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

[Xiaoye Sun]

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
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>
>
>
> --
> Vincenzo Maffione
>