Re: [PATCH v4 12/13] blk-mq: Use shared tags for shared sbitmap support

[John Garry]

+ Kashyap

On 24/09/2021 11:23, Hannes Reinecke wrote:
> On 9/24/21 10:28 AM, John Garry wrote:
> > Currently we use separate sbitmap pairs and active_queues atomic_t for
> > shared sbitmap support.
> >
> > However a full sets of static requests are used per HW queue, which is
> > quite wasteful, considering that the total number of requests usable at
> > any given time across all HW queues is limited by the shared sbitmap 
> > depth.
> >
> > As such, it is considerably more memory efficient in the case of shared
> > sbitmap to allocate a set of static rqs per tag set or request queue, and
> > not per HW queue.
> >
> > So replace the sbitmap pairs and active_queues atomic_t with a shared
> > tags per tagset and request queue, which will hold a set of shared static
> > rqs.
> >
> > Since there is now no valid HW queue index to be passed to the blk_mq_ops
> > .init and .exit_request callbacks, pass an invalid index token. This
> > changes the semantics of the APIs, such that the callback would need to
> > validate the HW queue index before using it. Currently no user of shared
> > sbitmap actually uses the HW queue index (as would be expected).
> >
> > Continue to use term "shared sbitmap" for now, as the meaning is known.
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> >   block/blk-mq-sched.c   | 82 ++++++++++++++++++-------------------
> >   block/blk-mq-tag.c     | 61 ++++++++++------------------
> >   block/blk-mq-tag.h     |  6 +--
> >   block/blk-mq.c         | 91 +++++++++++++++++++++++-------------------
> >   block/blk-mq.h         |  5 ++-
> >   include/linux/blk-mq.h | 15 ++++---
> >   include/linux/blkdev.h |  3 +-
> >   7 files changed, 125 insertions(+), 138 deletions(-)
> The overall idea to keep the full request allocation per queue was to 
> ensure memory locality for the requests themselves.
> When moving to a shared request structure we obviously loose that feature.
>
> But I'm not sure if that matters here; the performance impact might be 
> too small to be measurable, seeing that we'll be most likely bound by 
> hardware latencies anyway.
>
> Nevertheless: have you tested for performance regressions with this 
> patchset?

I have tested relatively lower rates, like ~450K IOPS, without any 
noticeable regression.

> I'm especially thinking of Kashyaps high-IOPS megaraid setup; if there 
> is a performance impact that'll be likely scenario where we can measure it.

I can test higher rates, like 2M IOPS, when I get access to the HW.

@Kashyap, Any chance you can help test performance here?

> But even if there is a performance impact this patchset might be 
> worthwhile, seeing that it'll reduce the memory footprint massively.

Sure, I don't think that minor performance improvements can justify the 
excessive memory.

Thanks,
John