[ovs-dev] [PATCH] vhost: Expose virtio interrupt requirement on rte_vhos API

Jan Scheurich jan.scheurich at ericsson.com
Fri Oct 13 22:57:13 UTC 2017


Hi Sugesh,

Actually the new API call in DPDK is not needed. A reply by Zhiyong Yang (http://dpdk.org/ml/archives/dev/2017-September/076504.html) pointed out that an existing API call provides access to the vring data structure that contains the interrupt flag. So I will abandon the DPDK patch.

Using the existing API I have created a patch on top of Ilya's output batching v4 series that automatically enables time-based batching on ports that should benefit from it most: vhostuser(client) using virtio interrupts as well as internal ports on the Linux (or BSD) host.

I still need to do careful testing that the interrupt detection works reliable. The performance should be the baseline performance of Ilya's patch.

BR, Jan

> -----Original Message-----
> From: ovs-dev-bounces at openvswitch.org [mailto:ovs-dev-bounces at openvswitch.org] On Behalf Of Chandran, Sugesh
> Sent: Friday, 13 October, 2017 17:34
> To: Jan Scheurich <jan.scheurich at ericsson.com>
> Cc: dev at openvswitch.org
> Subject: Re: [ovs-dev] [PATCH] vhost: Expose virtio interrupt requirement on rte_vhos API
> 
> Hi Jan,
> The DPDK changes are looks OK to me and will be useful. I am interested in testing this patch to see the impact on performance. Are
> you planning to share the changes in OVS for these APIs?
> 
> 
> 
> 
> 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 at ericsson.com<mailto:jan.scheurich at 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);
> +}
> 
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