/scsi.c memory for the SCSI command and the SCSI sense
  data are allocated separately. Would it be a good idea to combine
  scsi_host_cmd_pool.cmd_slab and scsi_host_cmd_pool.sense_slab into a
  single kmem_cache or would this break something ?
- While running an I/O workload that was using eight LUNs (two hosts
  with four LUNs each) I noticed huge lock contention on the spinlock in
  scsi_host_cmd_pool.cmd_slab. Has anyone already tried to modify the
  code in drivers/scsi/scsi.c such that there is one pool per LUN
  instead of one pool for all LUNs of all hosts ? Note: the
  scsi_sg_pools array in drivers/scsi/scsi_lib.c is also shared over
  all LUNs.

Thanks,

Bart.
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