On Fri, 2016-12-16 at 17:12 -0700, Jens Axboe wrote:

From the discussion last time, I looked into the feasibility of having
two sets of tags for the same request pool, to avoid having to copy
some of the request fields at dispatch and completion time. To do that,
we'd have to replace the driver tag map(s) with our own, and augment
that with tag map(s) on the side representing the device queue depth.
Queuing IO with the scheduler would allocate from the new map, and
dispatching would acquire the "real" tag. We would need to change
drivers to do this, or add an extra indirection table to map a real
tag to the scheduler tag. We would also need a 1:1 mapping between
scheduler and hardware tag pools, or additional info to track it.
Unless someone can convince me otherwise, I think the current approach
is cleaner.

Hello Jens,

Can you have a look at the attached patches? These implement the "two tags
per request" approach without a table that maps one tag type to the other
or any other ugly construct. __blk_mq_alloc_request() is modified such that
it assigns rq->sched_tag and sched_tags->rqs[] instead of rq->tag and
tags->rqs[]. rq->tag and tags->rqs[] are assigned just before dispatch by
blk_mq_assign_drv_tag(). This approach results in significantly less code
than the approach proposed in v4 of your blk-mq-sched patch series. Memory
usage is lower because only a single set of requests is allocated. The
runtime overhead is lower because request fields no longer have to be
copied between the requests owned by the block driver and the requests
owned by the I/O scheduler. I can boot a VM from the virtio-blk driver but
otherwise the attached patches have not yet been tested.

Thanks,

Bart.