On 11/22/2019 10:14 AM, Bart Van Assche wrote:
Thanks for having shared these numbers. I think this is very useful information. Do these results show the performance drop that happens if /sys/block/.../device/queue_depth exceeds .can_queue? What I am wondering about is how important these results are in the context of this discussion. Are there any modern SCSI devices for which a SCSI LLD sets scsi_host->can_queue and scsi_host->cmd_per_lun such that the device responds with BUSY? What surprised me is that only three SCSI LLDs call scsi_track_queue_full() (mptsas, bfa, esp_scsi). Does that mean that BUSY responses from a SCSI device or HBA are rare?
That's because most of the drivers, which had queue full ramp up/ramp down in them and would have called scsi_track_queue_full() converted over to the moved-queue-full-handling-in-the-mid-layer, indicated by sht->track_queue_depth = 1.
Yes - it is still hit a lot! -- james
