netdev - Re: BQL crap and wireless

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Message-ID: <CA+mtBx-j_+mrNpntR7CfYOromDnM-ZGzvebpUCjC8Qf14X6u9g@mail.gmail.com>
Date:	Mon, 29 Aug 2011 20:34:20 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Jim Gettys <jg@...edesktop.org>
Cc:	"Luis R. Rodriguez" <mcgrof@...il.com>,
	Dave Taht <dave.taht@...il.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Andrew McGregor <andrewmcgr@...il.com>,
	Matt Smith <smithm@....qualcomm.com>,
	Kevin Hayes <hayes@....qualcomm.com>,
	Derek Smithies <derek@...ranet.co.nz>, netdev@...r.kernel.org
Subject: Re: BQL crap and wireless

> Computing the buffering in bytes is better than in packets; but since on
> wireless multicast/broadcast is transmitted at a radically different
> rate than other packets, I expect something based on time is really the
> long term solution; and only the driver has any idea how long a packet
> of a given flavour will likely take to transmit.

The generalization of BQL would be to set the queue limit in terms of
a cost function implemented by the driver.  The cost function would
most likely be an estimate of time to transmit a packet.  So C(P)
could represent cost of a packet, sum(C(P) for P queued) is aggregate
cost of queue packets, and queue limit is the maximum cost sum.  For
wired Ethernet, number of bytes in packet might be a reasonable
function (although framing cost could be included, but I'm not sure
that would make a material difference).  For wireless, maybe the
function could be more complex possibly taking multicast, previous
history of transmission times, or other arbitrary characteristics of
the packet into account...

I can post a new patch with this generalization if this is interesting.

Tom
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