please be aware that digital receivers aren't really easy to make more CPU efficient; a real-world receiver not only has to convert received symbols to bits, but has to do the frequency and frame synchronization as well as a timing recovery; high-bandwidth transceivers must go through significant amounts of equalization (OFDM is one way to approach this, converting one giant channel into a lot of narrow, hence, nearly flat, channels that can be single-tap equalized, at the expense of doing FFTs and inserting pilots/redundancy), too. The Benchmark_* tools aren't really doing anything to equalize (if I remember correctly), so they aren't really applicable to wideband channels, and if you do equalization, you'll probably be even slower.
As said on numerous occassions, go ahead and try the rx_ofdm.grc and tx_ofdm.grc examples from gr-digital. They are much more modern.
If I had a diagnosis, still, your PC is probably simply too slow for receivers that aren't highly optimized; GNU Radio really does its best with efficient filter, FFT, synchronizer structures, but it might not be able to compensate that at 2.7GHz x 4 cores, you only have 432 CPU cycles per sample at 25MS/s, if your application was perfectly scalable (which it's not). That's not really much for a complicated receiver, if you consider that includes all the data handling (ie. getting data from the network card, converting it to something your CPU can deal with, writing the results somewhere), filtering, synchronization, channelizing (if doing *FDM), symbol decision, de-framing, channel decoding (no one would build a real-world transceiver without that), and as said, benchmark_rx (from the gr-digital/examples/ofdm directory, we're not talking about the narrowband one) isn't that optimally structured – it's really just a proof of concept.
Best regards,
Marcus
On 07.04.2016 13:39, SangHyuk Kim
wrote:
Dear Marcus,
I'm always appreciated your help.
I did ./benchmark_rate --rx_rate 25e6 and it worked well.
Then, I should make it simple.
Very thanks you!
2016-04-07 18:49 GMT+09:00 Marcus Müller <address@hidden>:
Dear SangHyuk,
On 07.04.2016 08:25, SangHyuk Kim wrote:
No, that is the maximum number of 16bit complex samples you can get over Gigabit ethernet; I think I've done the math before, but here it goes again :)I am using USRP N210 and CBX daughter board. As I know, my machine's maximum sample rate is up to 25 MSps.Hi all,I'm trying to solve Rx overflow problem.
and because the USRP can only give you integer fractions of its master clock rate 100MHz as sampling rate, 25MS/s is the maximum you get.
Yes, it works well at Tx (ex ./benchmark -f 2.5G -r 25000000).However, when I use USRP to Rx mode (ex ./benchmark -f 2.5G -r 25000000), letter 'D' (overflow) be occurred.
... and that to a degree that the network stack, not UHD, decides to drop packets (not to "O"verflow). That is a very bad sign.Rx overflow 'D' is happened when host cannot consume packet fast enough.
Which will pose a significant additional load on your CPU, something that will influence your measurement.I observed incoming packet from USRP to host using wireshark tool.
Of course.After Rx command be launched (ex ./benchmark -f 2.5G -r 25000000), USRP sends many packet to host very fast.
Why? How is that surprising? More than two cores under full load sounds like a reasonable load here.
So, I checked cpu utilization at those moments. As a result, cpu utilization is over 200% (PC has 2.7GHz quad-cores). I couldn't believe it.
Well, then these values simply don't help; honestly, your PC is overwhelmed, and that not only by the interrupt load.
However, I found "interrupt coalescing".
Incoming packet occurs interrupt to host and interrupt coalescing adjust how long/many packets make one interrupt to PC.
So, I changed these value using ethtool -C eth0 rx-usecs 1000 rx-frames 200.
But, it doesn't any effect for my case.
Does UHD's
I'm using tg3 network driver and my setting like below:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Link partner advertised link modes: 1000baseT/Half 1000baseT/Full
Link partner advertised pause frame use: Transmit-only
Link partner advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on
Supports Wake-on: g
Wake-on: g
Current message level: 0x000000ff (255)
drv probe link timer ifdown ifup rx_err tx_err
Link detected: yes
Did I miss something ?
How can I solve Rx overflow problem at high sample rate ?
benchmark_rate --rx_rate 25e6
work? If that's the case, the receiver flowgraph is simply too difficult for your PC to handle in real time. Nothing you can really do about that but get a faster PC, or design a less complex receiver.
Best regards,
Marcus
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![$\frac{\SI{1}{\giga\bit\per\second}}{\SI{16}{\bit\per
Integer}\text{[I]} + \SI{16}{\bit\per
Integer}\text{[Q]}} =
\frac{\SI{1e9}{\bit\per\second}}{\SI{32}{\bit\per
S}}=\SI{31.25}{\mega S\per\second}$](pngktT3Y3dTZ2.png)