Subject: Re: Default offload settings in Ethernet drivers

From: Ben Hutchings <[email-masked]>
Date: Thu, 11 Dec 2008 19:29:28 +0000

On Thu, 2008-12-11 at 10:54 -0800, Stephen Hemminger wrote:
> On Thu, 11 Dec 2008 19:41:31 +0100
> Jan Ceuleers <[email-masked]> wrote:
> 
> > Hi!
> > 
> > A discussion recently took place on the power mailing list on the 
> > subject of the impact of (hardware-assisted) offload functions on the 
> > power efficiency of the overall system.
> > 
> > The discussion was brought on by me noticing that not all drivers enable 
> >   all of their offload features by default (case in point: r8169).
> > 
> > Although the discussion may not be complete, early indications are that:
> > 
> > 1. Hardware-assisted offloads improve power efficiency unless 
> > implemented in a separate CPU (TOE / Total Offloading);
> > 
> > 2. It would probably be a good idea to enable hardware-assisted offloads 
> > other than TOE by default given the above.
> > 
> > I would therefore like to sollicit views here:
> > 
> > 1. Would changing default offload settings in Ethernet drivers help to 
> > save the planet?
> > 
> > 2. Which offload settings does it make sense to enable by default?
> 
> Go get a kill-a-watt meter and real hardware and measure.
[...]

Even then, the results will be highly dependent on the CPU's power-
saving capabilities and on settings that affect the pattern of IRQs like
interrupt moderation and number of queues used by multiqueue-capable
drivers, not just on the offload settings.  I would expect checksum
generation/validation and segmentation in an ASIC to take less power
than in a CPU, but on an already-busy CPU this might not be the case.

Power usage also depends on throughput, of course.  If the test involves
pushing data as fast as possible rather than simulating a specific
workload then offload features may well probably increase throughput
without reducing power consumption.  So maybe the metric should be
power/throughput... but there is unlikely to be a linear relationship
between the two, so a single figure for this may be misleading.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.