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Date:   Wed, 29 May 2019 08:59:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket


On 2019/5/29 上午12:45, Stefano Garzarella wrote:
> On Wed, May 15, 2019 at 10:48:44AM +0800, Jason Wang wrote:
>> On 2019/5/15 上午12:35, Stefano Garzarella wrote:
>>> On Tue, May 14, 2019 at 11:25:34AM +0800, Jason Wang wrote:
>>>> On 2019/5/14 上午1:23, Stefano Garzarella wrote:
>>>>> On Mon, May 13, 2019 at 05:58:53PM +0800, Jason Wang wrote:
>>>>>> On 2019/5/10 下午8:58, Stefano Garzarella wrote:
>>>>>>> +static struct virtio_vsock_buf *
>>>>>>> +virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_copy)
>>>>>>> +{
>>>>>>> +	struct virtio_vsock_buf *buf;
>>>>>>> +
>>>>>>> +	if (pkt->len == 0)
>>>>>>> +		return NULL;
>>>>>>> +
>>>>>>> +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
>>>>>>> +	if (!buf)
>>>>>>> +		return NULL;
>>>>>>> +
>>>>>>> +	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
>>>>>>> +	 * the new virtio_vsock_buf avoiding the copy, because we are sure that
>>>>>>> +	 * we are not use more memory than that counted by the credit mechanism.
>>>>>>> +	 */
>>>>>>> +	if (zero_copy && pkt->len == pkt->buf_len) {
>>>>>>> +		buf->addr = pkt->buf;
>>>>>>> +		pkt->buf = NULL;
>>>>>>> +	} else {
>>>>>> Is the copy still needed if we're just few bytes less? We meet similar issue
>>>>>> for virito-net, and virtio-net solve this by always copy first 128bytes for
>>>>>> big packets.
>>>>>>
>>>>>> See receive_big()
>>>>> I'm seeing, It is more sophisticated.
>>>>> IIUC, virtio-net allocates a sk_buff with 128 bytes of buffer, then copies the
>>>>> first 128 bytes, then adds the buffer used to receive the packet as a frag to
>>>>> the skb.
>>>> Yes and the point is if the packet is smaller than 128 bytes the pages will
>>>> be recycled.
>>>>
>>>>
>>> So it's avoid the overhead of allocation of a large buffer. I got it.
>>>
>>> Just a curiosity, why the threshold is 128 bytes?
>>
>>  From its name (GOOD_COPY_LEN), I think it just a value that won't lose much
>> performance, e.g the size two cachelines.
>>
> Jason, Stefan,
> since I'm removing the patches to increase the buffers to 64 KiB and I'm
> adding a threshold for small packets, I would simplify this patch,
> removing the new buffer allocation and copying small packets into the
> buffers already queued (if there is a space).
> In this way, I should solve the issue of 1 byte packets.
>
> Do you think could be better?


I think so.

Thanks


>
> Thanks,
> Stefano

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