QNetworkAccessManager - first GET very slow

Question

I have a problem with using the QNetworkAccessManager in Qt 5.5 on android. Downloading a simple, small graphic file via http GET results in a lot of garbage collection calls and a lockup of the UI during that time. Subsequent GETs work flawlessly and without these GC calls. The code is as follows:

void DownloadManager::downloadFile(QUrl fromUrl, QString toFilePath) {

    _currentFilePath = toFilePath;

    QNetworkRequest request;
    request.setUrl(fromUrl);

    qDebug() << "before";

    _currentReply = _mgr.get(request);

    qDebug() << "after";

    connect(_currentReply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(downloadError(QNetworkReply::NetworkError)));
    connect(_currentReply, SIGNAL(downloadProgress(qint64, qint64)), this, SLOT(downloadProgress(qint64,qint64)));
    connect(_currentReply, SIGNAL(finished()), this, SLOT(downloadFinished()));

}

DownloadManager is a custom QObject-derived class without any special features that are relevant to the get request. _mgr is a QNetworkAccessManager Object that's allocated during DownloadManagers cTor.

As you can see, this is just a textbook example of a get request, nothing too fancy about it. And as I said: it works, for the most part. Only the first get request ends up like this:

D/ .../downloadmanager.cpp:61 (void DownloadManager::downloadFile(QUrl, QString)): before
D/dalvikvm(13298): GC_CONCURRENT freed 2290K, 25% free 10911K/14407K, paused 2ms+3ms, total 29ms
D/dalvikvm(13298): GC_CONCURRENT freed 501K, 25% free 10884K/14407K, paused 13ms+2ms, total 35ms
D/dalvikvm(13298): GC_CONCURRENT freed 524K, 25% free 10892K/14407K, paused 12ms+3ms, total 36ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 6ms
D/dalvikvm(13298): GC_CONCURRENT freed 537K, 25% free 10887K/14407K, paused 2ms+9ms, total 32ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 14ms
D/dalvikvm(13298): GC_CONCURRENT freed 840K, 25% free 10899K/14407K, paused 12ms+3ms, total 38ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 11ms
D/dalvikvm(13298): GC_CONCURRENT freed 1294K, 25% free 10901K/14407K, paused 2ms+2ms, total 27ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 11ms
D/dalvikvm(13298): GC_CONCURRENT freed 1187K, 22% free 11330K/14407K, paused 2ms+2ms, total 30ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 15ms
D/dalvikvm(13298): GC_CONCURRENT freed 1459K, 19% free 11919K/14535K, paused 13ms+4ms, total 64ms
D/dalvikvm(13298): WAIT_FOR_CONCURRENT_GC blocked 18ms
D/ .../downloadmanager.cpp:65 (void DownloadManager::downloadFile(QUrl, QString)): after

I simply don't understand what causes that much GC to happen - it takes about a full to one and a half seconds for everything to work out (for a download that should take a split-second and, moreover, be asynchronus and not locking up the UI).

Additional Information:

• It is always only the first download that triggers this. Subsequent downloads, even for the exact same file, work flawlessly

• It doesn't matter if there's a file at the exact location with the exact name or not. Downloading the file, deleting it, getting back into the application and redownloading it provides the same results - the first get is slow and has the GC, the second works perfectly fine.

• I call all that from a QML File, that causes a singleton c++ object to call DownloadManager::downloadFile.

• Other than the QML UI, nothing else is running within the application. No heavy data exchanges, no background loading on other threads, nothing.

I'd be thankful for any pointers towards solving this.

Answer 1

I didn't try on Android, but I had the same issue on Windows. Because those are the same symptoms, I would say this is likely the same reason, which is that the implementation is lazily loading some shared library on the first get() call. This is particularly true when using an encrypted connection; in my case I can see in Visual Studio that 19 DLLs are loaded on the first get() call.

One way to work around that is to pre-connect to the server using connectToHost or connectToHostEncrypted, depending on whether you are using an encrypted connection (e.g. HTTPS) or not. I am calling that at application startup, but anytime the UI is idle should be fine. Then subsequent get() calls will all have the same performance, including the first one, since the libraries have been loaded and the connection is already established. I am assuming that connecting to any server will load the libraries.

See https://forum.qt.io/topic/65201/qnetworkaccessmanager-first-get-very-slow/14 for details on the generic error (not specific to Android).