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

Andrew Bainbridge andbain at microsoft.com
Mon Oct 15 13:20:57 CEST 2018


Is the feature you are describing is called packet "pacing"? Here's a Mellanox document describing it: 
  https://community.mellanox.com/docs/DOC-2551 

I grep'ed the DPDK source for "rate_limit" and found rte_eth_set_queue_rate_limit(). Is that the function you need?

From my quick grep'ing, it looks to me like this feature isn't supported on Mellanox drivers but is in the ixgbe driver. However, this is all guess work. I'm not an expert. I'd like to know the real answer!

- Andrew

-----Original Message-----
From: users <users-bounces at dpdk.org> On Behalf Of Philipp B
Sent: 15 October 2018 11:45
To: shaklee3 at gmail.com
Cc: users at dpdk.org
Subject: Re: [dpdk-users] rte_eth_tx_burst: Can I insert timing gaps

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 at 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 at 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


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