Why qt execution is so slow,this is a test

[newgod]

Testing machine:
win7
8G
I7 2600

qt4.7.4 php5.3.8 python 2.6

run with qt

QTime t;
    t.start();
    QString aa="";
    for(int i=0;i<=1000000;i++){
       aa += "1";
     }
     printf("used: %d ms", t.elapsed());

QTime t;
    t.start();
    QString aa="";
    for(int i=0;i<=1000000;i++){
       aa += "1";
     }
     printf("used: %d ms", t.elapsed());

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debug :240ms
release:138ms


run with php

$time = new runtime;
    $time->start();
    $str = "";
    for ($i = 0; $i <= 1000000; $i++) {
        $str.="1";
    }
    $time->stop();
    echo "used:" . $time->spent() . "ms";

$time = new runtime;
    $time->start();
    $str = "";
    for ($i = 0; $i <= 1000000; $i++) {
        $str.="1";
    }
    $time->stop();
    echo "used:" . $time->spent() . "ms";

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used time :92.9ms


run with python:

import os,sys
    from time import clock as now
    start = now()
    str = ""
    for i in range(0,1000000):
        str += "i"
    finish = now()
    print (finish-start)

import os,sys
    from time import clock as now
    start = now()
    str = ""
    for i in range(0,1000000):
        str += "i"
    finish = now()
    print (finish-start)

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used time:210ms

run with qt c++:

QTime t;
        t.start();
        char *pStr = new char[1000001];
        for(int i=0;i<=1000000;i++){
            pStr[i] = '1';
        }
        delete pStr;
        printf("used: %d ms",t.elapsed());

QTime t;
        t.start();
        char *pStr = new char[1000001];
        for(int i=0;i<=1000000;i++){
            pStr[i] = '1';
        }
        delete pStr;
        printf("used: %d ms",t.elapsed());

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debug,3ms
release,1ms

not just this one test,every test just faster to python,there is no php half of the fast
is this my qt configuration's problem?
some one answer me,please

[wysota]

Comparing apples to monkeys using a grandpa's clock doesn't yield any practical results.

By the way, yes -- appending single characters to QString is quite slow. However it's not a problem for me since I'm not often adding thousands of single characters to a string in a loop in my programs. Such apps are quite boring.

[newgod]

yes i know,this is about apple's question,so, don't tell me answer about grandpa

[stampede]

Do you have any practical, real world problem with the performance of your Qt-based app ? Because the only comment for your test I can think of is - "so what ?"

[newgod]

i just want enhance the performance of my code,i need a real about the question's answer,so please do not write some not relevant's comment.Again

[stampede]

You want to enhance the performance of appending 1000000 characters to a string ? Simple answer would be - don't do it this way. Use QString::reserve to preallocate the string buffer and then insert characters into given position, this should work faster.

[Lykurg]

...or use QByteArray which shouldn't have all the UTF-8 overhead.

[newgod]

thanks,i think i know that is where the problem,the string will be converted to unicode

[amleto]

Reading the docs was helpful? No way! They are just there to look pretty.

[wysota]

By the way... the "C++ test" is flawed. First of all the only "C++" thing in it is using "new" and "delete" instead of "malloc" and "free" and second of all all the other "tests" increment the buffer char by char instead of preallocating everything at once.

Here is a more proper test of this useless case and the results. If you analyze the results, you'll see what takes the most time and how to counteract it.

********* Start testing of StringTest *********
Config: Using QTest library 4.7.4, Qt 4.7.4
PASS   : StringTest::initTestCase()
RESULT : StringTest::testC():
     16 msecs per iteration (total: 66, iterations: 4)
PASS   : StringTest::testC()
RESULT : StringTest::testCPreallocated():
     0.60 msecs per iteration (total: 77, iterations: 128)
PASS   : StringTest::testCPreallocated()
RESULT : StringTest::testCPP():
     112,716 msecs per iteration (total: 112,716, iterations: 1)
PASS   : StringTest::testCPP()
RESULT : StringTest::testCPPPreallocated():
     0.63 msecs per iteration (total: 81, iterations: 128)
PASS   : StringTest::testCPPPreallocated()
RESULT : StringTest::testCPPString():
     4.3 msecs per iteration (total: 70, iterations: 16)
PASS   : StringTest::testCPPString()
RESULT : StringTest::testByteArray():
     5.8 msecs per iteration (total: 94, iterations: 16)
PASS   : StringTest::testByteArray()
RESULT : StringTest::testByteArrayPreallocated():
     2.7 msecs per iteration (total: 89, iterations: 32)
PASS   : StringTest::testByteArrayPreallocated()
RESULT : StringTest::testByteArrayPrefilled():
     0.11 msecs per iteration (total: 60, iterations: 512)
PASS   : StringTest::testByteArrayPrefilled()
RESULT : StringTest::testQString():
     10 msecs per iteration (total: 80, iterations: 8)
PASS   : StringTest::testQString()
RESULT : StringTest::testQStringPreallocated_reserve():
     9.3 msecs per iteration (total: 75, iterations: 8)
PASS   : StringTest::testQStringPreallocated_reserve()
RESULT : StringTest::testQStringPreallocated_resize():
     7.6 msecs per iteration (total: 61, iterations: 8)
PASS   : StringTest::testQStringPreallocated_resize()
RESULT : StringTest::testQStringPrefilled():
     1.0 msecs per iteration (total: 65, iterations: 64)
PASS   : StringTest::testQStringPrefilled()
PASS   : StringTest::cleanupTestCase()
Totals: 14 passed, 0 failed, 0 skipped
********* Finished testing of StringTest *********

********* Start testing of StringTest *********
Config: Using QTest library 4.7.4, Qt 4.7.4
PASS   : StringTest::initTestCase()
RESULT : StringTest::testC():
     16 msecs per iteration (total: 66, iterations: 4)
PASS   : StringTest::testC()
RESULT : StringTest::testCPreallocated():
     0.60 msecs per iteration (total: 77, iterations: 128)
PASS   : StringTest::testCPreallocated()
RESULT : StringTest::testCPP():
     112,716 msecs per iteration (total: 112,716, iterations: 1)
PASS   : StringTest::testCPP()
RESULT : StringTest::testCPPPreallocated():
     0.63 msecs per iteration (total: 81, iterations: 128)
PASS   : StringTest::testCPPPreallocated()
RESULT : StringTest::testCPPString():
     4.3 msecs per iteration (total: 70, iterations: 16)
PASS   : StringTest::testCPPString()
RESULT : StringTest::testByteArray():
     5.8 msecs per iteration (total: 94, iterations: 16)
PASS   : StringTest::testByteArray()
RESULT : StringTest::testByteArrayPreallocated():
     2.7 msecs per iteration (total: 89, iterations: 32)
PASS   : StringTest::testByteArrayPreallocated()
RESULT : StringTest::testByteArrayPrefilled():
     0.11 msecs per iteration (total: 60, iterations: 512)
PASS   : StringTest::testByteArrayPrefilled()
RESULT : StringTest::testQString():
     10 msecs per iteration (total: 80, iterations: 8)
PASS   : StringTest::testQString()
RESULT : StringTest::testQStringPreallocated_reserve():
     9.3 msecs per iteration (total: 75, iterations: 8)
PASS   : StringTest::testQStringPreallocated_reserve()
RESULT : StringTest::testQStringPreallocated_resize():
     7.6 msecs per iteration (total: 61, iterations: 8)
PASS   : StringTest::testQStringPreallocated_resize()
RESULT : StringTest::testQStringPrefilled():
     1.0 msecs per iteration (total: 65, iterations: 64)
PASS   : StringTest::testQStringPrefilled()
PASS   : StringTest::cleanupTestCase()
Totals: 14 passed, 0 failed, 0 skipped
********* Finished testing of StringTest *********

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