Re: [PATCH] vhost: Expose virtio interrupt requirement on rte_vhos API

[Stephen Hemminger <stephen@networkplumber.org>]

I heard Fd.io has a faster vhost driver. Has anyone investigated?

On Sep 23, 2017 8:16 PM, "Jan Scheurich" <jan.scheurich@ericsson.com> wrote:

> Performance tests with the OVS DPDK datapath have shown that the tx
> throughput over a vhostuser port into a VM with an interrupt-based virtio
> driver is limited by the overhead incurred by virtio interrupts. The OVS
> PMD spends up to 30% of its cycles in system calls kicking the eventfd.
> Also the core running the vCPU is heavily loaded with generating the virtio
> interrupts in KVM on the host and handling these interrupts in the
> virtio-net driver in the guest. This limits the throughput to about 500-700
> Kpps with a single vCPU.
>
> OVS is trying to address this issue by batching packets to a vhostuser
> port for some time to limit the virtio interrupt frequency. With a 50 us
> batching period we have measured an iperf3  throughput increase by 15% and
> a PMD utilization decrease from 45% to 30%.
>
> On the other hand, guests using virtio PMDs do not profit from time-based
> tx batching. Instead they experience a 2-3% performance penalty and an
> average latency increase of 30-40 us. OVS therefore intends to apply
> time-based tx batching only for vhostuser tx queues that need to trigger
> virtio interrupts.
>
> Today this information is hidden inside the rte_vhost library and not
> accessible to users of the API. This patch adds a function to the API to
> query it.
>
> Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
>
> ---
>
>  lib/librte_vhost/rte_vhost.h | 12 ++++++++++++
>  lib/librte_vhost/vhost.c     | 19 +++++++++++++++++++
>  2 files changed, 31 insertions(+)
>
> diff --git a/lib/librte_vhost/rte_vhost.h b/lib/librte_vhost/rte_vhost.h
> index 8c974eb..d62338b 100644
> --- a/lib/librte_vhost/rte_vhost.h
> +++ b/lib/librte_vhost/rte_vhost.h
> @@ -444,6 +444,18 @@ int rte_vhost_get_vhost_vring(int vid, uint16_t
> vring_idx,
>   */
>  uint32_t rte_vhost_rx_queue_count(int vid, uint16_t qid);
>
> +/**
> + * Does the virtio driver request interrupts for a vhost tx queue?
> + *
> + * @param vid
> + *  vhost device ID
> + * @param qid
> + *  virtio queue index in mq case
> + * @return
> + *  1 if true, 0 if false
> + */
> +int rte_vhost_tx_interrupt_requested(int vid, uint16_t qid);
> +
>  #ifdef __cplusplus
>  }
>  #endif
> diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
> index 0b6aa1c..bd1ebf9 100644
> --- a/lib/librte_vhost/vhost.c
> +++ b/lib/librte_vhost/vhost.c
> @@ -503,3 +503,22 @@ struct virtio_net *
>
>         return *((volatile uint16_t *)&vq->avail->idx) -
> vq->last_avail_idx;
>  }
> +
> +int rte_vhost_tx_interrupt_requested(int vid, uint16_t qid)
> +{
> +    struct virtio_net *dev;
> +    struct vhost_virtqueue *vq;
> +
> +    dev = get_device(vid);
> +    if (dev == NULL)
> +        return 0;
> +
> +    vq = dev->virtqueue[qid];
> +    if (vq == NULL)
> +        return 0;
> +
> +    if (unlikely(vq->enabled == 0 || vq->avail == NULL))
> +        return 0;
> +
> +    return !(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
> +}
>
>