netdev - Re: [PATCH net] openvswitch: meter: remove rate from the bucket size calculation

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Message-ID: <94576ef8-76b3-67bc-7b55-c1d9513ec31c@ovn.org>
Date:   Thu, 29 Apr 2021 23:15:06 +0200
From:   Ilya Maximets <i.maximets@....org>
To:     Ilya Maximets <i.maximets@....org>, jean.tourrilhes@....com
Cc:     Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Pravin B Shelar <pshelar@....org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andy Zhou <azhou@....org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        ovs dev <dev@...nvswitch.org>, William Tu <u9012063@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Davide Caratti <dcaratti@...hat.com>
Subject: Re: [PATCH net] openvswitch: meter: remove rate from the bucket size
 calculation

On 4/28/21 8:59 PM, Ilya Maximets wrote:
> On 4/28/21 6:31 PM, Jean Tourrilhes wrote:
>> On Wed, Apr 28, 2021 at 01:22:12PM +0200, Ilya Maximets wrote:
>>>
>>> I didn't test it, but I looked at the implementation in
>>> net/sched/act_police.c and net/sched/sch_tbf.c, and they should work
>>> in a same way as this patch, i.e. it's a classic token bucket where
>>> burst is a burst and nothing else.
>>
>> 	Actually, act_police.c and sch_tbf.c will behave completely
>> differently, even if they are both based on the token bucket
>> algorithm.
>> 	The reason is that sch_tbf.c is applied to a queue, and the
>> queue will smooth out traffic and avoid drops. The token bucket is
>> used to dequeue the queue, this is sometime called leaky bucket. I've
>> personally used sch_tbf.c with burst size barely bigger than the MTU,
>> and it works fine.
> 
> Makes sense.  Thanks for the clarification!
> 
>> 	This is why I was suggesting to compare to act_police.c, which
>> does not have a queue to smooth out traffic and can only drop
>> packets.
> 
> I see.  Unfortunately, due to the fact that act_police.c uses time
> instead of bytes as a measure for tokens, we will still see a difference
> in behavior.  Probably, not so big, but it will be there and it will
> depend on a line rate.

I found my mistake in calculations (see another reply).  So, there
should not be significant difference with OVS meters, indeed.

> 
>> 	I believe OVS meters are similar to policers, so that's why
>> they are suprising for people used to queues such as TBF and HTB.
>>
>> 	Regards,
>>
>> 	Jean
>>