netdev - Re: drop all fragments inside tx queue if one gets dropped

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Message-ID: <5717EA7E.1020606@hpe.com>
Date:	Wed, 20 Apr 2016 13:45:50 -0700
From:	Rick Jones <rick.jones2@....com>
To:	Michael Richardson <mcr@...delman.ca>, netdev@...r.kernel.org,
	linux-wpan@...r.kernel.org
Cc:	Alexander Aring <aar@...gutronix.de>
Subject: Re: drop all fragments inside tx queue if one gets dropped

For the "everything old is new again" files, back in the 1990s, it was 
noticed that on the likes of a netperf UDP_STREAM test on HP-UX, with 
fragmentation taking place, it was possible to consume 100% of the link 
bandwidth and have 0% effective throughput because the transmit queue 
was kept full with IP datagram fragments which could not possibly be 
reassembled (*) because one or more of the fragments of a datagram were 
dropped because the transmit queue was full.

HP-UX implemented "packet trains" where all the fragments of a 
fragmented datagram were presented to the driver, which then either 
queued them all, or none of them.

I don't recall seeing similar poor behaviour in Linux; I would have 
assumed that the intra-stack flow-control "took care" of it.  Perhaps 
there is something specific to wpan which precludes that?

happy benchmarking,

rick jones