I've been using the "System Load Viewer" widget with an update interval of 500ms (showing all cores), and noticed it causes the plasma-desktop process to have a constant ~2% cpu, whereas without this widget it's usually at 0, and with a larger interval such as 3000ms it's at ~0.7% cpu.

strace shows that the plasma-desktop process is wasting most of its time in the stat system call on the file /etc/localtime. A bit more digging shows the stack trace pasted below.

It looks like the data container's timeSinceLastUpdate calls QTime::elapsed, which internally does a timezone lookup that does the stat, which is a whole lot of overhead for measuring an elapsed time (which is done often).

A quick search shows that QT has a QElapsedTime class as a quick replacement for QTime's elapsed functionality, which uses a monotonic clock rather than the system time, so it doesn't deal with this timezone overhead, and should be much more efficient. As an added bonus, it doesn't reset every 24 hours like QTime::elapsed (which can potentially cause subtle bugs), is immune to timezone changes and system time changes, etc. (see its docs) so it should be much safer and more appropriate in measuring an elapsed time between data engine polls. It sounds like it would be a better choice, and probably fix this performance hog.

There may be other performance issues, but this is the first one that stood out.


Here is the stack trace:

#0  0x00007f3bc5490035 in __GI___xstat (vers=<optimized out>, name=0x7f3bc55230da "/etc/localtime", buf=0x7fff721e5e50)
    at ../sysdeps/unix/sysv/linux/wordsize-64/xstat.c:35
        resultvar = 18446744073709551578
#1  0x00007f3bc5457b5d in __tzfile_read (file=file@entry=0x7f3bc55230da "/etc/localtime", extra=extra@entry=0, 
    extrap=extrap@entry=0x0) at tzfile.c:161
        default_tzdir = "/usr/share/zoneinfo"
        num_isstd = <optimized out>
        num_isgmt = <optimized out>
        f = <optimized out>
        tzhead = {tzh_magic = "`_\036r", tzh_version = "\377", 
          tzh_reserved = "\177\000\000\360uB\305;\177\000\000\030\000\000", tzh_ttisgmtcnt = "\000\000\000", 
          tzh_ttisstdcnt = "\r\000\000", tzh_leapcnt = "\000\000\000", tzh_timecnt = "\r\000\000", 
          tzh_typecnt = "\000\000\000", tzh_charcnt = "&\271\367\301"}
        chars = <optimized out>
        i = <optimized out>
        total_size = <optimized out>
        types_idx = <optimized out>
        leaps_idx = <optimized out>
        was_using_tzfile = 1
        trans_width = 4
        tzspec_len = <optimized out>
        new = 0x0
        st = {st_dev = 140735107981184, st_ino = 99187242, st_nlink = 140735107981296, st_mode = 3254258449, st_uid = 32571, 
          st_gid = 99187248, __pad0 = 0, st_rdev = 139893110894019, st_size = 140735107980944, st_blksize = 4311194416, 
          st_blocks = 68633392, st_atim = {tv_sec = 140735107981296, tv_nsec = 140733193388032}, st_mtim = {
            tv_sec = 139894692656992, tv_nsec = 15}, st_ctim = {tv_sec = 20152577, tv_nsec = 139894684633696}, 
          __glibc_reserved = {6, 0, 139894689265136}}
        __PRETTY_FUNCTION__ = "__tzfile_read"
#2  0x00007f3bc5456eb4 in tzset_internal (always=always@entry=1, explicit=explicit@entry=1) at tzset.c:447
        is_initialized = 1
        tz = 0x7f3bc55230da "/etc/localtime"
#3  0x00007f3bc54577b0 in __tzset () at tzset.c:600
No locals.
#4  0x00007f3bc1f46910 in QTime::currentTime () at tools/qdatetime.cpp:3116
        tv = {tv_sec = 1403364091, tv_usec = 769201}
        ltime = 1403364091
        ct = {mds = -1}
        t = 0x0
        res = {tm_sec = 99187216, tm_min = 0, tm_hour = -1039948579, tm_mday = 32571, tm_mon = 112568816, tm_year = 0, 
          tm_wday = 1919, tm_yday = 37, tm_isdst = 61719552, tm_gmtoff = 133755200, tm_zone = 0x7fff721e5f8f ""}
#5  0x00007f3bc1f469dd in QTime::elapsed (this=0x21d28e0) at tools/qdatetime.cpp:2005
        n = -38
#6  0x00007f3bc4fb92ed in Plasma::DataContainer::timeSinceLastUpdate (this=this@entry=0x1b740e0)
    at ../../plasma/datacontainer.cpp:320
No locals.
#7  0x00007f3bc4fbae8f in Plasma::DataEnginePrivate::internalUpdateSource (this=0x21d3080, source=0x1b740e0)
    at ../../plasma/dataengine.cpp:570
No locals.
#8  0x00007f3bc4fbc9c0 in Plasma::DataEngine::qt_static_metacall (_o=0x7f3bc55230da, _id=1914592848, _a=0x7fff721e6170, 
    _c=<optimized out>) at ./dataengine.moc:107
No locals.
#9  0x00007f3bc204e87a in QMetaObject::activate (sender=0x1b740e0, 
    m=m@entry=0x7f3bc538e980 <Plasma::DataContainer::staticMetaObject>, local_signal_index=local_signal_index@entry=2, 
    argv=argv@entry=0x7fff721e6170) at kernel/qobject.cpp:3539
        receiverInSameThread = true
        previousSender = 0x0
        callFunction = 0x7f3bc4fbca60 <Plasma::DataEngine::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>
        receiver = 0x219b850
        currentSender = {sender = 0x1b740e0, signal = 6, ref = 1}
---Type <return> to continue, or q <return> to quit---
        method_relative = 6
        c = 0x21d2b50
        last = 0x21d2b50
        methodOffset = <optimized out>
        signal_absolute_index = 6
        empty_argv = {0x0}
        locker = {val = 20687936}
        connectionLists = 0x21d2ba0
        list = 0x21d2c20
        signalOffset = <optimized out>
        signal_index = <optimized out>
        currentThreadId = 139894697121728
#10 0x00007f3bc4fb93c2 in Plasma::DataContainer::updateRequested (this=<optimized out>, _t1=0x1b740e0)
    at ./datacontainer.moc:139
        _a = {0x0, 0x7fff721e6168}
#11 0x00007f3bc4feeb40 in Plasma::SignalRelay::timerEvent (this=0x21222c0, event=<optimized out>)
    at ../../plasma/private/datacontainer_p.cpp:154
        event = <optimized out>
        this = 0x21222c0
#12 0x00007f3bc2052a31 in QObject::event (this=0x21222c0, e=<optimized out>) at kernel/qobject.cpp:1156
No locals.
#13 0x00007f3bc13d3e2c in QApplicationPrivate::notify_helper (this=this@entry=0x1338130, receiver=receiver@entry=0x21222c0, 
    e=e@entry=0x7fff721e6540) at kernel/qapplication.cpp:4567
        consumed = <optimized out>
#14 0x00007f3bc13da4a0 in QApplication::notify (this=this@entry=0x13101b0, receiver=receiver@entry=0x21222c0, 
    e=e@entry=0x7fff721e6540) at kernel/qapplication.cpp:4353
        w = 0x21222c0
        extra = <optimized out>
        isProxyWidget = <optimized out>
        d = 0x1338130
        res = false
#15 0x00007f3bc2cffd1a in KApplication::notify (this=0x13101b0, receiver=0x21222c0, event=0x7fff721e6540)
    at ../../kdeui/kernel/kapplication.cpp:311
No locals.
#16 0x00007f3bc203a4dd in QCoreApplication::notifyInternal (this=0x13101b0, receiver=0x21222c0, event=0x7fff721e6540)
    at kernel/qcoreapplication.cpp:953
        threadData = 0x126b1f0
        returnValue = <optimized out>
        result = false
        cbdata = {0x21222c0, 0x7fff721e6540, 0x7fff721e64af}
        d = <optimized out>
#17 0x00007f3bc206a323 in sendEvent (event=<optimized out>, receiver=<optimized out>)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
No locals.
#18 QTimerInfoList::activateTimers (this=0x13392e0) at kernel/qeventdispatcher_unix.cpp:621
        e = {<QEvent> = {_vptr.QEvent = 0x7f3bc239b9f0 <vtable for QTimerEvent+16>, static staticMetaObject = {d = {
                superdata = 0x0, stringdata = 0x7f3bc212a9c0 <qt_meta_stringdata_QEvent> "QEvent", 
                data = 0x7f3bc212b3c0 <qt_meta_data_QEvent>, extradata = 0x0}}, d = 0x0, t = 1, posted = 0, spont = 0, 
            m_accept = 1, reserved = 3651}, id = 1107296358}
        currentTimerInfo = 0x215b9c0
        n_act = 54
        maxCount = 56
#19 0x00007f3bc20675f1 in timerSourceDispatch (source=<optimized out>) at kernel/qeventdispatcher_glib.cpp:193
        timerSource = <optimized out>
#20 0x00007f3bb89ece04 in g_main_dispatch (context=0x1338610) at /build/buildd/glib2.0-2.40.0/./glib/gmain.c:3064
        dispatch = 0x7f3bc20675d0 <timerSourceDispatch(GSource*, GSourceFunc, gpointer)>
        prev_source = 0x0
        was_in_call = 0
        user_data = 0x0
---Type <return> to continue, or q <return> to quit---
        callback = 0x0
        cb_funcs = <optimized out>
        cb_data = 0x0
        need_destroy = <optimized out>
        source = 0x1339280
        current = 0x1275f90
        i = 2
#21 g_main_context_dispatch (context=context@entry=0x1338610) at /build/buildd/glib2.0-2.40.0/./glib/gmain.c:3663
No locals.
#22 0x00007f3bb89ed048 in g_main_context_iterate (context=context@entry=0x1338610, block=block@entry=1, 
    dispatch=dispatch@entry=1, self=<optimized out>) at /build/buildd/glib2.0-2.40.0/./glib/gmain.c:3734
        max_priority = 0
        timeout = 0
        some_ready = 1
        nfds = <optimized out>
        allocated_nfds = 18
        fds = 0x3c77da0
#23 0x00007f3bb89ed0ec in g_main_context_iteration (context=0x1338610, may_block=1)
    at /build/buildd/glib2.0-2.40.0/./glib/gmain.c:3795
        retval = <optimized out>
#24 0x00007f3bc20677a1 in QEventDispatcherGlib::processEvents (this=0x126caf0, flags=...)
    at kernel/qeventdispatcher_glib.cpp:434
        d = 0x1338540
        canWait = <optimized out>
        savedFlags = {i = 0}
        result = <optimized out>
#25 0x00007f3bc1475bb6 in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=...)
    at kernel/qguieventdispatcher_glib.cpp:204
        d = 0x1338540
        saved_flags = {i = 0}
        returnValue = 218
#26 0x00007f3bc20390af in QEventLoop::processEvents (this=this@entry=0x7fff721e67b0, flags=...) at kernel/qeventloop.cpp:149
        d = 0x1477b00
#27 0x00007f3bc20393a5 in QEventLoop::exec (this=this@entry=0x7fff721e67b0, flags=...) at kernel/qeventloop.cpp:204
        d = 0x1477b00
        locker = {val = 19313448}
#28 0x00007f3bc203eb79 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1225
        threadData = 0x126b1f0
        eventLoop = {<QObject> = {_vptr.QObject = 0x7f3bc239e710 <vtable for QEventLoop+16>, static staticMetaObject = {d = {
                superdata = 0x0, stringdata = 0x7f3bc2117ca0 <qt_meta_stringdata_QObject> "QObject", 
                data = 0x7f3bc2117d40 <qt_meta_data_QObject>, 
                extradata = 0x7f3bc239bc60 <QObject::staticMetaObjectExtraData>}}, static staticMetaObjectExtraData = {
              objects = 0x0, 
              static_metacall = 0x7f3bc2053ae0 <QObject::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>}, 
            d_ptr = {d = 0x1477b00}, static staticQtMetaObject = {d = {superdata = 0x0, 
                stringdata = 0x7f3bc2120be0 <qt_meta_stringdata_Qt> "Qt", data = 0x7f3bc2124500 <qt_meta_data_Qt>, 
                extradata = 0x0}}}, static staticMetaObject = {d = {superdata = 0x7f3bc239bc40 <QObject::staticMetaObject>, 
              stringdata = 0x7f3bc212a7c0 <qt_meta_stringdata_QEventLoop> "QEventLoop", 
              data = 0x7f3bc212a7e0 <qt_meta_data_QEventLoop>, 
              extradata = 0x7f3bc239e7a0 <QEventLoop::staticMetaObjectExtraData>}}, static staticMetaObjectExtraData = {
            objects = 0x0, 
            static_metacall = 0x7f3bc2098df0 <QEventLoop::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>}}
        returnCode = <optimized out>
#29 0x00007f3bc13d237c in QApplication::exec () at kernel/qapplication.cpp:3828
No locals.
#30 0x00007f3bc57a667c in kdemain (argc=1, argv=0x7fff721e6a08) at ../../../../plasma/desktop/shell/main.cpp:126
        __PRETTY_FUNCTION__ = "int kdemain(int, char**)"
        associatedScreen = <optimized out>
        aboutData = {d = 0x1310630}
---Type <return> to continue, or q <return> to quit---
        app = 0x13101b0
        appName = {static shared_null = {ref = {_q_value = 11650}, alloc = 0, size = 0, 
            data = 0x7f3bc23a07d8 <QByteArray::shared_null+24> "", array = ""}, static shared_empty = {ref = {
              _q_value = 497}, alloc = 0, size = 0, data = 0x7f3bc23a07b8 <QByteArray::shared_empty+24> "", array = ""}, 
          d = 0x12d8990}
        rc = <optimized out>
#31 0x00007f3bc53c5ec5 in __libc_start_main (main=0x4006e0 <main(int, char**)>, argc=1, argv=0x7fff721e6a08, 
    init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fff721e69f8) at libc-start.c:287
        result = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {0, 166257367395818701, 4196069, 140735107983872, 0, 0, 
                -166547368227152691, -272150802962752307}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 
              0x4007e0 <__libc_csu_init>, 0x7fff721e6a08}, data = {prev = 0x0, cleanup = 0x0, canceltype = 4196320}}}
        not_first_call = <optimized out>
#32 0x000000000040070e in _start ()


Reproducible: Always

Steps to Reproduce:
1. Add the System Load Viewer widget to the panel.
2. Configure it to show all processors
3. Configure its update interval to 500ms
4. Watch the plasma-desktop process cpu usage.
Actual Results:  
plasma-desktop takes up a constant 2% of cpu resources.

Expected Results:  
There should be no such cpu resource usage for a simple 2Hz resource monitor.

Using KDE 4.13.2 on Kubuntu 14.04.