[Qt-interest] QNetworkAccessManager in GUI thread

[Nitin Mahajan]

Hello,

--- On Fri, 22/1/10, Thiago Macieira <thiago at kde.org> wrote:

> From: Thiago Macieira <thiago at kde.org>
> Subject: Re: [Qt-interest] QNetworkAccessManager in GUI thread
> To: qt-interest at trolltech.com
> Date: Friday, 22 January, 2010, 12:03 AM
> Em Quinta-feira 21 Janeiro 2010, às
> 12:33:14, Nitin Mahajan escreveu:
> > Hello,
> > 
> > In a GUI application like Demo Browser, I have a
> QNetworkAccessManager
> > object say "A" created in the GUI thread itself.
> > 
> > I want to do the actual file download in a separate
> thread, so that my
> > GUI/mouse cursor does not freeze. Can I create a new
> QNetworkAccessManager
> > "B" in the new thread and set the cookiejar and proxy
> from "A" and still
> > download?
> 
> No.
> 
> QNetworkAccessManager, all its QNetworkReply objects,
> QNetworkCookieJar, 
> QAbstractNetworkCache, etc., must all live in the same
> thread.
> 
> > While doing the download in the GUI thread itself I
> can see the mouse
> > cursor freezing, even though the replies of the
> requests sent through
> > NetworkAccessManager arrive asynchronously. Whether
> Network acccess
> > manager does some operation which is still Blocking?
> 
> Nothing is blocking. Everything is asynchronous.
> 
> My guess is that your device is just that much
> underpowered. It would be 
> interesting to profile the application and find out where
> the bottleneck is.
> 

I did run the browser through 'ltrace' to find out the bottleneck.
I did

#ltrace  -C -c -p  5190
% time     seconds  usecs/call     calls      function
------ ----------- ----------- --------- --------------------
 95.35    9.867098        4882      2021 QApplication::notify(QObject*, QEvent*)
  1.08    0.111444         165       674 QIODevice::write(char const*, long long)
  0.82    0.084883         125       674 QIODevice::readAll()
  0.71    0.073494         108       675 QApplication::compressEvent(QEvent*, QObject*, QPostEventList*)
  0.70    0.072156         107       674 qFree(void*)
  0.68    0.070630         104       674 QIODevice::isOpen() const
  0.67    0.068849         102       674 QWidget::qt_metacall(QMetaObject::Call, int, void**)
------ ----------- ----------- --------- --------------------
100.00   10.348554                  6066 total

and at another instance the output of ltrace is 

ltrace -C -c -p  32182
% time     seconds  usecs/call     calls      function
------ ----------- ----------- --------- --------------------
 78.47    2.219333        1392      1594 QIODevice::write(char const*,
long long)
  6.13    0.173351         108      1594 QIODevice::readAll()
  5.33    0.150667          94      1594 QIODevice::isOpen() const
  5.14    0.145472          90      1600
QApplication::compressEvent(QEvent*, QObject*, QPostEventList*)
  4.93    0.139285          87      1594
QWidget::qt_metacall(QMetaObject::Call, int, void**)
------ ----------- ----------- --------- --------------------
100.00    2.828108                  7976 total


With this I think the QIODevice::write() call is the culprit. 
Whether this understanding of mine is correct?

If that is true the QIODevice::write is called from 

1. QHttpNetworkReplyPrivate::transferRaw(writing to Buffer) and

2. from the application while writing file from DownloadItem::downloadReadyRead().

I tried making the connections as Qt::QueuedConnection, but did not help.
Since NAM and its related objects have to be in same thread, its difficult to seperate out these two operations in different threads.

If I disconnect DownloadItem::downloadReadyRead(), still the UI freezes.

Can I get some pointers on how to approach this?

regards

-Nitin




> -- 
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT)
> kde.org
>   Senior Product Manager - Nokia, Qt Development
> Frameworks
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