[USRP-users] rx_samples_to_file issue

Marcus Müller marcus.mueller at ettus.com
Sat Oct 4 09:14:25 EDT 2014


Hi Peter,

didn't mean to confuse you! Actually, my job is doing the opposite (ie.
providing useful information), and thus let me just shortly follow up on
this:
On 03.10.2014 17:44, Peter Witkowski via USRP-users wrote:
> So I'm confused.
>
> You state that if I can't use rx_samples_to_file, my system is failing to
> perform as specified to write data out, then you give an example of several
> things that can happen to create a stochastic write speed (which I totally
> understand and agree with).  Given that writes can be stochastic, why is
> there not a software buffer implemented in the UHD sample code to account
> for such issues?  
Well, because that's, in my opinion, an operating system's job. Being a
code example, rx_samples_to_file just *musn't* contain the complexity
introduced when you try to implement buffering functionality smarter
than what your OS can do. And, I do think it's nearly impossible to be
smarter than the linux kernel when optimizing writes -- *but* you'll
have to tell your kernel what you want, as a user. The kernel, as it is
configured by any modern distribution by default, won't do enormous
write buffers, because that's not what the user usually wants,
increasing the risk of data loss in case of system failure, and because
you usually don't want to spend all of your RAM on filesystem buffers.
In your 64GB RAM case, though, default buffer sizes should suffice, I
guess, so I'm a bit out of clues here.
It is definitely not very hard to increase these buffers' sizes[1], so I
encourage you to try it and see if that solves your problem. Now, I must
admit that up to here I was always assuming you hadn't already played
around with these values, if this is not the case, please accept my
apologies!

> I understand that it's meant to be an example, but I've
> also seen it referenced as being used effectively as a debugger or test for
> people having issues (i.e. recommendation to use the UHD programs in place
> of GNURadio to resolve issues).
...and it's done many users and thus Ettus a great job of supplying
basic functionality! The fact that it works in almost any situation with
this very minimalistic approach (repeated recv->write) proves that UHD
is in fact a rather nice driver interface, IMHO. The fact that GNU Radio
sometimes solves issues that rx_samples_to_file can't indicates exactly
the buffering approach to be helpful. But in that case, buffering is not
increased by increasing kernel buffer sizes, but by introducing GNU
Radio buffers between blocks. The USRP source (Martin, scold me if I say
something stupid) is not really much smarter than rx_samples_to_file: It
recv()s a packet of samples, and returns these samples from the work
function, and then GNU Radio takes care of shuffling and buffering that
data. Basically, GNU Radio behaves much like an operating system from
the source block's point of view.
>
> Also, in terms of benchmarking, I'm quoting minimum values, not averages.
> I agree with you that average values are pointless, and in reality the disk
> subsystem needs to perform when called up.  My minimum values for a 4 disk
> RAID0 with a dedicated controller are well within the data rate that I am
> pushing.
Well, I'll kind of disagree with you: If your minimum write rate of your
system was bigger than the rate rx_samples_to_file causes, then you
wouldn't see the problem. The point, I believe, here is that the storage
system does not only consist of the hardware side of your RAID, but also
on your complete operating environment. Something slows down how fast
data is written to the RAID.
I think we both would expect the following to happen:

repeatedly:

rx_samples_to_file:
uhd::rx_streamer::recv
    (blocks until a packet of samples has arrived. Instantly returns if
it has before the call)
write(file_handle, recv_buff)
    (instantly returns, because writing should hit a buffer that the
operating system transparently pushes out to a disk. If buffer is full,
then block until enough space in buffer -- unless your filesystem is
mounted with some sync option...)

Now, if your RAID is definitely fast enough, the write buffer should
never get full. My hypothesis here is that either, your buffer size is
just to small, and a block of samples doesn't fit and has to be written
out instantly (which is unlikely), or something else occupies your
system. That might be just the fact that 400MB/s (are we talking about
an X3x0?) inevitably places a heavy load on things like PCIe busses and
CPUs, and that introduces a bottleneck in your storage chain which isn't
there if you "just" benchmark without the USRP. Also, the rather
smallish sizes of network packets dictate that journalling file systems
introduce a very bad overhead -- I don't know if you benchmarked with
files on a journaling file system and a (network packet size - header)
block size...
>
> Is there an example system that can handle sustained data capture from the
> USRP at (or near the limits) of 10GigE or the PCIe interfaces (maybe the
> requirement is enterprise class PCIe SSDs)?  I'm running a two socket Xenon
> system (two hex core processors) with 64GB of RAM.  How much more hardware
> should I throw at the problem to be able to sample/write at 100MS (half of
> what is quoted on the website for bandwidth for the 10GigE kit) using the
> provided code?
Definitely a nice system! I must admit that I don't have access to a
comparable setup, and thus I can't really offer you any first-hand
experience. Maybe others can.
> I think the issue here is that the code itself can't simply get through
> it's main loop fast enough.  There's a difference between data bandwidth
> and CPU throughput.  The sequential nature of the code means that if any
> weird stuff happens (your example was a good set of kernel related hilarity
> that can lead to stochastic timing) you will have overflows since you
> cannot read fast enough.  This is why a 90% solution for my application was
> to just set the dirty_background_ratio to 0 and also why redirection to
> /dev/null makes overflows go away.
This is interesting, as dirty_background_ratio is the percentage at
which the kernel should start writing out dirty pages in the background.
Now, I'm the one who's confused, because I would have expected this to
negatively impact performance. On the other hand, 0 (at least in my
head) does not make very much sense, maybe it's semantically identical
to 100%? Are you swapping (64GB would tell me you shouldn't have swap or
extremly low swappiness)?
On the other hand, it might really be that storage is not the bottleneck
here, and in fact maybe the CPU gets saturated. Now, you said that
writing to /dev/null solves your problem. Do your RAID or filesystem
consume a lot of CPU cycles? This is an interesting mystery...
>   With either method I didn't have to
> wait for a large write cache to flush before moving on to the next read
> from the USRP.  Note that there can also be things that happen on the read
> side as well.  Does this mean that I can only run the code on an RTOS?
No :) UHD has it's own incoming buffer handlers, but as you already
said, in this high performance scenario, you might be totally right, and
our single-threaded approach just doesn't cut it. Maybe dropping in some
asynchronous storage IO would help -- but I hate seeing that blowing up
in example users' faces, so I guess the fact that it doesn't work with a
system as potent as yours with the sample rates as high as you demand
might actually be a shortcoming of the examples that isn't going to be
fixed.
> As a final note, my understanding is that GNURadio and the USRP were
> developed for domain experts in DSP to use.
These are SDR frameworks and devices, respectively. The idea is to offer
people with the opportunity to build awesome DSP systems using
universally usable SDR blocks (GNU Radio) and universal software radio
peripherals, so well, they certainly address DSP people, but they
shouldn't be hard to use.
>  These users may or may not
> have prior experience in software.  As a result, I'd recommend perhaps
> adding a buffered example or have the USRP GNURadio block allow for
> buffering.
That is something we might consider. On the other hand, when someone
goes as far as you do, maybe having an example that does the buffering
in a separate thread (or even process) isn't worth that much -- in the
end, one will want to write one's own high performance application, and
that will include handling such data rates.
> Otherwise, I just don't see how you can advertise 200 MS/s
> (maybe even a simple "buffer" block in GNURadio would do the trick?).
Well, the devices support these rates, and our driver is able to
withstand these rates and sustain them without hitting CPU barriers due
to having too much overhead. That's awesome (ok, I might be biased, but
*I* think it's awesome). I don't feel ashamed because on your specific
setup, we can't find a way to make any of our generic examples deliver
the full rate of rx streams to storage -- we sell RF hardware, and not
storage infrastructure, and the point of the examples is demonstrating
the usage of UHD, and not holding a lecture on high performance storage
handling. I wish, though, that we could solve your problem.

Now, GNU Radio/gr-uhd does in fact come with an application called
uhd_rx_cfile, which is more or less a clone of rx_samples_to_file using
gr-uhd and GNU Radio instead of raw UHD. Does that work out for you?

> I
> understand that this is theoretical limit of the bus, but if there doesn't
> exist a driver or other software to make use of this, the practical limit
> becomes much, much smaller.
Well, UHD seems to be able to sustain these rates, if you write to
/dev/null, right? So the practical limit for UHD is definitely not being
hit.
I have another --maybe even practical-- suggestion to make: Roll your
own buffer!

mkfifo /tmp/mybuffer #assuming tmpfs is in ram
dd if=/tmp/mybuffer of=/mount/raid_volume/data.dat & #start in
background; you could play around with block sizes using the bs= option
of dd
rx_samples_to_file --file /tmp/mybuffer [all the other options]

By the way: Thanks for bringing this up! We know that recording samples
is a core concern of many users.

Greetings,
Marcus

[1] https://www.kernel.org/doc/Documentation/sysctl/vm.txt

>
> On Fri, Oct 3, 2014 at 10:55 AM, Marcus Müller <usrp-users at lists.ettus.com>
> wrote:
>
>>  I have to agree with Marcus on this. Also, keep in mind that storage is
>> really what an operating system should take care of in any "general
>> purpose" scenario, ie. that as long as I just write to a file, I'd expect
>> that the thing in charge of storage (my kernel / the filesystems / block
>> device drivers) does the best it can to keep up. If I find myself in a
>> situation where my specific storage needs dictate a huge write buffer,
>> changing the application might be one way, but as I'm responsible for my
>> won storage subsystem, I could just as well increase the cache buffer
>> sizes, and let the operating system handle storage operation. If your RAID
>> is really performing as well as it is benchmarked to, then this should not
>> be one of your problems. All rx_samples_to_file does is really sequentially
>> writing out data at a constant rate, which is the most basic write
>> benchmark I can think of.
>>
>> If your storage subsystem (filesystem + storage abstraction + raid driver
>> + interface driver + hard drive interface + hard drives + hardware caches)
>> can't keep up, it's failing to perform as specified, simple as that. In
>> this case, saying that the application needs to be smarter when dealing
>> with storage seems like a bit of a cop-out to me ;)
>>
>> I'd like to point out that most benchmarks use heavily averaged numbers
>> for write speeds etc. UHD on the other hand kind of demands soft real-time
>> performance of a write subsystem, which is a lot harder to fulfill. This
>> comes up rather frequently, but I have to stress it: you need a fast
>> guaranteed write rate, not only an average one, and as soon as your
>> operating system has to postpone writing data[1], it has to have enough
>> performance to catch up whilst still meeting continued demand. This is
>> general purpose hardware running general purpose OS with dozens of
>> processes, and you can't just say "every single component is up to my task,
>> thus my system suffices", because everything potentially blocks everything!
>>
>> Greetings,
>> Marcus
>>
>> [1] e.g. because the filesystems needs to calculate checksums, update
>> tables, another process gets scheduled, a device blocks your PCIe bus, your
>> platters randomly need a bit longer to seek, you reach the physical end of
>> an LVM volume and have to move across a disk, an interrupt does what an
>> interrupt does, some process is getting noticed on a changing file
>> descriptor, DBUS is happening in the kernel, token ring has run out of
>> tokens, thermal throttling, bitflips on SATA leading to retransmission,
>> some page getting fetched from swap...
>>
>>
>> On 03.10.2014 15:34, Marcus D. Leech via USRP-users wrote:
>>
>>
>>
>> One has to keep firmly in mind that programs like rx_samples_to_file are
>> *examples* that show how to use
>>
>>  the underlying UHD API. They are not necessarily optimized for all
>> situations, and indeed, one could
>>
>>  restructure rx_samples_to_file to decouple UHD I/O from filesystem I/O,
>> using a large buffer between them.
>>
>> The fact is that dynamic performance of high-speed, real-time, flows is
>> something that almost-invariably needs
>>
>>  tweaking for any particular situation. There's no way for an example
>> application to meet all those requirements.
>>
>> But the fact also remains that for *some* systems, rx_samples_to_file
>> (and uhd_rx_cfile on the Gnu Radio side)
>>
>>  are able to stream high-speed data just fine as-is.
>>
>> On 2014-10-03 09:26, Peter Witkowski via USRP-users wrote:
>>
>>
>>  To say that the issue is just because the disk subsystem can't keep up is a bit of cop-out.
>>
>> I had issues writing to disk when the incoming stream was 400MB/s and my RAID0 system was benchmarked at being much higher than that.
>>
>> The issue that I've been seeing stems from the fact that it appears that you cannot concurrently read/write from the data stream as its coming in. In effect you have a main loop that reads from the device and then immediately tries to write that buffer to file. If you do not complete these operations in a timely fashion overflows occur.
>>
>> One way to solve (or at least band aid the issue) is to set your dirty_background_ratio to 0. I was able to get writing to disk working somewhat with this setting as it is more predictable to directly write to disk instead of having your write cache fill up and then having a large amount of data to push to disk. That said, my RAID0 array is capable of such speeds and even then I was getting a few (but much reduced) overflows.
>>
>> The one surefire way I know of getting this working (even on a slow disk system) is to buffer the data. The buffer can then be consumed by the disk writing process while being concurrently added onto by the device reader. The easiest way to test buffering (that I've found) is to simply set up a GNURadio Companion program with a stream-to-vector block between the USRP and file sink blocks. This is exactly what I am doing currently since even with a very powerful system, I could not get data saved to disk quickly enough given the aforementioned issues with the provided UHD software.
>>
>> On Thu, Oct 2, 2014 at 11:48 PM, gsmandvoip via USRP-users <usrp-users at lists.ettus.com> <usrp-users at lists.ettus.com> wrote:
>>
>> Thanks Marcus for your replies. Yes O gone away.
>>
>> On Thu, Oct 2, 2014 at 5:50 PM, Marcus D. Leech <mleech at ripnet.com> <mleech at ripnet.com> wrote:
>>
>> with rx_samples_to_file without _4rx.rbf, Initially I tried on my i3, 4GB ram, it gave me
>> some OOOO but was lesser than earlier, but I do not understand, my most of the ram capacity and processor was sitting idle while it shows OOOO, why is this strange behaviour The default format for uhd_rx_cfile is complex-float, thus doubling the amount of data written compared to rx_samples_to_file.
>>
>> You can't just use CPU usage as an indicator of loading--if you're writing to disk, the disk subsystem may be much slower than you think, so the
>> "rate limiting step" is writes to the disk, not computational elements.
>>
>> Try using /dev/null as the file that you write to. If the 'O' go away, even at higher sampling rates, then it's your disk subsystem.
>>
>> using uhd_rx_cfile getting similar result, but strangely, why it is low, at 4M sampling rate it was higher???
>>
>> On Thu, Oct 2, 2014 at 9:27 AM, Marcus D. Leech <mleech at ripnet.com> <mleech at ripnet.com> wrote:
>>
>> On 10/01/2014 11:46 PM, gsmandvoip wrote:
>>
>> Yes I am running single channel, but when trying to achieve my desired sampling rate without _4rx.rbf, it says, requested sampling rate is not valid, adjusting to some 3.9M or so. sorry for misleading info I gave earlier, I have i3, with 32 bit and i7 with 64 bit, but getting same result on both machines
>>
>> Here is my command to capture signal:
>>
>> ./rx_samples_to_file --args="fpga=usrp1_fpga_4rx.rbf, subdev=DBSRX" --freq "$FC" --rate="$SR" $FILE --nsamps "$NSAMPLES"
>>
>> and here is its output:
>>
>> Creating the usrp device with: fpga=usrp1_fpga_4rx.rbf, subdev=DBSRX...
>> -- Loading firmware image: /usr/share/uhd/images/usrp1_fw.ihx... done
>> -- Opening a USRP1 device...
>> -- Loading FPGA image: /usr/share/uhd/images/usrp1_
>> fpga_4rx.rbf... done
>> -- Using FPGA clock rate of 52.000000MHz...
>> ERROR: LOOKUPERROR: INDEXERROR: MULTI_USRP::GET_TX_SUBDEV_SPEC(0) FAILED TO MAKE DEFAULT SPEC - VALUEERROR: THE SUBDEVICE SPECIFICATION "A:0" IS TOO LONG.
>> The user specified 1 channels, but there are only 0 tx dsps on mboard 0.
>>
>> Don't use the _4rx image if you don't need it.
>>
>> The USRP1 only does strict-integer resampling, and with a master clock (NON STANDARD FOR USRP1) of 52.000MHz, 4Msps is not a sample rate
>> that it can produce. Try 5.2Msps or 4.3333Msps.
>>
>> At 5.2Msps, it's recording at roughly 20.8Mbytes/second, so your system needs to be able to sustain that for at least as long as the capture lasts.
>>
>>
>>
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