netdev - Re: [Cake] [PATCH net-next v3] Add Common Applications Kept Enhanced (cake) qdisc

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Message-Id: <E39A1AD2-CC8A-4DB0-BA97-B2B0108F91FA@gmx.de>
Date:   Wed, 25 Apr 2018 19:54:52 +0200
From:   Sebastian Moeller <moeller0@....de>
To:     Jonathan Morton <chromatix99@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Cake List <cake@...ts.bufferbloat.net>, netdev@...r.kernel.org
Subject: Re: [Cake] [PATCH net-next v3] Add Common Applications Kept Enhanced
 (cake) qdisc



> On Apr 25, 2018, at 18:52, Jonathan Morton <chromatix99@...il.com> wrote:
> 
>> We can see here the high cost of forcing software GSO :/
>> 
>> Really, this should be done only :
>> 1) If requested by the admin ( tc .... gso ....)
>> 
>> 2) If packet size is above a threshold.
>> The threshold could be set by the admin, and/or based on a fraction of the bandwidth parameter.
>> 
>> I totally understand why you prefer to segment yourself for < 100 Mbit links.
>> 
>> But this makes no sense on 10Gbit+
> 
> It is absolutely necessary, so far as I can see, to segment GSO superpackets when overhead compensation is selected - as it very often should be, even on pure Ethernet links.  Without that, the calculation of link occupancy time will be wrong.  (The actual transmission time of an Ethernet frame is rather more than just 14 bytes longer than the underlying IP packet.)

	To elaborate a bit: For most link technologies the number of on-the-wire segments (and the total IP size of the superpacket) would go a long way, but for ATM with its mandatory per packet padding (to fill an integer number of ATM cells) one really needs to know the precise packet size.


> 
> Another reason to apply GSO segmentation is to achieve maximal smoothness of flow isolation.  This should still be achievable within some tolerance at high link rates, but calculating this tolerance is complicated by the triple-isolate algorithm.
> 
> If there's a way to obtain the individual packet sizes without incurring the full segmentation overhead, it may be worth considering (at high link rates only).  I would want to leave it on by default, because some of Cake's demonstrably superior latency performance depends on seeing the real packets, not the aggregates, and the overhead only becomes significant above 100Mbps on weak MIPS CPUs and 1Gbps on vaguely modern x86 stuff.
> 
> - Jonathan Morton
> 
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