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| This article helps understanding UNIX memory management. 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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| = Terms =
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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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| <syntaxhighlight lang="cpp-qt">
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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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| </syntaxhighlight>
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| '''CMakeLists.txt'''
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| <pre>
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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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| </pre>
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| Compile and link this program:
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| <pre>
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| cmake . && make -j4
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| </pre>
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| Run it:
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| <pre>
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| ./tutorial1 &
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| [3] 22733
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| </pre>
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| In this case the program gets the process ID 22733. We look at its memory consumption:
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| <pre>
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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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| </pre>
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| If we change the "100000" in the program code to "1", we get a different picture:
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| <pre>
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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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| </pre>
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| So we see the heap is counted to <tt>VmData</tt> and contained in <tt>VmRSS</tt>.
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| == Why is it so big ==
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| Probably VmLib is so big because it contains all library code in memory needed for KDE. Let's see what the loader thinks are our program's dependencies:
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| # ldd tutorial1
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| linux-vdso.so.1 => (0x00007fff739f3000)
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| libkdeui.so.5 => /usr/local/lib64/libkdeui.so.5 (0x00007fdb6448c000)
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| libkdecore.so.5 => /usr/local/lib64/libkdecore.so.5 (0x00007fdb63f31000)
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| libQtDBus.so.4 => /usr/local/lib64/libQtDBus.so.4 (0x00007fdb63cb5000)
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| libQtCore.so.4 => /usr/local/lib64/libQtCore.so.4 (0x00007fdb6380b000)
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| [...]
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| We gonna remove the KDE and Qt stuff, so let's write a new main.cpp:
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| int main (int argc, char *argv[])
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| {
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| for ( int i=0; i<100000; i++ ) new int;
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| while (true);
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| return 0;
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| }
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| And compile and link it with the C++ libraries:
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| # g++ -o tutorial1 main.cpp
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| Why do we need the C++ libraries (g++ is basically gcc -lstdc++)? Because we have a call to the new keyword in the program. What are the dependencies now?
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| # ldd tutorial1
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| linux-vdso.so.1 => (0x00007fffd3bff000)
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| libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fe2437a5000)
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| libm.so.6 => /lib64/libm.so.6 (0x00007fe24354e000)
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| libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fe243338000)
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| libc.so.6 => /lib64/libc.so.6 (0x00007fe242fcb000)
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| /lib64/ld-linux-x86-64.so.2 (0x00007fe243aae000)
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| that's all.
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| Note that this program runs until you terminate it with CTRL_C as we are not using KDE's messagebox any more.
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| # cat /proc/21028/status | grep VmLib
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| VmLib: 2912 kB
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| You see - not using libraries save place in memory, but as libraries are shared, it does not make sense to deny using libraries that are in memory anyway.
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| == disassemble it ==
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| Now let's disassemble the small program using the command
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| # objdump -d tutorial1
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| [...]
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| 00000000004005b4 <main>:
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| 4005b4: 55 push %rbp
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| 4005b5: 48 89 e5 mov %rsp,%rbp
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| 4005b8: 48 83 ec 20 sub $0x20,%rsp
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| 4005bc: 89 7d ec mov %edi,-0x14(%rbp)
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| 4005bf: 48 89 75 e0 mov %rsi,-0x20(%rbp)
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| 4005c3: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
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| 4005ca: eb 0e jmp 4005da <main+0x26>
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| 4005cc: bf 04 00 00 00 mov $0x4,%edi
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| 4005d1: e8 ea fe ff ff callq 4004c0 <_Znwm@plt>
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| 4005d6: 83 45 fc 01 addl $0x1,-0x4(%rbp)
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| 4005da: 81 7d fc 9f 86 01 00 cmpl $0x1869f,-0x4(%rbp)
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| 4005e1: 0f 9e c0 setle %al
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| 4005e4: 84 c0 test %al,%al
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| 4005e6: 75 e4 jne 4005cc <main+0x18>
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| 4005e8: eb fe jmp 4005e8 <main+0x34>
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| 4005ea: 90 nop
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| 4005eb: 90 nop
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| 4005ec: 90 nop
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| 4005ed: 90 nop
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| 4005ee: 90 nop
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| 4005ef: 90 nop
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| [...]
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| You see this is the main function in real [http://www.staerk.de/thorsten/Tutorials/Assembler_Tutorial assembler code].
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| = See also =
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| * http://www-archive.mozilla.org/projects/footprint/footprint-guide.html
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