linux-kernel - Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq

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Date:   Fri, 13 Oct 2017 17:47:43 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "ming.lei@...hat.com" <ming.lei@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "himanshu.madhani@...ium.com" <himanshu.madhani@...ium.com>,
        "tom81094@...il.com" <tom81094@...il.com>,
        "paolo.valente@...aro.org" <paolo.valente@...aro.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "oleksandr@...alenko.name" <oleksandr@...alenko.name>,
        "john.garry@...wei.com" <john.garry@...wei.com>,
        "osandov@...com" <osandov@...com>,
        "loberman@...hat.com" <loberman@...hat.com>,
        "james.smart@...adcom.com" <james.smart@...adcom.com>
Subject: Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in
 blk_mq_ops

On Sat, 2017-10-14 at 01:29 +0800, Ming Lei wrote:
> ->can_queue is size of the whole tag space shared by all LUNs, looks it isn't
> reasonable to increase cmd_per_lun to .can_queue.

Sorry but I disagree. Setting cmd_per_lun to a value lower than can_queue
will result in suboptimal performance if there is only a single LUN per SCSI
host. If there are multiple LUNs per SCSI host then the blk-mq core tracks
the number of active LUNs through the blk_mq_tags.active_queues variable.
See also hctx_may_queue(). The comment above that function is as follows:

/*
 * For shared tag users, we track the number of currently active users
 * and attempt to provide a fair share of the tag depth for each of them.
 */

BTW, the ib_srp initiator driver sets cmd_per_lun to can_queue and is able
to achieve more than one million IOPS even in tests with multiple LUNs per
SCSI host.

Bart.