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| It is unfortunate that a lot of people don't understand the UNIX memory model. Unfortunately this is helped by tools like ps which are not able to provide accurate information about memory usage. In UNIX a process uses basically three kinds of memory segments: '''shared memory segments''', '''code segments''' and '''data segments'''.
| | {{Moved To Community | Guidelines_and_HOWTOs/Debugging }} |
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| '''Shared memory''' is used by shared libraries. This memory is shared
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| by all processes which use a certain library. Unfortunately there is no
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| easy way to determine how much shared memory is used by how many processes.
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| So a process can use 10Mb of shared memory, but you don't know whether this
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| memory is shared with 1, 2 or 10 processes. So if you have 10 processes who
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| each use 10Mb of shared memory this actually requires 10Mb in the best case
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| and 100Mb in the worst case.
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| '''Code segments''' contain the actual executable code of your program.
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| This memory is shared by all processes of this same program. If you start
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| your program 5 times, it needs to load the code segment of your program
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| only once.
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| '''Data segments''' contain the data of your program. This kind of memory
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| is very important because the data segments of a process are not shared
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| with other processes. Starting the same program 5 times makes that the data
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| segments are 5 times in memory.
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| The size reported by [http://man-wiki.net/index.php/Ps ps auxf] is typically just the numbers for shared, code and
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| data added. This is not a very accurate representation of the memory usage
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| of an application.
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| KDE applications tend to be reported as quite large because the numbers
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| reported include the size of the shared memory segments. This size is
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| added to the size of each KDE application while in practice the shared
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| memory segments appear in memory only once. This is rather illusive,
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| imagine how the output of ps would look like if it included the size of
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| the UNIX kernel for each process!
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| Instead of looking at the output of ps you get a better idea of the actual
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| memory usage of an application by looking at the output of
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| cat /proc/<pid-of-process>/status.
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| == Example program ==
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| To demonstrate this, let's write a memory leaking program: | |
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| '''main.cpp'''
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| <code cppqt>
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| #include <KAboutData>
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| #include <KApplication>
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| #include <KCmdLineArgs>
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| #include <KMessageBox>
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| int main (int argc, char *argv[])
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| {
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| KAboutData aboutData( "tutorial1", 0, ki18n("Tutorial 1"), "1.0",
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| ki18n("Displays a KMessageBox popup") );
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|
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| KCmdLineArgs::init( argc, argv, &aboutData );
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| KApplication app;
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| for ( int i=0; i<100000; i++ ) new QString();
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| KMessageBox::questionYesNo( 0, i18n( "Hello World" ) );
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| int* i;
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| return 0;
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| }
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| </code>
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| '''CMakeLists.txt'''
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| <code>
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| project (tutorial1)
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| find_package(KDE4 REQUIRED)
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| include (KDE4Defaults)
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| include_directories(${KDE4_INCLUDES})
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| set(tutorial1_SRCS main.cpp)
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| kde4_add_executable(tutorial1 ${tutorial1_SRCS})
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| target_link_libraries(tutorial1 ${KDE4_KDEUI_LIBS})
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| install(TARGETS tutorial1 ${INSTALL_TARGETS_DEFAULT_ARGS})
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| </code>
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| Compile and linkthis program:
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| cmake . && make -j4
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| Run it:
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| ./tutorial1 &
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| [3] 22733
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| In this case the program gets the process ID 22733. We look at its memory consumption:
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| cat /proc/22733/status
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| VmRSS: 19772 kB
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| VmData: 5776 kB
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| VmStk: 84 kB
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| VmExe: 8 kB
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| VmLib: 26804 kB
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| If we change the "100000" in the program code to "1", we get a different picture:
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| VmRSS: 16624 kB
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| VmData: 2652 kB
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| VmStk: 84 kB
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| VmExe: 8 kB
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| VmLib: 26804 kB
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| So we see the heap is counted to "VmData" and contained in VmRSS.
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| == Further Information ==
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| More information about this topic can be found in the article about [http://ktown.kde.org/~seli/memory/analysis.html memory usage analysis].
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