It seems we share a common understanding and we need to agree on a good wording for the most meaningful API. Questions inline below:
06/06/2022 19:15, Andrew Rybchenko: > On 6/6/22 16:16, Spike Du wrote: > > Hi Andrew, > > Please see below for "fill threshold" concept, I'm ok with other > > comments about code. > > > > Regards, > > Spike. > > > > > > From: Andrew Rybchenko <andrew.rybche...@oktetlabs.ru> > >> On 6/3/22 15:48, Spike Du wrote: > >>> Fill threshold describes the fullness of a Rx queue. If the Rx queue > >>> fullness is above the threshold, the device will trigger the event > >>> RTE_ETH_EVENT_RX_FILL_THRESH. > >> > >> Sorry, I'm not sure that I understand. As far as I know the process to add > >> more Rx buffers to Rx queue is called 'refill' in many drivers. So fill > >> level is a > >> number (or percentage) of free buffers in an Rx queue. > >> If so, fill threashold should be a minimum fill level and below the level > >> we > >> should generate an event. > >> > >> However reading the first paragraph of the descrition it looks like you > >> mean > >> oposite thing - a number (or percentage) of ready Rx buffers with received > >> packets. > >> > >> I think that the term "fill threshold" is suggested by me, but I did it > >> with mine > >> understanding of the added feature. Now I'm confused. > >> > >> Moreover, I don't understand how "fill threshold" could be in terms of > >> ready > >> Rx buffers. HW simply don't really know when ready Rx buffers are > >> processed by SW. So, HW can't say for sure how many ready Rx buffers are > >> pending. It could be calculated as Rx queue size minus number of free Rx > >> buffers, but it is imprecise. First of all not all Rx descriptors could be > >> used. > >> Second, HW ring size could differ queue size specified in SW. > >> Queue size specified in SW could just limit maximum nubmer of free Rx > >> buffers provided by the driver. > >> > > > > Let me use other terms because "fill"/"refill" is also ambiguous to me. > > In a RX ring, there are Rx buffers with received packets, you call it > > "ready Rx buffers", there is a RTE api rte_eth_rx_queue_count() to get the > > number, > > It's also called "used descriptors" in the code. > > Also there are Rx buffers provided by SW to allow HW "fill in" received > > packets, we can call it "usable Rx buffers" (here "usable" means usable for > > HW). > > May be it is better to stick to Rx descriptor status terminology? > Available - Rx descriptor available to HW to put received packet to > Done - Rx descriptor with received packet reported to Sw > Unavailable - other (e.g. gap which cannot be used or just processed > Done, but not refilled (made available to HW). > > > Let's define Rx queue "fullness": > > Fullness = ready-Rx-buffers/Rxq-size > > i.e. number of DONE descriptors divided by RxQ size > > > On the opposite, we have "emptiness" > > Emptiness = usable-Rx-buffers/Rxq-size > > i.e. number of AVAIL descriptors divided by RxQ size > Note, that AVAIL != RxQ-size - DONE > > HW really knows number of available descriptors by its nature. > It is a space between latest done and latest received on refill. > > HW does not know which descriptors are DONE, since some which > are DONE before could be already processed by SW, but not yet > made available again. > > > > Here "fill threshold" describes "fullness", it's not "refill" described in > > you above words. Because in your words, "refill" is the opposite, it's > > filling "usable/free Rx buffers", or "emptiness". > > > > I can only briefly explain how mlx5 works to get LWM, because I'm not a > > Firmware guy. > > Mlx5 Rx queue is basically RDMA queue. It has two indexes: producer index > > which increases when HW fills in packet, consumer index which increases > > when SW consumes the packet. > > The queue size is known when it's created. The fullness is something like > > (producer_index - consumer_index) (I don't consider in wrap-around here). > > So mlx5 has the way to get the fullness or emptiness in HW or FW. > > Another detail is mlx5 uses the term "LWM"(limit watermark), which > > describes "emptiness". When usable-Rx-buffers is below LWM, we trigger an > > event. > > But Thomas think "fullness" is easier to understand, so we use "fullness" > > in rte APIs and we'll translate it to LWM in mlx5 PMD. I may be wrong :) > HW simply does now know fullness and there can't generate any events > based on it. It is a problem on Rx when there is now available > descriptors. I.e. emptiness. So you think "empty_thresh" would be better? Or "avail_thresh"? > >>> Fill threshold is defined as a percentage of Rx queue size with valid > >>> value of [0,99]. > >>> Setting fill threshold to 0 means disable it, which is the default. > >>> Add fill threshold configuration and query driver callbacks in > >>> eth_dev_ops. > >>> Add command line options to support fill_thresh per-rxq configure. > >>> - Command syntax: > >>> set port <port_id> rxq <rxq_id> fill_thresh <fill_thresh_num> > >>> > >>> - Example commands: > >>> To configure fill_thresh as 30% of rxq size on port 1 rxq 0: > >>> testpmd> set port 1 rxq 0 fill_thresh 30 > >>> > >>> To disable fill_thresh on port 1 rxq 0: > >>> testpmd> set port 1 rxq 0 fill_thresh 0 > >>> > >>> Signed-off-by: Spike Du <spi...@nvidia.com>
