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Because we don't have complete control over the sending rate of the kernel module, we cannot easily line up monitor intervals such that a single monitor interval contains packets sent at exactly the same rate.
To illustrate the problem, consider two adjacent monitor intervals with three sending rates on a 100Mbps link.
MI 1:
t = 0.00 -> t = 0.75, rate = 120Mbps, latency = 200ms (full buffer)
t = 0.75 -> t = 1.00, rate = 80Mbps, latency = 100ms (empty buffer)
average rate: 110Mbps
latency inflation: -100ms
MI 2:
t = 1.00 -> t = 1.75, rate = 80Mbps, latency = 100ms
t = 1.75 -> t = 2.00, rate = 120Mbps, latency = 200ms
average rate: 90Mbps
latency inflation: 100ms
In this case, we would conclude that 110Mbps is a better sending rate because it decreases our latency, while 90Mbps increases our latency. If we had finer granularity in our monitor intervals or they lined up with observed sending rates, we could see that 80Mbps is preferable to 120Mbps and we could change our target rate accordingly.
PCC Vivace don't build for Linux 6.7.0. For < 6.7.0 - all ok. Allegro build ok for 6.7.
extra_rate = pcc->intervals[0].delivered * tsk->mss_cache; this line of code get me confused.
I'm seeing that pacing using the pcc_set_pacing_rate
function works reasonably well below the link capacity.
Here's an example of reasonable pacing (17.5Mbps on a 24Mbps link).
Here's an example of poor pacing (40Mbps on a 24Mbps link). Notice that the actual sending rate goes above the pacing rate at times as well.
This issue still has to be confirmed using the BBR codebase. It appears that this is less of an issue near the link capacity.
In function 'pcc_decide()'
if (new_rate != pcc->rate) {
printk(KERN_INFO "%d decide: on new rate %d %d (%d)\n",
pcc->id, pcc->rate < new_rate, new_rate,
pcc->decisions_count);
pcc->moving = true;
pcc_setup_intervals_moving(pcc, list);
} else {
printk(KERN_INFO "%d decide: stay %d (%d)\n", pcc->id,
pcc->rate, pcc->decisions_count);
pcc_setup_intervals_probing(pcc, list);
}
pcc->rate = new_rate; // update new rate
When the condition: new_rate != pcc->rate
is satisified, PCC will enter moving stage and set the next monitor interval's rate in function:
' pcc_setup_intervals_moving(...)'
/* Reset statistics and set the target rate for just one monitor interval */
static void pcc_setup_intervals_moving(struct pcc_data *pcc)
{
pcc->intervals[0].packets_sent_base = 0;
pcc->intervals[0].rate = pcc->rate;
pcc->send_index = 0;
pcc->recive_index = 0;
pcc->wait = false;
}
However, at this time, the value of pcc->rate has not been updated, and it is still runs the old rate for the next interval.
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