changing pacing gain in TCP BBR

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MUHAMMAD AHSAN

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Aug 20, 2021, 4:16:38 AM8/20/21
to BBR Development
hi there,

I was wondering , if there is anyway we can tweak with the pacing gain parameter and
check the result for certain flows on ethernet , 4G LTE and other network toplogies.

will be thankful.
Br,
Ahsan

Neal Cardwell

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Aug 20, 2021, 9:38:10 AM8/20/21
to MUHAMMAD AHSAN, BBR Development
Yes, it's possible to run experiments modifying the pacing gain. 

For BBRv1 you can modify bbr_pacing_gain, recompile, boot, and test. Or convert it into a module parameter and modify it dynamically.

Or use BBRv2, where it is already a module parameter (see this spot in tcp_bbr2.c).

Note that you would want to make compensating changes in the first and second pacing gain of the cycle, so that the average pacing gain is still 1. (e.g. (1.25 + 0.75)/2 = 1, and also (1.3 + 0.7)/2 = 1. Also please keep in mind that larger pacing gains cause larger queues (if the bottleneck link is full) and thus larger queuing delays and potentially larger loss impacts on cross-traffic.

best regards,
neal


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