netdev - Re: qlen check in tun.c

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Message-ID: <51C3F65F.3090909@redhat.com>
Date:	Fri, 21 Jun 2013 14:44:47 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Jerry Chu <hkchu@...gle.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: qlen check in tun.c

On 06/20/2013 03:39 AM, Jerry Chu wrote:
> Hi Jason,
>
> On Tue, Jun 18, 2013 at 8:29 PM, Jason Wang <jasowang@...hat.com> wrote:
>> On 06/19/2013 10:31 AM, Jerry Chu wrote:
>>> In tun_net_xmit() the max qlen is computed as
>>> dev->tx_queue_len / tun->numqueues. For multi-queue configuration the
>>> latter may be way too small, forcing one to adjust txqueuelen based
>>> on number of queues created. (Well the default txqueuelen of
>>> 500/TUN_READQ_SIZE already seems too small even for single queue.)
>> Hi Jerry:
>>
>> Do you have some test result of this? Anyway, tun allows userspace to
>> adjust this value based on its requirement.
> Sure, but the default size of 500 is just way too small. queue overflows even
> with a simple single-stream throughput test through Openvswitch due to CPU
> scheduler anomaly. On our loaded multi-stream test even 8192 can't prevent
> queue overflow. But then with 8192 we'll be deep into the "buffer
> bloat" territory.

Do the overflow also happens on bridge? If not, maybe a bug of openvswitch?

Btw, it the scheduler brings unexpected latency, increasing tx queue
length may just make things worse.
> We haven't figured out an optimal strategy for thruput vs latency, but
> suffice to
> say 500 is too small.
>
> Jerry
>
>>> Wouldn't it be better to simply use dev->tx_queue_len to cap the qlen of
>>> each queue? This also seems to be more consistent with h/w multi-queues.
>> Make sense. Michael, any ideas on this?
>>> Also is there any objection to increase MAX_TAP_QUEUES from 8 to 16?
>>> Yes it will take up more space in struct tun_struct. But we are
>>> hitting the perf limit of 8 queues.
>> Not only the tun_struct, another issue of this is sizeof(netdev_queue)
>> which is 320 currently, if we use 16, it may be greater than 4096 which
>> lead high order page allocation. Need a solution such as flex array or
>> array of pointers.
>>
>> Btw, I have draft patch on both, will post as rfc.
>>
>> Thanks
>>> Thanks,
>>>
>>> Jerry
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