Hello,
I'd like to describe what I'm trying to do, so people can comment.
Basically I'm just transfering data between two systems over a network
(possibly the Internet, but not necessarily).
(Systems A and B are synchronized by NTP.)
System A receives data from an input device on the PCI bus. This input
device provides data at constant bit rate R, meaning if the driver
requests B bytes, the DMA will take approximately B/R seconds.
I wrote a 'send' (user space) app on system A which requests 1316 bytes
from the input device, adds an RTP header, and sends this data in a UDP
datagram to system B. The RTP header provides, notably, a sequence
number and a timestamp.
The datagrams travel over several networks, a few get lost, some arrive
at system B, not necessarily in the order they were sent, and definitely
not every B/R seconds.
I wrote a 'recv' (user space) app on system B whose job is to receive
the packets from system A and re-send them (either to an output device
on the PCI bus, or to another system on the LAN) after having removed
the network jitter.
Here's my implementation:
Note: Packet P{i} contains a timestamp T{i}
buffer N packets
grab the current (absolute, CLOCK_REALTIME) time H{0}
send P0
for i=1 to infinity
{
recv() a few packets (e.g. 3 at most)
compute H{i} = H{i-1} + (T{i} - T{i-1})
sleep until H{i}
send packet P_i
}
The packets were timestamped on system A,
thus for every i, T{i} - T{i-1} = ~B/R
Therefore, most of the network jitter will be removed.
Any comments so far?
Now all this seems nice in theory, but I'd also like to make it work in
practice! (And I'm having some problems.)
R is typically 38 Mbit/s => B/R is typically 277 µs
Obviously, I'm going to need high-resolution timers if I want to sleep
for so small an interval.
As far as I understand, each packet received will generate one IRQ, and
each write to the PCI output device will generate one IRQ. This means
3600 IRQs per second from the NIC, and 3600 IRQs per second from the PCI
device. Is that reasonable? (Considering a 1267 MHz P3 with no IO-APIC.)
It is easy to reduce the number of IRQs from the PCI device by grouping
several packets for a single write. It might be worthwhile.
One problem I have is that the PCI device's driver blocks until the
device has acknowledged the data, and the write operation sometimes
blocks for 200, 300, even 400 µs (I have not been able to tell why).
I might not need -rt, if I'm willing to handle several packets every
time I wake up?
buffer N packets
grab the current (absolute) time H{0}
send P0
for i=1 to infinity
{
sleep for at least X ms
recv() some packets (how many? as much as I can?)
grab the current time Hnow
while H{i} < Hnow send P{i}
}
(Rough pseudo-code.)
I'm eager to hear anyone's comments and suggestions.
Regards.
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