linux-kernel - Re: [PATCH] virtio-net: parameterize min ring num

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Message-ID: <bdd30ef5-4f69-8218-eed0-38c6daac42db@redhat.com>
Date:   Thu, 18 Jul 2019 22:01:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        ? jiang <jiangkidd@...mail.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kafai@...com" <kafai@...com>,
        "songliubraving@...com" <songliubraving@...com>,
        "yhs@...com" <yhs@...com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "jiangran.jr@...baba-inc.com" <jiangran.jr@...baba-inc.com>
Subject: Re: [PATCH] virtio-net: parameterize min ring num_free for virtio
 receive


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@...mail.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