netdev - RE: [next-queue PATCH v4 3/4] net/sched: Introduce Credit Based Shaper (CBS) qdisc

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Date:   Thu, 5 Oct 2017 18:41:48 +0000
From:   Rodney Cummings <rodney.cummings@...com>
To:     David Miller <davem@...emloft.net>,
        "levipearson@...il.com" <levipearson@...il.com>
CC:     "jiri@...nulli.us" <jiri@...nulli.us>,
        "vinicius.gomes@...el.com" <vinicius.gomes@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "andre.guedes@...el.com" <andre.guedes@...el.com>,
        "ivan.briano@...el.com" <ivan.briano@...el.com>,
        "jesus.sanchez-palencia@...el.com" <jesus.sanchez-palencia@...el.com>,
        "boon.leong.ong@...el.com" <boon.leong.ong@...el.com>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        "henrik@...tad.us" <henrik@...tad.us>
Subject: RE: [next-queue PATCH v4 3/4] net/sched: Introduce Credit Based
 Shaper (CBS) qdisc

The IEEE Std 802.1Q specs for credit-based shaper require precise transmit decisions
within a 125 microsecond window of time.

Even with the Preempt RT patch or similar enhancements, that isn't very practical
as software-only. I doubt that software would conform to the standard's
requirements.

This is analogous to memory, or CPU.
.

> -----Original Message-----
> From: David Miller [mailto:davem@...emloft.net]
> Sent: Thursday, October 5, 2017 1:29 PM
> To: levipearson@...il.com
> Cc: jiri@...nulli.us; vinicius.gomes@...el.com; netdev@...r.kernel.org;
> intel-wired-lan@...ts.osuosl.org; jhs@...atatu.com;
> xiyou.wangcong@...il.com; andre.guedes@...el.com; ivan.briano@...el.com;
> jesus.sanchez-palencia@...el.com; boon.leong.ong@...el.com;
> richardcochran@...il.com; henrik@...tad.us; Rodney Cummings
> <rodney.cummings@...com>
> Subject: Re: [next-queue PATCH v4 3/4] net/sched: Introduce Credit Based
> Shaper (CBS) qdisc
> 
> From: Levi Pearson <levipearson@...il.com>
> Date: Thu, 5 Oct 2017 12:09:32 -0600
> 
> > It would be a shame if this were blocked due to a missing software
> > implementation.
> 
> Quite the contrary, I think a software implementation is a minimum
> requirement for inclusion of this feature.
> 
> Without a software implementation, there is no clear definition of
> what is supposed to happen, and no clear way for people to test those
> expectations unless they have the specific hardware.
> 
> I completely agree with Jiri.  Hardware offload first is _not_ how
> we do things in the Linux networking.