DPDK provides but does not define (that I can see at least) these TXQ config fields. What are these fields what do they do?

https://doc.dpdk.org/api/structrte__eth__thresh.html
uint8_t pthresh
uint8_t hthresh
uint8_t wthresh

And here: What is the RS bit?
https://doc.dpdk.org/api/structrte__eth__txconf.html
uint16_t tx_rs_thresh. 

And some what unrelated: What is a ``doorbell"? this is something I continue to run into esp. in Mellanox NIC papers? Doorbell counts are reported for AWS NICs at least with xstats api:

tx_q0_doorbells: 18812
tx_q1_doorbells: 18861