Date: Fri, 27 Oct 2017 18:52:46 +0200 From: Vincenzo Maffione <v.maffione@gmail.com> To: Xiaoye Sun <Xiaoye.Sun@rice.edu> Cc: FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: [netmap] when does a packet in the netmap ring send out exactly Message-ID: <CA%2B_eA9i5WOiA8j3y8fX65rzDLXEyt2B2wo8pK12jM2ZvEBURYg@mail.gmail.com> In-Reply-To: <CAJnByzh4Kzp6-DXXcB06QHSBJpHBKhtDnKUn7R%2BK0A_5VUThyw@mail.gmail.com> References: <CAJnByzh4Kzp6-DXXcB06QHSBJpHBKhtDnKUn7R%2BK0A_5VUThyw@mail.gmail.com>
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Hi, This is actually a limitation of the netmap API: ring->tail is exposed to the user so that it knows it can use the slots in the range "[ring->head..ring->tail[" for new transmissions (note that head is included, tail excluded, to prevent wraparound). However, there is no explicit indication of "up to what slots packets were transmitted". For hw NICs, however, ring->tail is an indication of where transmission was completed. Example: 1) at the beginning ring->tail = ring->head = ring->cur = 0 2) then your program moves head/cur forward: head = cur = 10 3) you call TXSYNC, to submit the packets to the NIC. 4) after the TXSYNC call, is very likely that tail is still 0, i.e. because no transmission has been completed by the NIC (and no interrupt generated). 5) say after 20 us you issue another TXSYNC, and in the meanwhile 6 packets had completed. In this case after TXSYNC you will find tail==5, meaning that packets in the slots 0,1,2,3,4 and 5 have been completed. Note that also the slot pointed by tail has been completed. But you are right that there is no way to receive completion notification if the queue is not full. You must use TXSYNC to check (by sleeping or busy wait) when tail moves forward. Cheers, Vincenzo 2017-10-27 3:06 GMT+02:00 Xiaoye Sun <Xiaoye.Sun@rice.edu>: > Hi > > I write a netmap program that sends packets to the network. my program > uses one netmap ring and fills the ring slots with packets. > My program needs to do something (action A) after a particular packet > (packet P) in the ring slot is sent to the network. so the program tracks > the position of the tail point and checks if the tail point has moved > across the slot I used to put that packet P. > However, I found that the tail pointer may not move forward even seconds > after the receiver side got packet P. > Sometimes the tail pointer never moves forward until the TX ring is full. > I try ioctl(NIOCTXSYNC), however, it cannot 100% solve the problem. > > My question is that is there a way to make the TX ring empty as early as > possible so that I can know when my packet is sent out. or is there another > way to know when the packet in the slot is sent to the network/NIC physical > queue? > > I am using Linux 3.16.0-4-amd64. > > Thanks! > > Best, > Xiaoye > _______________________________________________ > freebsd-net@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org" > -- Vincenzo Maffione
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