Re: [PATCH] ARM: NEON detected memcpy.

[Richard Earnshaw <rearnsha@arm.com>]

On 09/04/13 13:58, Carlos O'Donell wrote:
> On 04/09/2013 05:04 AM, Richard Earnshaw wrote:
>> On 03/04/13 16:08, Joseph S. Myers wrote:
>>> I was previously told by people at ARM that NEON memcpy wasn't a good idea
>>> in practice because of raised power consumption, context switch costs etc.
>>> from using NEON in processes that otherwise didn't use it, even if it
>>> appeared superficially beneficial in benchmarks.
>>
>> What really matters is system power increase vs performance gain and
>> what you might be able to save if you finish sooner.  If a 10%
>> improvement to memcpy performance comes at a 12% increase in CPU
>> power, then that might seem like a net loss.  But if the CPU is only
>> 50% of the system power, then the increase in system power increase
>> is just half of that (ie 6%), but the performance improvement will
>> still be 10%.  Note that 20% is just an example to make the figures
>> easier here, I've no idea what the real numbers are, and they will be
>> hightly dependent on the other components in the system: a back-lit
>> display, in particular, will use a significant amount of power.
>>
>> It's also necessary to think about how the Neon unit in the processor
>> is managed.  Is it power gated or simply clock gated.  Power gated
>> regions are likely to have long power-up times (relative to normal
>> CPU operations), but clock-gated regions are typically
>> instantaneously available.
>>
>> Finally, you need to consider whether the unit is likely to be
>> already in use.  With the increasing trend to using the hard-float
>> ABI, VFP (and Neon) are generally much more widely used in code now
>> than they were, so the other potential cost of using Neon (lazy
>> context switching) is also likely to be a non-issue, than if the unit
>> is almost never touched.
>
> My expectation here is that downstream integrators run the
> glibc microbenchmarks, or their own benchmarks, measure power,
> and engage the community to discuss alternate runtime tunings
> for their systems.
>
> The project lacks any generalized whole-system benchmarking,
> but my opinion is that  microbenchmarks are the best "first step"
> towards achieving measurable performance goals (since whole-system
> benchmarking is much more complicated).
>
> At present the only policy we have as a community is that faster
> is always better.


You still have to be careful how you measure 'faster'.  Repeatedly 
running the same fragment of code under the same boundary conditions 
will only ever give you the 'warm caches' number (I, D and branch 
target), but if the code is called cold (or with different boundary 
conditions in the case of the Branch target cache) most of the time in 
real life, that's unlikely to be very meaningful.

R.