C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  global variables, memory and performance Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message axis Guest Posted: Mon Oct 13, 2003 4:26 am    Post subject: global variables, memory and performance As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? Are there any limits that aren't there when allocating on the stack? In my case, I am making a large openGL application, and am finding that I must keep lots of data in the global scope because the callback function that draws the window doesn't take any parameters. I haven't run into problems up to this point, and I try to keep the memory use at a minimum by allocating only when the data is necessary and deallocating when it's not needed any more, but it would surprise me to find out that Quake 2 held all it's map data in global memory (or maybe it shouldn't surprise me if it's not that big of a deal). Thanks! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Richter Guest Posted: Mon Oct 13, 2003 8:40 pm    Post subject: Re: global variables, memory and performance Hi, Quote: As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? This very much depends on the architecture you're developing on. At least, it is off-topic as far as C++ is concerned. Memory issues: The memory required for the object is reserved before main() is called, in a way that it is of no concern for C++. The lifetime of the objects end as soon as the program ends, i.e. they last *at least* until main() exits. Performance issues: Depends on the platform. On the platforms I know (i386 and co, Motorola 68K), it doesn't make a difference except possibly for caching issues (i.e. placement of data such that it fits well into the processor cache). Typically, the memory for these issues is part of the program binary and gets allocated by the loader of the operating system. Their constructors are called implicitly by the startup code of your binary, and their destructors are called implicitly by the shut-down code. Quote: Are there any limits that aren't there when allocating on the stack? The life time of the objects are different, namely it is global. Whether this is a "limit" depends on your application. So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Tue Oct 14, 2003 4:12 pm    Post subject: Re: global variables, memory and performance "axis" <google (AT) luisma (DOT) com> wrote in message Quote: In my case, I am making a large openGL application, and am finding that I must keep lots of data in the global scope because the callback function that draws the window doesn't take any parameters. I haven't run into problems up to this point, and I try to keep the memory use at a minimum by allocating only when the data is necessary and deallocating when it's not needed any more, but it would surprise me to find out that Quake 2 held all it's map data in global memory (or maybe it shouldn't surprise me if it's not that big of a deal). So you set some global variables, then call a function that takes no arguments, and that function kind of takes arguments in the form of the global variables, right? Will you program work correctly in a multi-threaded environment? Other than that, I can't think of any problem with the use of global variables, but others may have better ideas. -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Jørgensen Guest Posted: Tue Oct 14, 2003 4:30 pm    Post subject: Re: global variables, memory and performance "axis" <google (AT) luisma (DOT) com> wrote Quote: In my case, I am making a large openGL application, and am finding that I must keep lots of data in the global scope because the callback function that draws the window doesn't take any parameters. Just a suggestion. To avoid cluttering the global space, you could keep all your global data in a singleton class, see below. -Michael. ================================================ #include <iostream> class Global { public: Global() { a = 1; } static Global& GetGlobal() { static Global global; return global; } int a; }; int main() { std::cout << "a=" << Global::GetGlobal().a << std::endl; exit(0); } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top spipyeah Guest Posted: Wed Oct 15, 2003 10:41 am    Post subject: Re: global variables, memory and performance Quote: As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? Yes. By using global variables, you have poor data locality. If these variables are simply passed to function calls, it doesn't make any difference I guess. But global variables can always be aliased. They are not local to a function. Therefore, if you refer to them in a function for multiple calculations, they will probably have to be refetched to cache each time they are accessed. In other words, avoid globals if you use them for intensive calculations. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top NJS Guest Posted: Wed Oct 15, 2003 10:42 am    Post subject: Re: global variables, memory and performance Doesn't it make more sense to use namespaces for this? The syntax would certainly be cleaner. I don't see any benefits to using a singleton unless you are using a compiler that doesn't comform well to the standard. - Niek Quote: [snip] Just a suggestion. To avoid cluttering the global space, you could keep all your global data in a singleton class, see below. -Michael. [snip] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Wed Oct 15, 2003 10:46 am    Post subject: Re: global variables, memory and performance "Thomas Richter" <thor (AT) cleopatra (DOT) math.tu-berlin.de> wrote Quote: Memory issues: The memory required for the object is reserved before main() is called, in a way that it is of no concern for C++. The lifetime of the objects end as soon as the program ends, i.e. they last *at least* until main() exits. The OP is using maps, vectors, and such which manage dynamic memory. The sizeof vector includes just the start pointer, size, and capacity (a pointer and two integers, or two pointers and one integer). Map is similar. The memory actually used by the object is reserved at runtime. -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Wed Oct 15, 2003 12:51 pm    Post subject: Re: global variables, memory and performance axis wrote: Quote: As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? Are there any limits that aren't there when allocating on the stack? The primiary issue to worry about is initialization/destruction order dependencies. That is: correctness. For performance, you may want to look at your program's memory access patterns to see if you have locality-of-reference issues to worry about (CPU cache, page faults), and whether using global variables have any material effect on them. Here you have two dimensions to analyze: call graphs and data access patterns, and their interactions. As for limits: there are limits on static storage space, stack size, and heap space, on most platforms. On embedded platforms (such as PalmOS), these limits can be quite stringent. On general purpose platforms, such as Win32/Linux/Unix, you usually get to use the whole virtual address space. The stack is sometimes capped at some static limit that can be modified with a linker option (Win32) and/or operating environment parameters. If your static storage exceeds some limit, usually the linker or compiler will tell you; however, if your heap or stack exceeds the limit, you'll have to discover that at runtime. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Wed Oct 15, 2003 4:06 pm    Post subject: Re: global variables, memory and performance In article <eaf86cc1.0310121317.229101c0 (AT) posting (DOT) google.com>, axis wrote: Quote: As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? Are there any limits that aren't there when allocating on the stack? In my case, I am making a large openGL application, and am finding that I must keep lots of data in the global scope because the callback function that draws the window doesn't take any parameters. snip I assume you're talking about GLUT? Have a look at this C++ wrapper for it: <http://www.duke.edu/~stetten/GlutMaster/GlutMaster.html>. I don't see any licence terms on that, so you can't just go copying that without asking. However, the key idea I saw in it was to get an ID and use that as an index into an array of pointers to an abstract class. There is apparently a function called glutGetWindow() that returns an ID number for the window currently being processed. So with the following declarations: class window { public: virtual /*result*/ foo(/*params*/); virtual /*result*/ bar(/*params*/); // ... }; static const int MAX_WINDOWS = /* don't know */; window * window_table[MAX_WINDOWS]; each of the callback functions can look like this: /*result*/ foo(/*params*/) { return window_table[glutGetWindow()]->foo(/*params*/); } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Thu Oct 16, 2003 2:02 pm    Post subject: Re: global variables, memory and performance In article <882fe461.0310141405.4a498943 (AT) posting (DOT) google.com>, spipyeah <spip_yeah (AT) yahoo (DOT) com> writes Quote: As I understand it, global variables are allocated in a memory area outside of the stack and the heap. I know design-wise, global variables should be avoided when possible. But are there any memory or performance issues to consider when using global variables? Yes. By using global variables, you have poor data locality. If these variables are simply passed to function calls, it doesn't make any difference I guess. But global variables can always be aliased. They are not local to a function. Therefore, if you refer to them in a function for multiple calculations, they will probably have to be refetched to cache each time they are accessed. In other words, avoid globals if you use them for intensive calculations. Actually it is worse. Global variables are very bad news because of aliasing. Any function that has a reference/pointer parameter which could refer to a visible global variable and that uses global variable in its definition must assume that aliasing is possible between the (possibly dereferenced) parameter and the global variable. That completely shoots a wide range of optimisation opportunities. -- Francis Glassborow ACCU If you are not using up-to-date virus protection you should not be reading this. Viruses do not just hurt the infected but the whole community. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Thu Oct 16, 2003 2:37 pm    Post subject: Re: global variables, memory and performance "spipyeah" <spip_yeah (AT) yahoo (DOT) com> wrote in message Quote: Yes. By using global variables, you have poor data locality. If these variables are simply passed to function calls, it doesn't make any difference I guess. But global variables can always be aliased. They are not local to a function. Therefore, if you refer to them in a function for multiple calculations, they will probably have to be refetched to cache each time they are accessed. In other words, avoid globals if you use them for intensive calculations. But the OP is using vectors and maps. Even if you declare these as local or member variables and pass them by const reference, you still have aliasing problems. It's only when you pass by value that you can get caching optimizations due to lack of aliasing. We're unlikely to pass vector by reference because it would be expensive, and also because the vector has a pointer to an array of elements and even if we receive a vector by value the pointer means that data access to these elements can probably not be cached anyway. -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Oct 16, 2003 2:40 pm    Post subject: Re: global variables, memory and performance [email]njs8030 (AT) yahoo (DOT) com[/email] (NJS) wrote in message news:<c57a219f.0310141437.4ec45ba0 (AT) posting (DOT) google.com>... Quote: [snip] Just a suggestion. To avoid cluttering the global space, you could keep all your global data in a singleton class, see below. Doesn't it make more sense to use namespaces for this? The syntax would certainly be cleaner. I don't see any benefits to using a singleton unless you are using a compiler that doesn't comform well to the standard. Controlling order of initialization, perhaps. The one time I did something like this was before namespaces, but even then, the motivation wasn't to avoid namespace pollution, but to control the order of initialization. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Thu Oct 16, 2003 8:59 pm    Post subject: Re: global variables, memory and performance Hi, spipyeah wrote: Quote: But global variables can always be aliased. They are not local to a function. Therefore, if you refer to them in a function for multiple calculations, they will probably have to be refetched to cache each time they are accessed. Why? int i; void fun() { // some intensive calculations using i } There is no reason to refetch "i" every time it is used, IF fun() does not call other functions as part of computations. On the other hand, IF fun() does call other functions, then yes - the compiler cannot assume that the other function does not change the value of "i" and it should ensure that all functions operate on a single value. Note that if fun() does not call other functions during calculations, then the only aliasing can probably result from having multiple threads. Even then, the compiler should be thread-agnostic by its own, because even with refetching it cannot guarantee proper access synchronization. Quote: In other words, avoid globals if you use them for intensive calculations. Depends on the structure of code used for calculations. If the code operates only on global data, then there is no problem, because the locality of reference can be as good as with local data. Design issues aside, of course. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top spipyeah Guest Posted: Fri Oct 17, 2003 11:43 am    Post subject: Re: global variables, memory and performance Quote: Why? int i; void fun() { // some intensive calculations using i } There is no reason to refetch "i" every time it is used, IF fun() does not call other functions as part of computations. On the other hand, IF fun() does call other functions, then yes - the compiler cannot assume that the other function does not change the value of "i" and it should ensure that all functions operate on a single value. Ok, now take: int i; void fun( int *a ) { do { // some stuff i = a * something; } while( go_on ); // some intensive calculations involving both i and (*a) // among which: } a can potentially alias i, so because of the assignement to i, a has to be refetched each iteration. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top spipyeah Guest Posted: Sat Oct 18, 2003 10:08 am    Post subject: Re: global variables, memory and performance Quote: But the OP is using vectors and maps. You're right. I lost the scope of the discussion and talked about globals and aliasing in general. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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