Raspberry Pi 4 and bad latency.. SOLUTIONS?

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22 Mar 2025 13:34 #324548 by hitchhiker
Hi guys!I am on a new Project and found a mesa 7c81 an 7i92 on the flewmarket... dont know why someone sell it on a flewmarket...i start reading about the 7c81 and look to my boxes and found a raspberry pi 4 b version with 8gb ram.. oki load the image for the sd card from the linuxcnc website and bring it up.i isolate cpu 1 2 3 but latency doesnt go down.

Any tips?
in the config i disable audio blueetooth and overclock to 6v and 2000mhz..

which information i can share to get the cow frome the ice?

thanks all


 

 

 
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  • tommylight
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22 Mar 2025 13:56 #324552 by tommylight
Replied by tommylight on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
try this
latency-histogram --nobase --sbinsize 1000 --show

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22 Mar 2025 13:59 #324553 by hitchhiker
Replied by hitchhiker on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
I forget 1 thing.. i use a 64gb micro sd card from the brand kexin from amazon.

amzn.eu/d/hXGglyV

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22 Mar 2025 14:25 #324554 by hitchhiker
Replied by hitchhiker on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
same values or i am wrong??


 


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22 Mar 2025 16:52 #324564 by tommylight
Replied by tommylight on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
Much better, move on to wiring and making chips!
Keep an eye when starting LinuxCNC if it complains about latency, it shows a warning usually when enabling it or later during use.

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22 Mar 2025 21:25 #324586 by rodw
Your latency is a pass, move on

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23 Mar 2025 06:35 #324628 by hitchhiker
Replied by hitchhiker on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
on ny m73 i have a jitter of 1000... so what is the reason thst you say that this latency is good???? thanks

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23 Mar 2025 07:27 #324630 by rodw
Assuming you use the 7c81's FGPA for stepgen and don't sofware step, you don't need a base thread so latency requirements is much relaxed. I'd be happy with anything under 130,000 with 200,000 at a pinch

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23 Mar 2025 07:47 #324633 by hitchhiker
Replied by hitchhiker on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?
or a 7i92... i payed for both cards 100 euros...

so for my information... on a normal pc like optiplex 3020m or lenovo m73 tiny.. i have latency under 2000... but i never looked this way in the histogram.

can you explain me the spike?! please

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23 Mar 2025 07:52 - 23 Mar 2025 07:53 #324634 by Aciera
Replied by Aciera on topic Raspberry Pi 4 and bad latency.. SOLUTIONS?

same values or i am wrong??


Note the missing spikes (colored in blue/red) at the extreme left and right of the graph.
Last edit: 23 Mar 2025 07:53 by Aciera.

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