netdev - Re: qlen check in tun.c

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Date:	Tue, 25 Jun 2013 15:23:38 -0700
From:	Jerry Chu <hkchu@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: qlen check in tun.c

On Thu, Jun 20, 2013 at 1:07 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Wed, Jun 19, 2013 at 12:39:34PM -0700, 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.
>> We haven't figured out an optimal strategy for thruput vs latency, but
>> suffice to
>> say 500 is too small.
>>
>> Jerry
>
> Maybe TSO is off for you?
> With TSO you can get 64Kbyte packets, 500 of these is 30 Mbytes!
> We really should consider setting byte limits, not packet limits.

Sorry for the delay. TSO was on when I was seeing lots of pkts drops.
But I realized the catch was GRO was off, which caused lots of MTU
size pkts to pile up on the receive side overflowing the small tuntap
queue.

I just finished implementing GRE support in the GRO stack. When I
turned it on, there were much less pkt drops. I do notice now the many
acks triggered by the thruput tests will cause the tuntap queue to
overflow.

In any case, with a large tx queue there should probably have some
queue mgmt or BQL logic going with it.

Jerry
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