Re: [dpdk-users] rte_eth_tx_burst: Can I insert timing gaps

[Philipp B <philippb.ontour@gmail.com>]

Maybe I should explain with some ASCII art. It's not about sleeping
just the remaining period of 20ms. This is exactly what I am doing
already, and this works fine.

Think of 20ms intervals like this

|XXXXXXXXXXXXXXXXXX_____|XXXXXXXXXXXXXXXXXX_____

Where '|' is the start of the 20ms interval, X is a packet, and _ is
an idle gap of one packet. (Let's just pretend there are 1000s of Xs
per interval).

As I said, I let the CPU sleep upto the beginning of the interval
('|'). This beginning of interval is the only moment where CPU timing
controls adapter timing. Then I send out a few 1000 packets. In this
phase, I have a few 100 packets buffered by DPDK, so it will not help
to sleep on the CPU.

The pattern above is what I can easily produce just with an OS sleep,
a single buffer pool and rte_eth_tx_burst. What I am looking for, is a
way to e.g. remove every second packet from that pattern, while
keeping the other packet's timings unchanged:

|X_X_X_X_X_X_X_X_X______|X_X_X_X_X_X_X_X_X______

Basically, I do not need to transmit anything in the gaps. I just need
the delay. However, as my timing of CPU isn't coupled tightly to the
adapter, sleeping on the cpu will not help. This is intended by
design: I want to blow out a massive number of packets with exact
timing and virtually no CPU requirement.

What I look for is a sleep instruction executed by the adapter, which
is buffered in order with the packets issued by rte_eth_tx_burst.
(Plus some basic math rules how to convert packet sizes to durations,
based on line speeds).

Am Sa., 13. Okt. 2018 um 23:05 Uhr schrieb Cliff Burdick <shaklee3@gmail.com>:
>
> Maybe I'm misunderstanding the problem, but do you need to transmit anything? Can you just use the rte_cycles functions to sleep for the remaining period in 20ms?
>
> On Thu, Oct 11, 2018 at 2:04 AM Philipp B <philippb.ontour@gmail.com> wrote:
>>
>> Hi all!
>>
>> I am working on an RTP test traffic generator. The basic idea is
>> clock_nanosleep providing a 20ms clock cycle to start a (big) number
>> of rte_eth_tx_bursts, sending equally sized buffers. As long as the
>> timing within a 20ms cycle is dictated primarily by the line speed, I
>> can be sure that not just the first buffer of each cycle has a period
>> of 20ms, but also the n-th buffer. (I have sent n-1 buffers before
>> with the same size.)
>>
>> Basically, I see one 20ms interval as a series of time slots, each
>> capable to store an active RTP stream. My question now is, what to to
>> with inactive time slots? As long as all active streams are located at
>> consecutive time slots from the start of the 20ms interval, everything
>> is fine. But I cannot guarantee this.
>>
>> What I need is some kind of dummy buffer, which is not transmitted but
>> generates a tx timing gap as a buffer of X bytes would take to be
>> transferred.
>>
>> Is such a functionality provided? As a workaround, I already thought
>> about sending invalid packets (bad IP Header checksum?). However, this
>> won't be optimal when multiple lines are aggregated.
>>
>> Thanks!
>> Philipp Beyer