netdev - Re: Bypass qdiscs?

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Message-ID: <CAGXJAmzn0vFtkVT=JQLQuZm6ae+Ms_nOcvebKPC6ARWfM9DwOw@mail.gmail.com>
Date: Sat, 4 Nov 2023 19:47:30 -0700
From: John Ousterhout <ouster@...stanford.edu>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org
Subject: Re: Bypass qdiscs?

I haven't tried creating a "pass through" qdisc, but that seems like a
reasonable approach if (as it seems) there isn't something already
built-in that provides equivalent functionality.

-John-

P.S. If hardware starts supporting Homa, I hope that it will be
possible to move the entire transport to the NIC, so that applications
can bypass the kernel entirely, as with RDMA.

On Sat, Nov 4, 2023 at 8:08 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Fri, Nov 03, 2023 at 04:55:35PM -0700, John Ousterhout wrote:
> > Is there a way to mark an skb (or its socket) before invoking
> > ip_queue_xmit/ip6_xmit so that the packet will bypass the qdiscs and
> > be transmitted immediately? Is doing such a thing considered bad
> > practice?
> >
> > (Homa has its own packet scheduling mechanism so the qdiscs are just
> > getting in the way and adding delays)
>
> Hi John
>
> One thing to think about is what happens when hardware starts
> supporting Homa. Can the packet scheduling be moved into the hardware?
> Ideally you want to make use of the existing mechanisms to offload
> scheduling to the hardware, rather than add a Homa specific one.
>
> Did you try adding a Homa specific qdisc implementing the scheduling
> algorithm? Did it kill performance? We prefer to try to fix problems,
> rather than bypass them.
>
>        Andrew