# Throttle question

**URL:** <https://www.ruby-forum.com/t/throttle-question/239835>\
**Category:** GNU Radio\
**Created:** [March 16, 2014, 5:36pm UTC](https://www.ruby-forum.com/t/throttle-question/239835 "2014-03-16T17:36:29Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\
**Post date:** [March 16, 2014, 5:36pm UTC](https://www.ruby-forum.com/t/throttle-question/239835/1 "2014-03-16T17:36:29Z")

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For testing purposes I use  
vector source (byte ) -\> vector to stream -\> throttle -\> [blocks to  
test]  
In the vector source “repeat” is set to “yes” and the sample rate is set  
to 1024 ( and in the throttle block , too ).

I would expect that the throttle delivers 1024 samples every second. But  
I see that the throttle delivers 32768 bytes about every 32 seconds. (  
Which is on average 1024 samples per second ).  
Is this the expected behavior or should the samples be delivered every  
second ?

– Volker

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**Author:** ![Volker\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/b5a626/32.png) [@Volker\_S](https://www.ruby-forum.com/u/Volker_S)\
**Post date:** [March 16, 2014, 5:53pm UTC](https://www.ruby-forum.com/t/throttle-question/239835/2 "2014-03-16T17:53:37Z")

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Hi,

> For testing purposes I use  
> vector source (byte ) → vector to stream → throttle → [blocks to test]  
> In the vector source “repeat” is set to “yes” and the sample rate is set to  
> 1024 ( and in the throttle block , too ).
> 
> I would expect that the throttle delivers 1024 samples every second. But I  
> see that the throttle delivers 32768 bytes about every 32 seconds. ( Which  
> is on average 1024 samples per second ).  
> Is this the expected behavior or should the samples be delivered every  
> second ?

Excpected I wouldn’t say so, but there is no guarantee basically. Only  
long term average rate is correct, the rest is kind of up to the OS  
scheduler that says when thread runs …

At every iteration of the work() function, the block will let as much  
sample pass depending on the time since the last call.  
But for very very low rates (like 1k/s) that’s probably not going to  
go over very well because things tend to be processed by “block”.

Cheers,

```
Sylvain

```

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**Author:** ![Volker\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/b5a626/32.png) [@Volker\_S](https://www.ruby-forum.com/u/Volker_S)\
**Post date:** [March 16, 2014, 6:02pm UTC](https://www.ruby-forum.com/t/throttle-question/239835/3 "2014-03-16T18:02:10Z")

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Hi Volker,

yes, that is expected behaviour. Most probably 😉  
For explanation: Since GNU Radio itself always passes around multiple  
items at once (so your work doesn’t get called), all throttle can do is  
limit the average rate, unless it would consume one input item at a  
time, stall work as long as necessary and then output that one item.  
For higher sample rates (where the flow graph is nearly CPU-limited),  
which is the original use case of throttle (stopping GNU Radio from  
completely clogging the CPU) this would imply a considerable overhead.

What you can try is call the public  
gr::block::set\_max\_noutput\_items(int) method on your throttle block  
_prior_ to starting the flow graph. This should tell the scheduler to  
call the throttle’s work function with smaller numbers of input  
samples (also, it reduces the output buffer size if that is feasible  
for the downstream block).  
Also, try smaller values for the max\_output\_buffer size of the vector  
source, maybe (that could be even accessible in the GNU Radio  
companion block property dialog).

one more question in that context: why the vector to stream? the  
vector source can output single samples, too. Just set the vector  
length to 1.

Greetings,  
Marcus

On 16.03.2014 17:35, Volker S. wrote:

> – Volker
> 
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<div class="post-metadata">

**Author:** ![Volker\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/b5a626/32.png) [@Volker\_S](https://www.ruby-forum.com/u/Volker_S)\
**Post date:** [March 16, 2014, 6:11pm UTC](https://www.ruby-forum.com/t/throttle-question/239835/4 "2014-03-16T18:11:58Z")

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Thanks for the tips.

Setting the Max Output bufferin the throttle block did the trick.  
Yes, the vector to stream block can be avoided. The default vector  
length is 1 in grc.

Now testing works much smoother.

Good to know.

– Volker

Am 16.03.2014 18:00, schrieb Marcus M.: