Re: [PATCH v4 1/2] virtio-blk: support polling I/O

Suwan Kim
2022-04-05T05:21:52Z

On Mon, Apr 04, 2022 at 06:28:04PM +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 = 385K, avg latency = 165.94us
> -- numjobs=2 : IOPS = 408K, avg latency = 313.28us
> -- numjobs=4 : IOPS = 424K, avg latency = 613.05us
>
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
> drivers/block/virtio_blk.c | 112 +++++++++++++++++++++++++++++++++++--
> 1 file changed, 108 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 8c415be86732..c2d955da0006 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");

Sorry. I forgot to fix the parameter name.
I resended this path.

Regards,
Suwan Kim