netdev - Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max

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Date:	Sun, 24 Nov 2013 15:28:13 +0800
From:	Yang Yingliang <zjutyyl@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Yang Yingliang <yangyingliang@...wei.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org, brouer@...hat.com,
	jpirko@...hat.com, jbrouer@...hat.com
Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size


On 2013/11/24 3:06, Eric Dumazet wrote:
> On Tue, 2013-11-19 at 15:25 +0800, Yang Yingliang wrote:
>> commit b757c9336d63f94c6b57532(tbf: improved accuracy at high rates)
>> introduce a regression.
>>
>> With the follow command:
>> tc qdisc add dev eth1 root handle 1: tbf latency 50ms burst 10KB rate 30gbit mtu 64k
>>
>> Without this patch, the max_size value is 10751(bytes).
>> But, in fact, the real max_size value should be smaller than 7440(bytes).
>> Or a packet whose length is bigger than 7440 will cause network congestion.
>> Because the packet is so big that can't get enough tokens. Even all the tokens
>> in the buffer is given to the packet.
>>
>> With this patch, the max_size value is 7440(bytes).
>> The packets whose length is bigger than 7440(bytes) will be dropped or reshape
>> in tbf_enqueue().
> This changelog makes no sense.
>
> If userland asks 'burst 10KB', then burst is 10KB, not 7440.
>
Ideally burst should be 10KB in kernel space.
But at hight rates, when burst is converted to
time in tick in userland, it gets much more loss
than low rates. So the burst can't actually
reach 10KB in kernel.

>
>
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