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Re: [Discuss-gnuradio] Volk branch on github

From: Philip Balister
Subject: Re: [Discuss-gnuradio] Volk branch on github
Date: Fri, 17 Feb 2012 05:43:40 -0800
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On 02/16/2012 11:39 AM, Josh Blum wrote:
> On 02/16/2012 11:32 AM, Josh Blum wrote:
>> On 02/16/2012 11:24 AM, Tom Rondeau wrote:
>>> On Thu, Feb 16, 2012 at 2:08 PM, Josh Blum <address@hidden> wrote:
>>>>> Also, you never want to work on the smallest amount of memory possible.
>>>>> This is covered in my discussion on my blog. Making arbitrarily small
>>>> calls
>>>>> to work functions causes much more overhead than just running the
>>>> unaligned
>>>>> version of a Volk call. I found this out pretty quickly when I started
>>>>> looking into things. Better to process a large chunk to get back into
>>>>> alignment than try to handle calls to small amounts of data.
>>>> Perhaps this is because you have a processor that doesn't penalize you
>>>> for unaligned loads/stores.
>>>> -Josh
>>> I tested this on a handful of different processors: Core2Due, QuadCore, i7
>>> (first get), i7 (second gen) and they all told me the same thing. You are
>> For most if not all recent x86 processors there is no unaligned penalty.
>> You should be able to always call the unaligned volk routine and see no
>> difference in performance. I'm wondering about neon for example, which
>> has a penalty. And I suppose to a lesser extent, older x86 processors. I
>> dont have numbers now, but I think the volk profiler can confirm this
>> about said processors.
> The answer for neon is probably a case of the "don't do that". In other
> words, keep your blocks fed with aligned multiples, regardless of how
> the scheduler handles things.

The answer is more like:

Aligned is better


if you are forced to chose between aligned loads versus stores, align stores


start by using NEON and do not worry about alignment since the penalty
for unaligned access is not dreadful.


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