Question

I tried to profiling my program, which is a file downloader. I use 5 QNetworkRequest to enhance transfer speed, they all connect to the same readyRead() slot.

I use QMap to buffer those byte arrays.

When downloading speed reaches, e.g 5 MiB/s, I found CPU costs rises up significantly, nearly 100%, readyRead() was calling 30 times per seconds.

Is there any suggestion to this, e.g can I let QNetworkReply buffer its io device, reduce the amount of signals emitted?

UPDATA

I made some debug output in readyReady():
(reply here was casted from sender())

qDebug() << "Got: " << reply->readAll().length() << " bytes";

And about 30 output per seconds , each contains 1500 bytes

Answer 1

I have one hint after quick look on your code. You’re using QByteArray::append member (downloadBuffers) which realocates internal buffer for appended data. If you’re able to predict how much data you will store in each download buffer then use QByteArray::reserve with exact amount of data. If you can’t you could simply reserve i.e. 1mb and reserve memory progresively 1mb each time your next append will exceed current QByteArray::capacity. You can for example reserve downbufferSize for each buffer. It’s a slight optimization, but should give you some time