netdev - Re: qlen check in tun.c

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Message-ID: <51C12592.6050503@redhat.com>
Date:	Wed, 19 Jun 2013 11:29:22 +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 06/19/2013 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.
>
> 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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