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

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Message-ID: <20210428163124.GA28950@labs.hpe.com>
Date:   Wed, 28 Apr 2021 09:31:24 -0700
From:   Jean Tourrilhes <jean.tourrilhes@....com>
To:     Ilya Maximets <i.maximets@....org>
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 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.
	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 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