How can I open a single webcam in multiple programs written in openCV simultaneously. Btw I have attached 3 webcams and all are working fine in any single program of openCV, but why two program can't use them both, simultaneously?
Is this a restriction or is there any workaround?
Yes, this is an intention to restrict
Why? The conceptual view is related to the hardware control layer. Operating system assumes, there are some peripherals, that can be used on-demand, but keeps their context-of-use non-share-able.
As an example, one may assume a USB-mouse. While it can be used within several processes, some reasoning told the architects, it would not be the case one mouse should feed events into more than one-( window )-framed -context ( Yes, right, a.k.a. process )
Some other peripherals are even instances of both an EventSENSOR and an EventCOLLECTOR, USB-Cameras for example can receive and process signals from operating system to re-adjust their physical state ( Pan-Tilt-Zoom as an example ).
The more obvious becomes the 1:1 relation assumption, which is something we may sometimes wish not to have hardcoded there. On the other hand, what would the poor device do, if one process instructs it go-left and another go-right at the same time?
Similarly, who would be happy if one USB-mouse will send it's motion and other MMI-interaction events to all currently running processes? At least Drag&Drop UI-navigation policy will become a funny lottery.
Yes, there is a workaround
A simplest "just-enough" scenario includes aCameraControllerPROCESS which keeps the 1:1 relation with USB-device and has the visual data-acquisition responsibility. Here nanoseconds matter, so do not spend any CPU_CLK on anything other but on moving bytes into a buffer. All other processing shall be left on aVisualDataViewConsumerPROCESS where openCV may ( and will ) spend tens and hundreds microseconds on it's own.
Plus it has a communication / service provisioning part, which allows other distributed processes, access the acquired visual data in parallel manner ( in a non-blocking, concurrent view manner, which is a must for distributed soft-real-time processing ).
If one's architecture requires more features or alternatives, this layered approach allows one to add features while retaining both the control and performance overheads within acceptable envelopes.
A good implementation may remain quite smart, fast, low-latency and slim (resources-wise), if one uses ZeroMQ with it's Zero-Copy inproc: virtually abstract transport class, where one does lose an almost Zero-Latency as a cost of the trick, but gains an immense power of having a robust, massively-distribute-able one may scale-out beyond one, two, dozen, thousands, ... you name it ... hosts for free
Related
I stumbled upon PF_RING while reading about PACKET_MMAP kernel documentation (https://www.kernel.org/doc/Documentation/networking/packet_mmap.txt)
Can someone explain the difference between the actual technology (implementation details and differences) between PF_RING and PACKET_RX_RING/PACKET_TX_RING in PACKET_MMAP
PF_RING has two very different modes of operation.
The one called "vanilla" operates "above" driver level, so it should be mostly similar to PACKET_MMAP. They both simply share a buffer between the user application and the network stack. I think PF_RING also discards the packets, so it could be say it's exclusive. PACKET_MMAP, on the contrary, lets the kernel stack process the packets after the copy to userspace.
The "DNA" or "zero-copy" mode implements kernel bypassing. Instead of copying data to a shared ring buffer, the driver's buffers themselves are shared. This, obviously, requires custom drivers and means no other processes will be able to receive traffic from the affected interfaces. Many commonplace cards are supported. Due to this reduced copying and context switches and interrupts (you can do polling if you want to) you can squeeze quite a lot more of performance. The upstream technology that comes the closest is AF_XDP.
I may have gotten some things wrong (I just Googled for a bit out of curiosity and am by no means an expert in PF_RING), so watch out for other answers. I do think most of what I wrote is accurate.
I wondered how memory access is handled "in general" if ,for example, 2 cores of CPU try to access memory at the same time (over the memory controller)? Actually the same applies when a core and an DMA-enabled IO device try to access in the same way.
I think, memory controller is smart enough to utilise the address bus and handle those requests concurrently, however I'm not sure what happens when they try to access to same location or when the IO operation monopolises the address bus and there's no room for CPU to move on.
Thx
The short answer is "it's complex, but access can certainly potentially occur in parallel in certain situations".
I think your question is a bit too black and white: you may be looking for an answer like "yes, multiple devices can access memory at the same time" or "no they can't", but the reality is that first you'd need to describe some specific hardware configuration, including some of the low-level implementation details and optimization features to get an exact answer. Finally you'd need to define exactly what you mean by "the same time".
In general, a good first-order approximation is that hardware will make it appear that all hardware can access memory approximately simultaneously, possibly with an increase in latency and a decrease in bandwidth due to contention. At the very fine-grained timing level access one device may indeed postpone access by another device, or it may not, depending on many factors. It is extremely unlikely you would need this information to implement software correctly, and quite unlikely you need to know the details even to maximize performance.
That said, if you really need to know the details, read on and I can give some general observations on some kind of idealized latpop/desktop/server scale hardware.
As Matthias mentioned, you first have to consider caching. Caching means that any read or write operation subject to caching (which includes nearly all CPU requests and many other types of requests as well) may not touch memory at all, so in that sense many cores can "access" memory (at least the cache image of it) simultaneous.
If you then consider requests that miss in all cache levels, you need to know about the configuration of the memory subsystem. In general a RAM chips can only do "one thing" at a time (i.e., commands1 such a read and write apply to the entire module) and that usually extends to DRAM modules comprised of several chips and also to a series of DRAMs connected via a bus to a single memory controller.
So you can say that electrically speaking, the combination of one memory controller and its attached RAM is likely to be doing only on thing at once. Now that thing is usually something like reading bytes out of a physically contiguous span of bytes, but that operation could actually help handle several requests from different devices at once: even though each devices sends separate requests to the controller, good implementations will coalesce requests to the same or nearby2 area of memory.
Furthermore, even the CPU may have such abilities: when a new request occurs it can/must notice that an existing request is in progress for an overlapping region and tie the new request to an old one.
Still, you can say that for a single memory controller you'll usually be serving the request of one device at a time, absent unusual opportunities to combine requests. Now the requests themselves are typically on the order of nanoseconds, so many separate requests can be served in a small unit of time, so this "exclusiveness" fine-grained and not generally noticeable3.
Now above I was careful to limit the discussion to a single memory-controller - when you have multiple memory controllers4 you can definitely have multiple devices accessing memory simultaneously even at the RAM level. Here each controller is essentially independent, so if the requests from two devices map to different controllers (different NUMA regions) they can proceed in parallel.
That's the long answer.
1 In fact, the command stream is lower level and more complex than things like "read" or "write" and involves concepts such as opening a memory page, streaming bytes from it, etc. What every programmer should know about memory serves as an excellent intro to the topic.
2 For example, imagine two requests for adjacent bytes in memory: it is possible the controller can combine them into a single request if they fit within the bus width.
3 Of course if you are competing for memory across several devices, the overall impact may be very noticeable: a reduction in per-device bandwidth and an increase in latency, but what I mean is that the sharing is fine-grained enough that you can't generally tell the difference between finely-sliced exclusive access and some hypothetical device which makes simultaneous progress on each request in each period.
4 The most common configuration on modern hardware is one memory controller per socket, so on a 2P system you'd usually have two controllers, also other rations (both higher and lower) are certainly possible.
There are dozens of things that come into play. E.g. on the lowest level there are bus arbitration mechanisms which allow that multiple participants can access a shared address and data bus.
On a higher level there are also things like CPU caches that need to be considered: If a CPU reads from memory it might only read from it's local cache, which might not reflect that state that exists in another CPU cores local cache. To synchronize memory between cache instances in multicore systems there exist cache coherence protocols which are are implemented in the CPUs. These have to guarantee that if one CPU writes to shared memory the caches of all other CPUs (which might also contain a copy of the memory locations content) get updated.
I have a Beaglebone running Ubuntu. We want to continuously sample from 3 on-board ATD converters at 100KS/s, and every window of samples we will run a cross correlation DSP algorithm. Once we find a correlation value above a threshold, we will send the value to a PC.
My concern is the process scheduling in Ubuntu. If our process gets swapped out and an ATD sample becomes available during this time, the process will miss the sample. We need to ensure that our process will capture every sample and save it in memory.
With this being said, is there a way to trigger interrupts on the Beaglebone so that if an ATD sample is ready, the sample will be saved in the memory of our program even if the program does not have the processor at the time?
Thanks!
You might be able to trigger the EDMA or use the PRUSS. Probably best to ask on beagleboard#googlegroups.com. There isn't a DSP per-se on the BeagleBone.
This is not exactly an answer to your question, but hopefully it explains how the process works. Since you didn't mention what hardware you are running for AD conversion, maybe this is the best that can be done:
With audio hardware, which faces the same problem, the solution comes from the hardware and the drivers working together: whenever the hardware has filled up enough of the buffer it signals the driver (via an interrupt or some similar mechanism). In some cases, it's also possible that the driver polls the hardware or something like that, but that's a less efficient solution, and I'm not sure anyone does it that way anymore (maybe on cheaper hardware?). From there, the driver process may call right into the end-user process, or it may simply mark the relevant end-user process as "runnable". Either way, control needs to be transferred to the end user process.
For that to happen, the end user process must be running at a higher priority than anything else occupying the CPUs at that moment. To guarantee that your process will always be first in the queue, you can run it at a high priority, with the appropriate permissions, you can even run in very high priorities.
The time it takes for the top priority process to go from runnable to running is sometimes called the "latency" of the OS, though I am sure there's a more specific technical term. The latency of Linux is on the order of 1 ms, but since it's not a "hard" real-time OS, this is not a guarantee. If this is too long to handle your chunks of data, you may have to buffer some of it in your driver.
A faithful implementation of the actor message-passing semantics means that message contents are deep-copied from a logical point-of-view, even for immutable types. Deep-copying of message contents remains a bottleneck for implementations the actor model, so for performance some implementations support zero-copy message passing (although it's still deep-copy from the programmer's point-of-view).
Is zero-copy message-passing implemented at all in Erlang? Between nodes it obviously can't be implemented as such, but what about between processes on the same node? This question is related.
I don't think your assertion is correct at all - deep copying of inter-process messages isn't a bottleneck in Erlang, and with the default VM build/settings, this is exactly what all Erlang systems are doing.
Erlang process heaps are completely separate from each other, and the message queue is located in the process heap, so messages must be copied. This is also true for transferring data into and out of ETS tables as their data is stored in a separate allocation area from process heaps.
There are a number of shared datastructures however. Large binaries (>64 bytes long) are generally allocated in a node-wide area and are reference counted. Erlang processes just store references to these binaries. This means that if you create a large binary and send it to another process, you're only sending the reference.
Sending data between processes is actually worse in terms of allocation size than you might imagine - sharing inside a term isn't preserved during the copy. This means that if you carefully construct a term with sharing to reduce memory consumption, it will expand to its unshared size in the other process. You can see a practical example in the OTP Efficiency Guide.
As Nikolaus Gradwohl pointed out, there was an experimental hybrid heap mode for the VM which did allow term sharing between processes and enabled zero-copy message passing. It hasn't been a particularly promising experiment as I understand it - it requires extra locking and complicates the existing ability of processes to independently garbage collect. So not only is copying inter-process messages not the usual bottleneck in Erlang systems, allowing it actually reduced performance.
AFAIK there was/is experimental support for zero-copy message-passing in erlang using the -shared or -hybrid modell. I read a blog post in 2009 claiming that it's broken on smp machines, but I have no idea about the current status
As has been mentioned here and in other questions current versions of Erlang basically copy everything except for larger binaries. In older pre-SMP times it was feasible to not copy but pass references. While this resulted in very fast message passing it created other problems in the implementation, primarily it made garbage collection more difficult and complicated implementation. I think that today passing references and having shared data could result in excessive locking and synchronisation which is, of course, not a Good Thing.
I wrote the accepted answer to that other question you're referencing, and in it I give you a direct pointer to this line of code:
message = copy_struct(message, msize, &hp, &bp->off_heap);
This is in a function called when the Erlang run-time system needs to send a message, and it's not inside any kind of "if" that could cause it to be skipped. So, as far as I can tell, the answer is "yes, it's always copied." (That's not strictly true -- there is an "if", but it seems to be dealing with exceptional cases, not the normal code-flow path.)
(I'm ignoring the hybrid heap option brought up by Nikolaus. It looks like he's right, but since this isn't the way Erlang is normally built and it has its own penalties, I don't see that it's worth considering as a way to answer your concern.)
I don't know why you're considering 10 GByte/sec a bottleneck, though. Nothing short of registers or CPU cache goes faster in the computer, and such memories are small, thus constituting a kind of bottleneck themselves. Besides which, the zero-copy idea you're proposing would require locking in the case of cross-CPU message passing in a multi-core system, which is also a bottleneck. We're already paying the locking penalty once in this function to copy the message into the other process's message queue; why pay it again later when that process gets around to reading the message?
Bottom line, I don't think your ideas of ways to make it go faster would actually help much.
In looking at Go and Erlang's approach to concurrency, I noticed that they both rely on message passing.
This approach obviously alleviates the need for complex locks because there is no shared state.
However, consider the case of many clients wanting parallel read-only access to a single large data structure in memory -- like a suffix array.
My questions:
Will using shared state be faster and use less memory than message passing, as locks will mostly be unnecessary because the data is read-only, and only needs to exist in a single location?
How would this problem be approached in a message passing context? Would there be a single process with access to the data structure and clients would simply need to sequentially request data from it? Or, if possible, would the data be chunked to create several processes that hold chunks?
Given the architecture of modern CPUs & memory, is there much difference between the two solutions -- i.e., can shared memory be read in parallel by multiple cores -- meaning there is no hardware bottleneck that would otherwise make both implementations roughly perform the same?
One thing to realise is that the Erlang concurrency model does NOT really specify that the data in messages must be copied between processes, it states that sending messages is the only way to communicate and that there is no shared state. As all data is immutable, which is fundamental, then an implementation may very well not copy the data but just send a reference to it. Or may use a combination of both methods. As always, there is no best solution and there are trade-offs to be made when choosing how to do it.
The BEAM uses copying, except for large binaries where it sends a reference.
Yes, shared state could be faster in this case. But only if you can forgo the locks, and this is only doable if it's absolutely read-only. if it's 'mostly read-only' then you need a lock (unless you manage to write lock-free structures, be warned that they're even trickier than locks), and then you'd be hard-pressed to make it perform as fast as a good message-passing architecture.
Yes, you could write a 'server process' to share it. With really lightweight processes, it's no more heavy than writing a small API to access the data. Think like an object (in OOP sense) that 'owns' the data. Splitting the data in chunks to enhance parallelism (called 'sharding' in DB circles) helps in big cases (or if the data is on slow storage).
Even if NUMA is getting mainstream, you still have more and more cores per NUMA cell. And a big difference is that a message can be passed between just two cores, while a lock has to be flushed from cache on ALL cores, limiting it to the inter-cell bus latency (even slower than RAM access). If anything, shared-state/locks is getting more and more unfeasible.
in short.... get used to message passing and server processes, it's all the rage.
Edit: revisiting this answer, I want to add about a phrase found on Go's documentation:
share memory by communicating, don't communicate by sharing memory.
the idea is: when you have a block of memory shared between threads, the typical way to avoid concurrent access is to use a lock to arbitrate. The Go style is to pass a message with the reference, a thread only accesses the memory when receiving the message. It relies on some measure of programmer discipline; but results in very clean-looking code that can be easily proofread, so it's relatively easy to debug.
the advantage is that you don't have to copy big blocks of data on every message, and don't have to effectively flush down caches as on some lock implementations. It's still somewhat early to say if the style leads to higher performance designs or not. (specially since current Go runtime is somewhat naive on thread scheduling)
In Erlang, all values are immutable - so there's no need to copy a message when it's sent between processes, as it cannot be modified anyway.
In Go, message passing is by convention - there's nothing to prevent you sending someone a pointer over a channel, then modifying the data pointed to, only convention, so once again there's no need to copy the message.
Most modern processors use variants of the MESI protocol. Because of the shared state, Passing read-only data between different threads is very cheap. Modified shared data is very expensive though, because all other caches that store this cache line must invalidate it.
So if you have read-only data, it is very cheap to share it between threads instead of copying with messages. If you have read-mostly data, it can be expensive to share between threads, partly because of the need to synchronize access, and partly because writes destroy the cache friendly behavior of the shared data.
Immutable data structures can be beneficial here. Instead of changing the actual data structure, you simply make a new one that shares most of the old data, but with the things changed that you need changed. Sharing a single version of it is cheap, since all the data is immutable, but you can still update to a new version efficiently.
What is a large data structure?
One persons large is another persons small.
Last week I talked to two people - one person was making embedded devices he used the word
"large" - I asked him what it meant - he say over 256 KBytes - later in the same week a
guy was talking about media distribution - he used the word "large" I asked him what he
meant - he thought for a bit and said "won't fit on one machine" say 20-100 TBytes
In Erlang terms "large" could mean "won't fit into RAM" - so with 4 GBytes of RAM
data structures > 100 MBytes might be considered large - copying a 500 MBytes data structure
might be a problem. Copying small data structures (say < 10 MBytes) is never a problem in Erlang.
Really large data structures (i.e. ones that won't fit on one machine) have to be
copied and "striped" over several machines.
So I guess you have the following:
Small data structures are no problem - since they are small data processing times are
fast, copying is fast and so on (just because they are small)
Big data structures are a problem - because they don't fit on one machine - so copying is essential.
Note that your questions are technically non-sensical because message passing can use shared state so I shall assume that you mean message passing with deep copying to avoid shared state (as Erlang currently does).
Will using shared state be faster and use less memory than message passing, as locks will mostly be unnecessary because the data is read-only, and only needs to exist in a single location?
Using shared state will be a lot faster.
How would this problem be approached in a message passing context? Would there be a single process with access to the data structure and clients would simply need to sequentially request data from it? Or, if possible, would the data be chunked to create several processes that hold chunks?
Either approach can be used.
Given the architecture of modern CPUs & memory, is there much difference between the two solutions -- i.e., can shared memory be read in parallel by multiple cores -- meaning there is no hardware bottleneck that would otherwise make both implementations roughly perform the same?
Copying is cache unfriendly and, therefore, destroys scalability on multicores because it worsens contention for the shared resource that is main memory.
Ultimately, Erlang-style message passing is designed for concurrent programming whereas your questions about throughput performance are really aimed at parallel programming. These are two quite different subjects and the overlap between them is tiny in practice. Specifically, latency is typically just as important as throughput in the context of concurrent programming and Erlang-style message passing is a great way to achieve desirable latency profiles (i.e. consistently low latencies). The problem with shared memory then is not so much synchronization among readers and writers but low-latency memory management.
One solution that has not been presented here is master-slave replication. If you have a large data-structure, you can replicate changes to it out to all slaves that perform the update on their copy.
This is especially interesting if one wants to scale to several machines that don't even have the possibility to share memory without very artificial setups (mmap of a block device that read/write from a remote computer's memory?)
A variant of it is to have a transaction manager that one ask nicely to update the replicated data structure, and it will make sure that it serves one and only update-request concurrently. This is more of the mnesia model for master-master replication of mnesia table-data, which qualify as "large data structure".
The problem at the moment is indeed that the locking and cache-line coherency might be as expensive as copying a simpler data structure (e.g. a few hundred bytes).
Most of the time a clever written new multi-threaded algorithm that tries to eliminate most of the locking will always be faster - and a lot faster with modern lock-free data structures. Especially when you have well designed cache systems like Sun's Niagara chip level multi-threading.
If your system/problem is not easily broken down into a few and simple data accesses then you have a problem. And not all problems can be solved by message passing. This is why there are still some Itanium based super computers sold because they have terabyte of shared RAM and up to 128 CPU's working on the same shared memory. They are an order of magnitude more expensive then a mainstream x86 cluster with the same CPU power but you don't need to break down your data.
Another reason not mentioned so far is that programs can become much easier to write and maintain when you use multi-threading. Message passing and the shared nothing approach makes it even more maintainable.
As an example, Erlang was never designed to make things faster but instead use a large number of threads to structure complex data and event flows.
I guess this was one of the main points in the design. In the web world of google you usually don't care about performance - as long as it can run in parallel in the cloud. And with message passing you ideally can just add more computers without changing the source code.
Usually message passing languages (this is especially easy in erlang, since it has immutable variables) optimise away the actual data copying between the processes (of course local processes only: you'll want to think your network distribution pattern wisely), so this isn't much an issue.
The other concurrent paradigm is STM, software transactional memory. Clojure's ref's are getting a lot of attention. Tim Bray has a good series exploring erlang and clojure's concurrent mechanisms
http://www.tbray.org/ongoing/When/200x/2009/09/27/Concur-dot-next
http://www.tbray.org/ongoing/When/200x/2009/12/01/Clojure-Theses