SCSI Multi-Queue (scsi-mq)

Bobby9999 · 01-22-2020, 08:38 AM

Hi all,

I have a question regarding multi-queue (MQ) in SCSI layer. Since Linux kernel 3.13 (2014), the linux block layer has multi-queue a.k.a mq-blk.

Post mq-blk in block layer, the SCSI IO submission path had to be updated. As a result, SCSI multi-queue a.k.a scsi-mq work has been functional since Linux kernel 3.17.

My question is: How actually multi-queuing is achieved in SCSI? Traditionally, the SCSI mid-level layer used to create queuecommand (). Now when there is multi-queuing implemented in SCSI, does multi-queuing actually means creating multi queuecommand ()?

Please help me understand it. Thanks.

syg00 · 01-23-2020, 06:23 PM

I saw a paper on this ages ago - spend some time searching the net, there will be plenty of performance papers out there by now.
But from memory, the simple answer is that a queue is established per core (per execution thread probably) so the I/O can be q'd on the same CPU as the application that issued the I/O request.
But go look for definitive answers - even the posts on lkml (the kernel mailing list) that submitted the patches will have a discussion of the internals methinks.