Re: [PATCH v3 1/2] virtio-blk: support polling I/O

[Suwan Kim <suwan.kim027@gmail.com>]

On Thu, Mar 24, 2022 at 10:32:02AM -0400, Michael S. Tsirkin wrote:
> On Thu, Mar 24, 2022 at 11:04:49PM +0900, Suwan Kim wrote:
> > This patch supports polling I/O via virtio-blk driver. Polling
> > feature is enabled by module parameter "num_poll_queues" and it
> > sets dedicated polling queues for virtio-blk. This patch improves
> > the polling I/O throughput and latency.
> > 
> > The virtio-blk driver doesn't not have a poll function and a poll
> > queue and it has been operating in interrupt driven method even if
> > the polling function is called in the upper layer.
> > 
> > virtio-blk polling is implemented upon 'batched completion' of block
> > layer. virtblk_poll() queues completed request to io_comp_batch->req_list
> > and later, virtblk_complete_batch() calls unmap function and ends
> > the requests in batch.
> > 
> > virtio-blk reads the number of poll queues from module parameter
> > "num_poll_queues". If VM sets queue parameter as below,
> > ("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
> > It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
> > as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
> > queues, the poll queues have no callback function.
> > 
> > Regarding HW-SW queue mapping, the default queue mapping uses the
> > existing method that condsiders MSI irq vector. But the poll queue
> > doesn't have an irq, so it uses the regular blk-mq cpu mapping.
> > 
> > For verifying the improvement, I did Fio polling I/O performance test
> > with io_uring engine with the options below.
> > (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
> > I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
> > queues for VM.
> > 
> > As a result, IOPS and average latency improved about 10%.
> > 
> > Test result:
> > 
> > - Fio io_uring poll without virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
> > 	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
> > 	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us
> > 
> > - Fio io_uring poll with virtio-blk poll support
> > 	-- numjobs=1 : IOPS = 380K, avg latency = 167.87us
> > 	-- numjobs=2 : IOPS = 409K, avg latency = 312.6us
> > 	-- numjobs=4 : IOPS = 413K, avg latency = 619.72us
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> > ---
> >  drivers/block/virtio_blk.c | 101 +++++++++++++++++++++++++++++++++++--
> >  1 file changed, 97 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 8c415be86732..3d16f8b753e7 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
> >  		 "0 for no limit. "
> >  		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
> >  
> > +static unsigned int num_poll_queues;
> > +module_param(num_poll_queues, uint, 0644);
> > +MODULE_PARM_DESC(num_poll_queues, "The number of dedicated virtqueues for polling I/O");
> > +
> >  static int major;
> >  static DEFINE_IDA(vd_index_ida);
> >
> 
> Is there some way to make it work reasonably without need to set
> module parameters? I don't see any other devices with a num_poll_queues
> parameter - how do they handle this?

Hi Michael,

NVMe driver uses module parameter.

Please refer to this.
-----
drivers/nvme/host/pci.c

static unsigned int poll_queues;
module_param_cb(poll_queues, &io_queue_count_ops, &poll_queues, 0644);
MODULE_PARM_DESC(poll_queues, "Number of queues to use for polled IO.");
-----

Regards,
Suwan Kim