[netmap] when does a packet in the netmap ring send out exactly

[Xiaoye Sun]

Hi Luigi,

Thanks!
I was using the most recent netmap on Github and I believe the tail pointer
only moves forward when there are less than half of the total slots
available in the netmap ring.
Then I switch to the version of v11.3
<https://github.com/luigirizzo/netmap/tree/v11.3>, it behaves as what you
described.

Linux Kernel: 3.16.0-4-amd64

Best,
Xiaoye


On Mon, Nov 20, 2017 at 6:11 PM, Luigi Rizzo <rizzo at iet.unipi.it> wrote:

> Hi,
> I think if you call the TXSYNC ioctl without advancing the head
> pointer, then the tail is advanced
> as much as possible.
>
> Cheers
> luigi
>
> On Mon, Nov 20, 2017 at 3:35 PM, Xiaoye Sun <Xiaoye.Sun at rice.edu> wrote:
> > Hi,
> >
> > I found that the tail pointer only moves when the ring has less than half
> > of the slots available. This prevents me from knowing the accurate time
> > when the packet in a slot is processed. Is there a way to move the tail
> > pointer as long as the packet in the slot is processed? Is this a
> > configurable feature?
> >
> > Best,
> > Xiaoye
> >
> > On Fri, Oct 27, 2017 at 11:52 AM, Vincenzo Maffione <
> v.maffione at gmail.com>
> > wrote:
> >
> >> 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 at 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 at freebsd.org mailing list
> >>> https://lists.freebsd.org/mailman/listinfo/freebsd-net
> >>> To unsubscribe, send any mail to "freebsd-net-unsubscribe at freebsd.org"
> >>>
> >>
> >>
> >>
> >> --
> >> Vincenzo Maffione
> >>
> > _______________________________________________
> > freebsd-net at freebsd.org mailing list
> > https://lists.freebsd.org/mailman/listinfo/freebsd-net
> > To unsubscribe, send any mail to "freebsd-net-unsubscribe at freebsd.org"
>
>
>
> --
> -----------------------------------------+-------------------------------
>  Prof. Luigi RIZZO, rizzo at iet.unipi.it  . Dip. di Ing. dell'Informazione
>  http://www.iet.unipi.it/~luigi/        . Universita` di Pisa
>  TEL      +39-050-2217533               . via Diotisalvi 2
>  Mobile   +39-338-6809875               . 56122 PISA (Italy)
> -----------------------------------------+-------------------------------
>