Development/Tutorials/Debugging/Shared Memory Usage in KDE: Difference between revisions

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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 }}
     
    = Terms =
    '''Shared memory''' is used by shared libraries. This memory is shared
    by all processes which use a certain library. Unfortunately there is no
    easy way to determine how much shared memory is used by how many processes.
    So a process can use 10Mb of shared memory, but you don't know whether this
    memory is shared with 1, 2 or 10 processes. So if you have 10 processes who
    each use 10Mb of shared memory this actually requires 10Mb in the best case
    and 100Mb in the worst case.
     
    '''Code segments''' contain the actual executable code of your program.
    This memory is shared by all processes of this same program. If you start
    your program 5 times, it needs to load the code segment of your program
    only once.
     
    '''Data segments''' contain the data of your program. This kind of memory
    is very important because the data segments of a process are not shared
    with other processes. Starting the same program 5 times makes that the data
    segments are 5 times in memory.
     
    The size reported by [http://man-wiki.net/index.php/Ps ps auxf] is typically just the numbers for shared, code and
    data added. This is not a very accurate representation of the memory usage
    of an application.
     
    KDE applications tend to be reported as quite large because the numbers
    reported include the size of the shared memory segments. This size is
    added to the size of each KDE application while in practice the shared
    memory segments appear in memory only once. This is rather illusive,
    imagine how the output of ps would look like if it included the size of
    the UNIX kernel for each process!
     
    Instead of looking at the output of ps you get a better idea of the actual
    memory usage of an application by looking at the output of
    cat /proc/<pid-of-process>/status.
     
    = Example program =
    To demonstrate this, let's write a memory leaking program:
     
    '''main.cpp'''
    <syntaxhighlight lang="cpp-qt">
    #include <KAboutData>
    #include <KApplication>
    #include <KCmdLineArgs>
    #include <KMessageBox>
     
    int main (int argc, char *argv[])
    {
        KAboutData aboutData( "tutorial1", 0, ki18n("Tutorial 1"), "1.0",
                              ki18n("Displays a KMessageBox popup") );
        KCmdLineArgs::init( argc, argv, &aboutData );
        KApplication app;
     
        for ( int i=0; i<100000; i++ ) new QString();
     
        KMessageBox::questionYesNo( 0, i18n( "Hello World" ) );
        int* i;
        return 0;
    }
    </syntaxhighlight>
    '''CMakeLists.txt'''
    <pre>
    project (tutorial1)
    find_package(KDE4 REQUIRED)
    include (KDE4Defaults)
    include_directories(${KDE4_INCLUDES})
    set(tutorial1_SRCS main.cpp)
    kde4_add_executable(tutorial1 ${tutorial1_SRCS})
    target_link_libraries(tutorial1 ${KDE4_KDEUI_LIBS})
    install(TARGETS tutorial1  ${INSTALL_TARGETS_DEFAULT_ARGS})
    </pre>
    Compile and link this program:
    <pre>
    cmake . && make -j4
    </pre>
    Run it:
    <pre>
    ./tutorial1 &
    [3] 22733
    </pre>
    In this case the program gets the process ID 22733. We look at its memory consumption:
    <pre>
    cat /proc/22733/status
    VmRSS:    19772 kB
    VmData:    5776 kB
    VmStk:        84 kB
    VmExe:        8 kB
    VmLib:    26804 kB
    </pre>
    If we change the "100000" in the program code to "1", we get a different picture:
    <pre>
    VmRSS:    16624 kB
    VmData:    2652 kB
    VmStk:        84 kB
    VmExe:        8 kB
    VmLib:    26804 kB
    </pre>
    So we see the heap is counted to <tt>VmData</tt> and contained in <tt>VmRSS</tt>.
     
    == Why is it so big ==
    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:
    # ldd tutorial1
            linux-vdso.so.1 =>  (0x00007fff739f3000)
            libkdeui.so.5 => /usr/local/lib64/libkdeui.so.5 (0x00007fdb6448c000)
            libkdecore.so.5 => /usr/local/lib64/libkdecore.so.5 (0x00007fdb63f31000)
            libQtDBus.so.4 => /usr/local/lib64/libQtDBus.so.4 (0x00007fdb63cb5000)
            libQtCore.so.4 => /usr/local/lib64/libQtCore.so.4 (0x00007fdb6380b000)
    [...]
    We gonna remove the KDE and Qt stuff, so let's write a new main.cpp:
    int main (int argc, char *argv[])
    {
        for ( int i=0; i<100000; i++ ) new int;
        while (true);
        return 0;
    }
    And compile and link it with the C++ libraries:
    # g++ -o tutorial1 main.cpp
    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?
    # ldd tutorial1
            linux-vdso.so.1 =>  (0x00007fffd3bff000)
            libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fe2437a5000)
            libm.so.6 => /lib64/libm.so.6 (0x00007fe24354e000)
            libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fe243338000)
            libc.so.6 => /lib64/libc.so.6 (0x00007fe242fcb000)
            /lib64/ld-linux-x86-64.so.2 (0x00007fe243aae000)
    that's all.
     
    Note that this program runs until you terminate it with CTRL_C as we are not using KDE's messagebox any more.
    # cat /proc/21028/status | grep VmLib
    VmLib:      2912 kB
    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.
     
    == disassemble it ==
    Now let's disassemble the small program using the command
    # objdump -d tutorial1
    [...]
    00000000004005b4 <main>:
      4005b4:      55                      push  %rbp
      4005b5:      48 89 e5                mov    %rsp,%rbp
      4005b8:      48 83 ec 20            sub    $0x20,%rsp
      4005bc:      89 7d ec                mov    %edi,-0x14(%rbp)
      4005bf:      48 89 75 e0            mov    %rsi,-0x20(%rbp)
      4005c3:      c7 45 fc 00 00 00 00    movl  $0x0,-0x4(%rbp)
      4005ca:      eb 0e                  jmp    4005da <main+0x26>
      4005cc:      bf 04 00 00 00          mov    $0x4,%edi
      4005d1:      e8 ea fe ff ff          callq  4004c0 <_Znwm@plt>
      4005d6:      83 45 fc 01            addl  $0x1,-0x4(%rbp)
      4005da:      81 7d fc 9f 86 01 00    cmpl  $0x1869f,-0x4(%rbp)
      4005e1:      0f 9e c0                setle  %al
      4005e4:      84 c0                  test  %al,%al
      4005e6:      75 e4                  jne    4005cc <main+0x18>
      4005e8:      eb fe                  jmp    4005e8 <main+0x34>
      4005ea:      90                      nop
      4005eb:      90                      nop
      4005ec:      90                      nop
      4005ed:      90                      nop
      4005ee:      90                      nop
      4005ef:      90                      nop
    [...]
    You see this is the main function in real [http://www.staerk.de/thorsten/Tutorials/Assembler_Tutorial assembler code].
     
    == find out its symbols ==
    Let's find out what symbols it contains:
    <pre>
    # nm tutorial1
    0000000000600e10 d _DYNAMIC
    0000000000600fe8 d _GLOBAL_OFFSET_TABLE_
    00000000004006e0 R _IO_stdin_used
                    w _Jv_RegisterClasses
                    U _Znwm@@GLIBCXX_3.4
    0000000000600df0 d __CTOR_END__
    0000000000600de8 d __CTOR_LIST__
    0000000000600e00 D __DTOR_END__
    0000000000600df8 d __DTOR_LIST__
    00000000004007a8 r __FRAME_END__
    0000000000600e08 d __JCR_END__
    0000000000600e08 d __JCR_LIST__
    0000000000601020 A __bss_start
    0000000000601010 D __data_start
    0000000000400690 t __do_global_ctors_aux
    0000000000400520 t __do_global_dtors_aux
    0000000000601018 D __dso_handle
                    w __gmon_start__
    0000000000600de4 d __init_array_end
    0000000000600de4 d __init_array_start
    0000000000400680 T __libc_csu_fini
    00000000004005f0 T __libc_csu_init
                    U __libc_start_main@@GLIBC_2.2.5
    0000000000601020 A _edata
    0000000000601030 A _end
    00000000004006c8 T _fini
    0000000000400488 T _init
    00000000004006d8 t _real_fini
    00000000004004d0 T _start
    00000000004004fc t call_gmon_start
    0000000000601020 b completed.5939
    0000000000601010 W data_start
    0000000000601028 b dtor_idx.5941
    0000000000400590 t frame_dummy
    00000000004005b4 T main
    </pre>
    Now according to [http://man-wiki.net/index.php/Nm nm's man page] T stands for text which stands for code segment, and D stands for the data segment.
     
    = See also =
    * http://www-archive.mozilla.org/projects/footprint/footprint-guide.html

    Latest revision as of 09:00, 5 August 2016

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