netdev - Re: the side effect of using copy skb instead of skb

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Message-ID: <87zjgih66u.fsf@nemi.mork.no>
Date:	Wed, 09 Jul 2014 18:01:45 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	joey ming <joey.zming@...il.com>
Cc:	jim_baxter@...tor.com, Alexey Orishko <alexey.orishko@...il.com>,
	netdev@...r.kernel.org, zhao.ming9@....com.cn
Subject: Re: the side effect of using copy skb instead of skb_clone in cdc ncm/mbim driver

[didn't notice earlier, but Alexey's address was wrong - I fixed it on
this reply]

joey ming <joey.zming@...il.com> writes:

> thanks for your reply.
> from my test results, perhaps the ncm protocol is the same efficiency with
> cdc-ecm. But Alexey(alexey.orishko@...ricsson.com) said his experiment two
> years ago:"One real-world example was modem for 21+6Mbit/s what used 100%
> CPU with ECM responsible for approx. 40% of the MIPS used. Using NCM
> instead CPU was only at approx. 65% utilization. Which allowed multiple
> other functions to be added and significantly increased the usability and
> value of the modem". I don't know why the test result was differ so large.
> Is that correct that cdc-ncm is effctive than cdc-ecm for low speed device
> but not for high speed device?

Alexey's results were on modem hardware, and I am guessing the OS wasn't
Linux.  I have no doubt that you can increase efficiency if you can take
a fixed size big NCM buffer, and make the radio interface write packets
directly into it using the alignment of your choice, before you just
give the whole buffer to a USB controller.  You mostly don't have to
involve the CPU at all.  So NCM is probably a great win for the modems,
and you are right: That is likely why this aggregating protocol was
invented.

But little of this is applicable to the typical Linux implementation,
whether it runs on a host or a device.  Big USB buffers do not help much
on the USB controllers, and they are just a hassle other places because
it's difficult to pass partial buffers around.

And I think modem hardware now has become so much more powerful that the
same applies to it as well.

Bjørn
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