From: Christoph Hellwig <hch@infradead.org>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>, Asias He <asias@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] virtio_blk: blk-mq support
Date: Mon, 28 Oct 2013 01:52:06 -0700 [thread overview]
Message-ID: <20131028085206.GB31270@infradead.org> (raw)
In-Reply-To: <87ob6aglgl.fsf@rustcorp.com.au>
On Mon, Oct 28, 2013 at 01:17:54PM +1030, Rusty Russell wrote:
> Let's pretend I'm stupid.
>
> We don't actually have multiple queues through to the host, but we're
> pretending to, because it makes the block layer go faster?
>
> Do I want to know *why* it's faster? Or should I look the other way?
You shouldn't. To how multiple queues benefit here I'd like to defer to
Jens, given the single workqueue I don't really know where to look here.
The real benefit that unfortunately wasn't obvious from the description
is that even with just a single queue the blk-multiqueue infrastructure
will be a lot faster, because it is designed in a much more streaminline
fashion and avoids lots of lock roundtrips both during submission itself
and for submission vs complettion. Back when I tried to get virtio-blk
to perform well on high-end flash (the work that Asias took over later)
the queue_lock contention was the major issue in virtio-blk and this
patch gets rid of that even with a single queue.
A good example are the patches from Nick to move scsi drivers over to
the infrastructure that only support a single queue. Even that gave
over a 10 fold improvement over the old code.
Unfortunately I do not have access to this kind of hardware at the
moment, but I'd love to see if Asias or anyone at Red Hat could redo
those old numbers.
next prev parent reply other threads:[~2013-10-28 8:52 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-25 13:04 [PATCH 0/2] block multiqueue support for virtio-blk Christoph Hellwig
2013-10-25 13:05 ` [PATCH 1/2] blk-mq: add blk_mq_stop_hw_queues Christoph Hellwig
2013-10-25 13:44 ` Jens Axboe
2013-10-25 13:05 ` [PATCH 2/2] virtio_blk: blk-mq support Christoph Hellwig
2013-10-25 13:51 ` Jens Axboe
2013-10-25 16:06 ` Asias He
2013-10-28 2:47 ` Rusty Russell
2013-10-28 8:52 ` Christoph Hellwig [this message]
2013-10-29 21:34 ` Jens Axboe
2013-10-30 8:51 ` Asias He
2013-11-01 16:45 ` Christoph Hellwig
2013-11-01 17:00 ` Jens Axboe
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