Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

[Jason Wang <jasowang@redhat.com>]

On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
>> This change makes ring buffer reclaim threshold num_free configurable
>> for better performance, while it's hard coded as 1/2 * queue now.
>> According to our test with qemu + dpdk, packet dropping happens when
>> the guest is not able to provide free buffer in avail ring timely.
>> Smaller value of num_free does decrease the number of packet dropping
>> during our test as it makes virtio_net reclaim buffer earlier.
>>
>> At least, we should leave the value changeable to user while the
>> default value as 1/2 * queue is kept.
>>
>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
> That would be one reason, but I suspect it's not the
> true one. If you need more buffer due to jitter
> then just increase the queue size. Would be cleaner.
>
>
> However are you sure this is the reason for
> packet drops? Do you see them dropped by dpdk
> due to lack of space in the ring? As opposed to
> by guest?
>
>

Besides those, this patch depends on the user to choose a suitable 
threshold which is not good. You need either a good value with 
demonstrated numbers or something smarter.

Thanks