UDP/IP via Ethernet in real time (config-preempt patch)

[M. Koehrer]

Hi everybody,

I have a question concerning Ingo Molnar's realtime preempt patch.
This patch is great go get low latency on timer threads and a really
good real time behaviour.
I want to connect an external, embedded device via Ethernet (UDP/IP)=20
to my Linux PC. This communication should be in real time as well.
=46or this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network adapter=
s.

How can I get this UDP networking real time capable?

Any feedback is welcome.

Thanks a lot!

Regards

Mathias


--=20
Mathias Koehrer
***@arcor.de


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[Kok, Auke]

Post by M. Koehrer
Hi everybody,
I have a question concerning Ingo Molnar's realtime preempt patch.
This patch is great go get low latency on timer threads and a really
good real time behaviour.
I want to connect an external, embedded device via Ethernet (UDP/IP)
to my Linux PC. This communication should be in real time as well.
For this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network adapters.
How can I get this UDP networking real time capable?

In general, you'll have to find a non-NAPI ethernet driver and disable any form
of interrupt throttling. This will likely adversely affect throughput of course...


Auke
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[Stephen Hemminger]

On Tue, 17 Jul 2007 10:23:30 -0700

Post by Kok, Auke

Post by M. Koehrer
Hi everybody,
I have a question concerning Ingo Molnar's realtime preempt patch.
This patch is great go get low latency on timer threads and a really
good real time behaviour.
I want to connect an external, embedded device via Ethernet (UDP/IP)
to my Linux PC. This communication should be in real time as well.
For this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network adapters.
How can I get this UDP networking real time capable?

In general, you'll have to find a non-NAPI ethernet driver and disable any form
of interrupt throttling. This will likely adversely affect throughput of course...

RT kernel will also change the adapter hard irq into a work queue so it will
actually make the receive latency worse not better.
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[Christopher Fowler]

Post by Stephen Hemminger
RT kernel will also change the adapter hard irq into a work queue so it will
actually make the receive latency worse not better.

Is there any such thing as "Real-Time IP"? I thought the nature of
TCP/IP lead to latency and the any RT type communications need to take
place over something like serial.


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[Dan Searle]

Hi,

I would dump the idea of using UDP and inject raw ethernet frames onto
the LAN segment. You can do this by bypassing almost all of the
net-stack I'm sure of it. I think the netconsole kernel code does
it...

Documentation/networking/netconsole.txt

Dan...

Post by Christopher Fowler

Post by Stephen Hemminger
RT kernel will also change the adapter hard irq into a work queue so it will
actually make the receive latency worse not better.

Is there any such thing as "Real-Time IP"? I thought the nature of
TCP/IP lead to latency and the any RT type communications need to take
place over something like serial.
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[Stephen Hemminger]

On Wed, 18 Jul 2007 09:15:46 -0400

Post by Christopher Fowler

Post by Stephen Hemminger
RT kernel will also change the adapter hard irq into a work queue so it will
actually make the receive latency worse not better.

Is there any such thing as "Real-Time IP"? I thought the nature of
TCP/IP lead to latency and the any RT type communications need to take
place over something like serial.

People use UDP/IP for time critical applications. But like any real time system it
comes down to what the allowable latency for the application is, and on
what systems and topology it is deployed.

The RT Linux patch seems to be focused on giving faster response
to applications/threads that need servicing. I.e soft real time.
The tradeoff is shortening the possible thread service latency of
an application real time thread due interrupts from devices. In other
words, with RT you can have a real time thread that gets microsecond
response even though the normal e1000 irq could spend 10's of microseconds
walking the receive ring.
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[Christopher Fowler]

Post by Stephen Hemminger
People use UDP/IP for time critical applications. But like any real time system it
comes down to what the allowable latency for the application is, and on
what systems and topology it is deployed.

So basically if you have the right topology,the right hardware, and the
right requirements then it is possible.

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[M. Koehrer]

Hi,

I had a short look at the netconsole.txt and the correspondent code.
It seems to be based on netpoll. This looks quite interesting for my ne=
eds.
However, it seems to be only accessible from kernel space and not=20
from user space.
I want to run a real time application in user space and do the UDP acce=
sses=20
from there.=20
Is this somehow possible with the netpoll code? I did not see a callabl=
e interface
from user space side.


Thanks for any feedback on this topic.

Regards

Mathias

I would dump the idea of using UDP and inject raw ethernet frames ont=

o

the LAN segment. You can do this by bypassing almost all of the
net-stack I'm sure of it. I think the netconsole kernel code does
it...
=20
Documentation/networking/netconsole.txt
=20

--=20
Mathias Koehrer
***@arcor.de


Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonieren
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT JETZT bei Arcor: g=FC=
nstig
und schnell mit DSL - das All-Inclusive-Paket f=FCr clevere Doppel-Spar=
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[Stephen Hemminger]

AF_PACKET will let you send/receive raw frames. The processing occurs
first in the receive side, so it will be quick. Also there is memory map interface
to avoid copy.
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[Neil Horman]

Post by M. Koehrer
Hi,
=20
I had a short look at the netconsole.txt and the correspondent code.
It seems to be based on netpoll. This looks quite interesting for my =

needs.

Post by M. Koehrer
However, it seems to be only accessible from kernel space and not=20
from user space.
I want to run a real time application in user space and do the UDP ac=

cesses=20

Post by M. Koehrer
from there.=20
Is this somehow possible with the netpoll code? I did not see a calla=

ble interface

Post by M. Koehrer
from user space side.
=20
=20

Raw or Packet sockets then (AF_RAW / AF_PACKET)?

Neil

Post by M. Koehrer
Thanks for any feedback on this topic.
=20
Regards
=20
Mathias

I would dump the idea of using UDP and inject raw ethernet frames o=

nto

Post by M. Koehrer

the LAN segment. You can do this by bypassing almost all of the
net-stack I'm sure of it. I think the netconsole kernel code does
it...
=20
Documentation/networking/netconsole.txt
=20

=20
=20
--=20
Mathias Koehrer
=20
=20
Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonier=

en

Post by M. Koehrer
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT JETZT bei Arcor: g=FC=

nstig

Post by M. Koehrer
und schnell mit DSL - das All-Inclusive-Paket f=FCr clevere Doppel-Sp=

arer,

Post by M. Koehrer
nur 39,85 =80 inkl. DSL- und ISDN-Grundgeb=FChr!
http://www.arcor.de/rd/emf-dsl-2
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in

Post by M. Koehrer
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--=20
/***************************************************
*Neil Horman
*Software Engineer
*Red Hat, Inc.
****@tuxdriver.com
*gpg keyid: 1024D / 0x92A74FA1
*http://pgp.mit.edu
***************************************************/
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[M. Koehrer]

Post by M. Koehrer
How can I get this UDP networking real time capable?

=20
In general, you'll have to find a non-NAPI ethernet driver and disa=

ble any

form=20

of interrupt throttling. This will likely adversely affect throughp=

ut of

course...

=20

=20
RT kernel will also change the adapter hard irq into a work queue so =

it

will
actually make the receive latency worse not better.
=20

It is possible somehow to "boost" the work queue for this interrupt to
run with real time priority? In this case, the latency should be good e=
nough.

Mathias

--=20
Mathias Koehrer
***@arcor.de


Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonieren
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT JETZT bei Arcor: g=FC=
nstig
und schnell mit DSL - das All-Inclusive-Paket f=FCr clevere Doppel-Spar=
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[M. Koehrer]

Hi,

thanks for the feedback. I had a closer look at the receive path of an =
Ethernet frame.
The driver (e1000 in my case) receives the raw frame and calls netif_rx=
=2E
=46rom netif_rx the sk_buff is passed on to netif_rx_schedule and here =
a raise_soft_irqoff(NET_RX_SOFTIRQ)
is called to trigger the task that does the remaining parts of the rece=
ption.
Even if I use a AF_PACKET / AF_RAW socket, it will be handled by the NE=
T_RX_SOFTIRQ task.
As this task is not running in real time priority, I loose real time be=
haviour.

Within netif_rx there is a call to netpoll_rx which could be used as a =
kind filter to=20
separate interesting packets. However, currently netpoll_rx is used by =
the netconsole which allows=20
to register exactly one UDP per device that is taken into account.
I can not handle a real time network. Neither I do have a inteface to u=
ser space to the netconsole.
One approach could be to implement something similar to netpoll_rx that=
is used as switch
to separate standard Ethernet frames and frames that are treated as rea=
l time relevant.
In this case something similar to netif_rx_schedule is needed that call=
s a real time priority=20
irq thread like NET_RX_SOFTIRQ...

Any comments or feedback on this is highly welcome...

Thanks a lot

Mathias

Post by Neil Horman

Hi,
=20
I had a short look at the netconsole.txt and the correspondent code=

=2E

Post by Neil Horman

It seems to be based on netpoll. This looks quite interesting for m=

y

Post by Neil Horman
needs.

However, it seems to be only accessible from kernel space and not=20
from user space.
I want to run a real time application in user space and do the UDP

accesses=20

from there.=20
Is this somehow possible with the netpoll code? I did not see a cal=

lable

Post by Neil Horman
interface

from user space side.
=20
=20

Raw or Packet sockets then (AF_RAW / AF_PACKET)?
=20
Neil
=20

Thanks for any feedback on this topic.
=20
Regards
=20
Mathias

I would dump the idea of using UDP and inject raw ethernet frames=

onto

Post by Neil Horman

the LAN segment. You can do this by bypassing almost all of the
net-stack I'm sure of it. I think the netconsole kernel code does
it...
=20
Documentation/networking/netconsole.txt
=20

=20
=20
--=20
Mathias Koehrer
=20
=20
Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefoni=

eren

Post by Neil Horman
g=FCnstig

und schnell mit DSL - das All-Inclusive-Paket f=FCr clevere Doppel-=

Sparer,

Post by Neil Horman

nur 39,85 =80 inkl. DSL- und ISDN-Grundgeb=FChr!
http://www.arcor.de/rd/emf-dsl-2
-
To unsubscribe from this list: send the line "unsubscribe linux-net=

" in

Post by Neil Horman

More majordomo info at http://vger.kernel.org/majordomo-info.html

=20
--=20
/***************************************************
*Neil Horman
*Software Engineer
*Red Hat, Inc.
*gpg keyid: 1024D / 0x92A74FA1
*http://pgp.mit.edu
***************************************************/
=20

--=20
Mathias Koehrer
***@arcor.de


Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonieren
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT JETZT bei Arcor: g=FC=
nstig
und schnell mit DSL - das All-Inclusive-Paket f=FCr clevere Doppel-Spar=
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[Christopher Fowler]

Since you have the e1000 code in source I guess it could be possible to
write your own e1000 driver and create a device in /dev that would allo=
w
you to interface with it. You then could do anything you want and
bypass all the Linux networking code.

Create an API to build UDP/IP packets to drop on the wire and program
your real-time app to use that.

Just trying to be creative....

Hi,
=20
thanks for the feedback. I had a closer look at the receive path of a=

n Ethernet frame.

The driver (e1000 in my case) receives the raw frame and calls netif_=

rx.

From netif_rx the sk_buff is passed on to netif_rx_schedule and here=

a raise_soft_irqoff(NET_RX_SOFTIRQ)

is called to trigger the task that does the remaining parts of the re=

ception.

Even if I use a AF_PACKET / AF_RAW socket, it will be handled by the =

NET_RX_SOFTIRQ task.

As this task is not running in real time priority, I loose real time =

behaviour.

=20
Within netif_rx there is a call to netpoll_rx which could be used as =

a kind filter to=20

separate interesting packets. However, currently netpoll_rx is used b=

y the netconsole which allows=20

to register exactly one UDP per device that is taken into account.
I can not handle a real time network. Neither I do have a inteface to=

user space to the netconsole.

One approach could be to implement something similar to netpoll_rx th=

at is used as switch

to separate standard Ethernet frames and frames that are treated as r=

eal time relevant.

In this case something similar to netif_rx_schedule is needed that ca=

lls a real time priority=20

irq thread like NET_RX_SOFTIRQ...
=20
Any comments or feedback on this is highly welcome...
=20
Thanks a lot
=20
Mathias
=20

Hi,
=20
I had a short look at the netconsole.txt and the correspondent co=

de.

It seems to be based on netpoll. This looks quite interesting for=

my

needs.

However, it seems to be only accessible from kernel space and not=

=20

from user space.
I want to run a real time application in user space and do the UD=

P

accesses=20

from there.=20
Is this somehow possible with the netpoll code? I did not see a c=

allable

interface

from user space side.
=20
=20

Raw or Packet sockets then (AF_RAW / AF_PACKET)?
=20
Neil
=20

Thanks for any feedback on this topic.
=20
Regards
=20
Mathias

I would dump the idea of using UDP and inject raw ethernet fram=

es onto

the LAN segment. You can do this by bypassing almost all of the
net-stack I'm sure of it. I think the netconsole kernel code do=

es

it...
=20
Documentation/networking/netconsole.txt
=20

=20
=20
--=20
Mathias Koehrer
=20
=20
Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefo=

nieren

ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT JETZT bei Arcor=

g=C3=BCnstig

und schnell mit DSL - das All-Inclusive-Paket f=C3=BCr clevere Do=

ppel-Sparer,

nur 39,85 =E2=82=AC inkl. DSL- und ISDN-Grundgeb=C3=BChr!
http://www.arcor.de/rd/emf-dsl-2
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et" in

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l

=20
--=20
/***************************************************
*Neil Horman
*Software Engineer
*Red Hat, Inc.
*gpg keyid: 1024D / 0x92A74FA1
*http://pgp.mit.edu
***************************************************/
=20

=20

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[Neil Horman]

Hi,
thanks for the feedback. I had a closer look at the receive path of an Ethernet frame.
The driver (e1000 in my case) receives the raw frame and calls netif_rx.

From netif_rx the sk_buff is passed on to netif_rx_schedule and here a raise_soft_irqoff(NET_RX_SOFTIRQ)

is called to trigger the task that does the remaining parts of the reception.
Even if I use a AF_PACKET / AF_RAW socket, it will be handled by the NET_RX_SOFTIRQ task.
As this task is not running in real time priority, I loose real time behaviour.
Within netif_rx there is a call to netpoll_rx which could be used as a kind filter to
separate interesting packets. However, currently netpoll_rx is used by the netconsole which allows
to register exactly one UDP per device that is taken into account.
I can not handle a real time network. Neither I do have a inteface to user space to the netconsole.
One approach could be to implement something similar to netpoll_rx that is used as switch
to separate standard Ethernet frames and frames that are treated as real time relevant.
In this case something similar to netif_rx_schedule is needed that calls a real time priority
irq thread like NET_RX_SOFTIRQ...
Any comments or feedback on this is highly welcome...
Thanks a lot
Mathias

This is exactly why previous posters were telling you that ethernet was not
really what you were looking for if you want hard real time behavior. Perhaps
you should look into the RtNet project or something simmilar:

http://sourceforge.net/projects/rtnet/


Regards
Neil
--
/***************************************************
*Neil Horman
*Software Engineer
*Red Hat, Inc.
****@tuxdriver.com
*gpg keyid: 1024D / 0x92A74FA1
*http://pgp.mit.edu
***************************************************/
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[Bill Davidsen]

Post by M. Koehrer
Hi everybody,
I have a question concerning Ingo Molnar's realtime preempt patch.
This patch is great go get low latency on timer threads and a really
good real time behaviour.
I want to connect an external, embedded device via Ethernet (UDP/IP)
to my Linux PC. This communication should be in real time as well.

What is your real-time requirement? What's the response time upper
bound? I agree with the people who say this may not be right way to go.

Post by M. Koehrer
For this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network adapters.
How can I get this UDP networking real time capable?
Any feedback is welcome.
Thanks a lot!
Regards
Mathias

[M. Koehrer]

Hi,=20

What is your real-time requirement? What's the response time upper=20
bound? I agree with the people who say this may not be right way to g=

o.
I want to close a control loop. The faster it is the better it is...
Well, to tell concrete numbers, I want to have a response within 300 mi=
croseconds.
The request and the response amount of data is about 250 byte UDP=20
data each (100 MBit full duplex).

Currently, I am using rtnet (http://www.rtnet.org) and the Xenomai real=
time framework
for Linux. This works fine.
The drawback of this approach is that rtnet supports=20
only a limited number of support network adapters. (Special network dri=
vers
are required).
And the preempt patch seems to be closer to standard Linux than a=20
a Xenomai patched kernel.
One major problem is that shared interrupts do not really work smoothly=
with Xenomai...
My goal is to replace the Xenomai/rtnet combination with just the kerne=
l-preempt patch.

=20

Post by M. Koehrer
For this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network ada=

pters.

Post by M. Koehrer
How can I get this UDP networking real time capable?

Regards

Mathias

--=20
Mathias Koehrer
***@arcor.de


Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonieren
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT F=DCR ALLE NEUEINSTEI=
GER
Jetzt bei Arcor: g=FCnstig und schnell mit DSL - das All-Inclusive-Pake=
t
f=FCr clevere Doppel-Sparer, nur 34,95 =80 inkl. DSL- und ISDN-Grundg=
eb=FChr!
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[Neil Horman]

Post by M. Koehrer
Hi,=20
=20

What is your real-time requirement? What's the response time upper=20
bound? I agree with the people who say this may not be right way to=

go.

Post by M. Koehrer
I want to close a control loop. The faster it is the better it is...
Well, to tell concrete numbers, I want to have a response within 300 =

microseconds.

Post by M. Koehrer
The request and the response amount of data is about 250 byte UDP=20
data each (100 MBit full duplex).
=20
Currently, I am using rtnet (http://www.rtnet.org) and the Xenomai re=

al time framework

Post by M. Koehrer
for Linux. This works fine.
The drawback of this approach is that rtnet supports=20
only a limited number of support network adapters. (Special network d=

rivers

Post by M. Koehrer
are required).
And the preempt patch seems to be closer to standard Linux than a=20
a Xenomai patched kernel.
One major problem is that shared interrupts do not really work smooth=

ly with Xenomai...

Post by M. Koehrer
My goal is to replace the Xenomai/rtnet combination with just the ker=

nel-preempt patch.

Post by M. Koehrer
=20

Thats all well and good, but the preempt patch isn't going to provide t=
he cap on
network latency that you're after. I would invest your time in develop=
ing
drivers for those network cards that you want supported under rtnet.


Regards
Neil

Post by M. Koehrer

=20

Post by M. Koehrer
For this I have a separate network adapter in my PC that is used
solely for this UDP Ethernet traffic.
However, I think all Ethernet traffic is be handled by
the soft IRQ threads [softirq-net-rx] [softirq-net-tx] but I do
not see a way to do real time networking via one of the network a=

dapters.

Post by M. Koehrer

Post by M. Koehrer
How can I get this UDP networking real time capable?

Regards
=20
Mathias
=20
--=20
Mathias Koehrer
=20
=20
Viel oder wenig? Schnell oder langsam? Unbegrenzt surfen + telefonier=

en

Post by M. Koehrer
ohne Zeit- und Volumenbegrenzung? DAS TOP ANGEBOT F=DCR ALLE NEUEINST=

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Post by M. Koehrer
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Post by M. Koehrer
f=FCr clevere Doppel-Sparer, nur 34,95 =80 inkl. DSL- und ISDN-Grun=

dgeb=FChr!

Post by M. Koehrer
http://www.arcor.de/rd/emf-dsl-2
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Post by M. Koehrer
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--=20
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