I would like to delete an ovm object (and its children) so that I can recreate it with different configs. Is there a way to do this in OVM?
Currently, when I try to create the object a second time with new, I get the following VCS runtime error:
[CLDEXT] Cannot set 'ap' as a child of 'instance', which already has a child by that name.
I realize that I can simply use a different name to "re-create" the instance, but then I'll still have the old instance sitting around and soaking up memory.
OVM is just a SystemVerilog library. That means that all the rules of SystemVerilog apply to OVM. So, yes, you can use new() with OVM. Sometimes it's preferable to use the factory, and sometimes it's preferable to use new() (that's a topic for a different discussion).
SystemVerilog does not have a delete operator or a destructor like C++. Instead, when you are done with an object you just remove all references to it and the garbage collector will clean up the memory. Here's a quote from the SystemVerilog reference manual (IEEE 1800-2009) section 8.7:
SystemVerilog does not require the complex memory allocation and deallocation of C++. Construction of an object is straightforward; and garbage collection, as in Java, is implicit and automatic. There can be no memory leaks or other subtle behaviors, which are so often the bane of C++ programmers.
It's not entirely true that you cannot have a memory leak. You can forget to remove all references to an object and the garbage collector will not know to pick it up. However, you do not have to worry about memory with the same detail as you do in C++.
The particular error you received with id CLDEXT is from ovm_component class. From the message it appears that you attempted to create two components with the same name and the same parent. Components in OVM are typically static. That is, you create and elaborate them once, usually at time 0, and don't delete or add components after that. Because of this model there are no methods in ovm_component to remove child components. So there really isn't a good way to replace a component once it has been instantiated. By the way, this only applies to components. Other types of objects can be re-allocated.
If you feel that you need to replace a component with a different one after time 0 you should re-think the architecture of your testbench. There are probably betters ways to accomplish what you are trying to do without replacing components.
I have only UVM experience but I think OVM is similar. I would have liked to reply to #Victor Lyuboslavsky's comment but I can't add comments.
The issue is with the name 'ap' which evidently has already been used for a child of 'instance'. Use this code instead.
static int instNum = 0;
instance_ap = my_ovm_extended_class::type_id::create
($sformatf ("ap%0d", instNum), this);
The first time an object is created & the handle assigned to 'instance_ap', the object would have the name 'instance.ap0'. The next time the code executes an object called 'instance.ap1', and so on.
As mentioned by other posters this ought to be done only for non-component objects, and components should be static and must be created during/before the build phase & connected to each other during/before the connect phase.
Try assigning null to the object before calling new again.
Unless I see someone else answer this question, I'd say there is no easy way to deallocate objects in OVM framework.
OVM testbenches are static and created when the testbench is created.
When the environment class is instantiated, it will call new(create), build, connect, end_of_elaboration, start_of_simulation, run and check on all components.
By the end of the environment build phase all components must be created.
By the end of the environment connect phase all components must have their TLM ports connected.
Because of these requirements, you can not change components (or port connections) except for during the phase.
As part of the static nature of the testbench environment, every component must have a unique get_full_name() response. This is because string lookups are used to identify components in the hierarchy.
Assigning an object to null should deallocate memory. If there is no other handle pointing to that memory location, then it should get reclaimed.
Related
Problem
Example use case:
I have a control which displays a status gauge. The visual status of the gauge is bound to a property of the control
The control is part of a topology graph. So depending on the topology e. g. a 100 of these controls may be displayed at once
There are several topologies. Every time you switch to another topology view the whole graph is regenerated
Question
Could this cause a memory leak and do you have to perform a manual unbind in the old topology view before you create the new one? Similar to the bindings, do you have to remove event handlers manually?
The bindings and the event handlers are inside the control. So once the control isn't accessible anymore it should be possible that it's garbage collected. So I think you don't have to do anything, but I don't know.
Thank you very much for the expertise!
If you look into the JavaDocs:
[...] All bindings in our implementation use instances of WeakInvalidationListener, which means usually a binding does not need to be disposed. But if you plan to use your application in environments that do not support WeakReferences you have to dispose unused Bindings to avoid memory leaks.
So if you use or extend the default Bindings the Garbage Collector should be able to do its work.
If you do not, be sure do implement and call Binding.dispose().
As always: If an object is no longer referenced by any other object it gets garbage collected (at some point in the future). So usually one does not need to specifically implement in this direction, as it tends to clutter the code.
I'd like to add two extra fields of type StgWord32 to the thread state object (TSO). Based on the information I found on the GHC-Wiki and from looking at the source code, I have extended the struct in /includes/rts/storage/TSO.h and changed the program that creates different offsets (creating DerivedConstants.h). The compiler, the rts, and a simple application re-compile, but at the end of the execution (in hs_exit_) the garbage collector complains:
internal error: scavenge_stack: weird activation record found on stack: 45
I guess it has to to with cmm and/or the STG implementation details (the offsets are generated since the structs are not visible at cmm level, correct me if I'm wrong). Is the order of fields significant? Have I missed a file that should be changed?
I use a debug build of the compiler and RTS and a rather dated ghc 6.12.3 on a 64bit architecture. Any hints to relevant documentation and comments
on the difference between ghc 6 and 7 regarding TSO handling are welcome, too.
The error that you are getting comes from: ghc/rts/sm/Scav.c. Specifically at line 1917:
default:
barf("scavenge_stack: weird activation record found on stack: %d", (int)(info->i.type));
It looks like you need to also modify ClosureTypes.h, which you can find in ghc/includes/rts/storage. This file seems to contain the different kinds of headers that can appear in a heap object. I've also run into some strange bootstrapping errors, where if I try to rebuild using the stage-1 compiler, I get the error you mentioned, but if I do a clean build, then it compiles just fine.
A workaround that turned out good enough for me was to introduce a separate data structure for each Capability that would hold the additional information for each lightweight thread. I have used a HashTable (see rts/Hash.h and .c) mapping from thread id to the custom info struct. The entries were added when the threads were created from sparks (in schduleActiveteSpark).
Timing the creation, insertion, lookup and destruction of the entries and the table showed negligible overhead for small programs. The main overhead results from the actual usage of the information and should ideally be kept outside of the innermost scheduler loop. For the THREADED_RTS build one needs to ensure that other Capabilities don't access tables that are not their own (or use a mutex if such access is required, which is potential source of additional overhead).
I'm using [self retain] to hold an object itself, and [self release] to free it elsewhere. This is very convenient sometimes. But this is actually a reference-loop, or dead-lock, which most garbage-collection systems target to solve. I wonder if objective-c's autorelease pool may find the loops and give me surprises by release the object before reaching [self release]. Is my way encouraged or not? How can I ensure that the garbage-collection, if there, won't be too smart?
This way of working is very discouraged. It looks like you need some pointers on memory management.
Theoretically, an object should live as long as it is useful. Useful objects can easily be spotted: they are directly referenced somewhere on a thread stack, or, if you made a graph of all your objects, reachable through some path linked to an object referenced somewhere on a thread stack. Objects that live "by themselves", without being referenced, cannot be useful, since no thread can reach to them to make them perform something.
This is how a garbage collector works: it traverses your object graph and collects every unreferenced object. Mind you, Objective-C is not always garbage-collected, so some rules had to be established. These are the memory management guidelines for Cocoa.
In short, it is based over the concept of 'ownership'. When you look at the reference count of an object, you immediately know how many other objects depend on it. If an object has a reference count of 3, it means that three other objects need it to work properly (and thus own it). Every time you keep a reference to an object (except in rare conditions), you should call its retain method. And before you drop the reference, you should call its release method.
There are some other importants rule regarding the creation of objects. When you call alloc, copy or mutableCopy, the object you get already has a refcount of 1. In this case, it means the calling code is responsible for releasing the object once it's not required. This can be problematic when you return references to objects: once you return it, in theory, you don't need it anymore, but if you call release on it, it'll be destroyed right away! This is where NSAutoreleasePool objects come in. By calling autorelease on an object, you give up ownership on it (as if you called release), except that the reference is not immediately revoked: instead, it is transferred to the NSAutoreleasePool, that will release it once it receives the release message itself. (Whenever some of your code is called back by the Cocoa framework, you can be assured that an autorelease pool already exists.)
It also means that you do not own objects if you did not call alloc, copy or mutableCopy on them; in other words, if you obtain a reference to such an object otherwise, you don't need to call release on it. If you need to keep around such an object, as usual, call retain on it, and then release when you're done.
Now, if we try to apply this logic to your use case, it stands out as odd. An object cannot logically own itself, as it would mean that it can exist, standalone in memory, without being referenced by a thread. Obviously, if you have the occasion to call release on yourself, it means that one of your methods is being executed; therefore, there's gotta be a reference around for you, so you shouldn't need to retain yourself in the first place. I can't really say with the few details you've given, but you probably need to look into NSAutoreleasePool objects.
If you're using the retain/release memory model, it shouldn't be a problem. Nothing will go looking for your [self retain] and subvert it. That may not be the case, however, if you ever switch over to using garbage collection, where -retain and -release are no-ops.
Here's another thread on SO on the same topic.
I'd reiterate the answer that includes the phrase "overwhelming sense of ickyness." It's not illegal, but it feels like a poor plan unless there's a pretty strong reason. If nothing else, it seems sneaky, and that's never good in code. Do heed the warning in that thread to use -autorelease instead of -release.
I think this might be a fairly easy question.
I found a lot of examples using threads and shared variables but in no example a shared variable was created inside a thread. I want to make sure I don't do something that seems to work and will break some time in the future.
The reason I need this is I have a shared hash that maps keys to array refs. Those refs are created/filled by one thread and read/modified by another (proper synchronization is assumed). In order to store those array refs I have to make them shared too. Otherwise I get the error Invalid value for shared scalar.
Following is an example:
my %hash :shared;
my $t1 = threads->create(
sub { my #ar :shared = (1,2,3); $hash{foo} = \#ar });
$t1->join;
my $t2 = threads->create(
sub { print Dumper(\%hash) });
$t2->join;
This works as expected: The second thread sees the changes the first made. But does this really hold under all circumstances?
Some clarifications (regarding Ian's answer):
I have one thread A reading from a pipe and waiting for input. If there is any, thread A will write this input in a shared hash (it maps scalars to hashes... those are the hashes that need to be declared shared as well) and continues to listen on the pipe. Another thread B gets notified (via cond_wait/cond_signal) when there is something to do, works on the stuff in the shared hash and deletes the appropriate entries upon completion. Meanwhile A can add new stuff to the hash.
So regarding Ian's question
[...] Hence most people create all their shared variables before starting any sub-threads.
Therefore even if shared variables can be created in a thread, how useful would it be?
The shared hash is a dynamically growing and shrinking data structure that represents scheduled work that hasn't yet been worked on. Therefore it makes no sense to create the complete data structure at the start of the program.
Also the program has to be in (at least) two threads because reading from the pipe blocks of course. Furthermore I don't see any way to make this happen without sharing variables.
The reason for a shared variable is to share. Therefore it is likely that you will wish to have more than one thread access the variable.
If you create your shared variable in a sub-thread, how will you stop other threads accessing it before it has been created? Hence most people create all their shared variables before starting any sub-threads.
Therefore even if shared variables can be created in a thread, how useful would it be?
(PS, I don’t know if there is anything in perl that prevents shared variables being created in a thread.)
PS A good design will lead to very few (if any) shared variables
This task seems like a good fit for the core module Thread::Queue. You would create the queue before starting your threads, push items on with the reader, and pop them off with the processing thread. You can use the blocking dequeue method to have the processing thread wait for input, avoiding the need for signals.
I don't feel good answering my own question but I think the answers so far don't really answer it. If something better comes along, I'd be happy to accept that. Eric's answer helped though.
I now think there is no problem with sharing variables inside threads. The reasoning is: Threads::Queue's enqueue() method shares anthing it enqueues. It does so with shared_clone. Since enqueuing should be good from any thread, sharing should too.
I have written a managed class that wraps around an unmanaged C++ object, but I found that - when using it in C# - the GC kicks in early while I'm executing a method on the object. I have read up on garbage collection and how to prevent it from happening early. One way is to use a "using" statement to control when the object is disposed, but this puts the responsibility on the client of the managed object. I could add to the managed class:
MyManagedObject::MyMethod()
{
System::Runtime::InteropServices::GCHandle handle =
System::Runtime::InteropServices::GCHandle::Alloc(this);
// access unmanaged member
handle.Free();
}
This appears to work. Being new to .NET, how do other people deal with this problem?
Thank you,
Johan
You might like to take a look at this article: http://www.codeproject.com/Tips/246372/Premature-NET-garbage-collection-or-Dude-wheres-my. I believe it describes your situation exactly. In short, the remedies are either ausing block or a GC.KeepAlive. However, I agree that in many cases you will not wish to pass this burden onto the client of the unmanaged object; in this case, a call to GC.KeepAlive(this) at the end of every wrapper method is a good solution.
You can use GC.KeepAlive(this) in your method's body if you want to keep the finalizer from being called. As others noted correctly in the comments, if your this reference is not live during the method call, it is possible for the finalizer to be called and for memory to be reclaimed during the call.
See http://blogs.microsoft.co.il/blogs/sasha/archive/2008/07/28/finalizer-vs-application-a-race-condition-from-hell.aspx for a detailed case study.