netdev - Re: [PATCH] atm: br2684: Fix excessive queue bloat

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Message-ID: <1353880892.26346.300.camel@shinybook.infradead.org>
Date:	Sun, 25 Nov 2012 22:01:32 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Krzysztof Mazur <krzysiek@...lesie.net>
Cc:	netdev@...r.kernel.org, John Crispin <blogic@...nwrt.org>,
	Dave Täht <dave.taht@...il.com>,
	"Chas Williams (CONTRACTOR)" <chas@....nrl.navy.mil>
Subject: Re: [PATCH] atm: br2684: Fix excessive queue bloat

On Sun, 2012-11-25 at 22:43 +0100, Krzysztof Mazur wrote:
> On Sat, Nov 24, 2012 at 12:01:32AM +0000, David Woodhouse wrote:
> > There's really no excuse for an additional wmem_default of buffering
> > between the netdev queue and the ATM device. Two packets (one in-flight,
> > and one ready to send) ought to be fine. It's not as if it should take
> > long to get another from the netdev queue when we need it.
> > 
> > If necessary we can make the queue space configurable later, but I don't
> > think it's likely to be necessary.
> 
> Maybe some high-speed devices will require larger queue, especially for
> smaller packets, but 2 packet queue should be sufficient in almost all cases.

Yeah. This is fairly much the same conversation I ended up having when I
did the same for PPPoATM.

Some day *perhaps* we might look at doing something adaptive, so it'll
detect a TX underrun and increase the amount of buffering. But this
seems perfectly good for now.

> Maybe this magic "2" and the comment should be moved to some #define.

Doesn't make it any less magic. I'm happier with it as it is, with a
clear comment describing why it's done that way.

-- 
dwmw2


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