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Re: [Discuss-gnuradio] Volk branch on github

From: Josh Blum
Subject: Re: [Discuss-gnuradio] Volk branch on github
Date: Thu, 16 Feb 2012 11:39:25 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111229 Thunderbird/9.0

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.


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