iPhone network performance

Question

Hi all,

I have a question and I am very open to suggestions (even very odd ones!)

I am writing an iPhone app, which does a request (URL with parameters) to a server. As a response, the iPhone receives XML. All is well.

Right now, I am looking to improve my application's speed by measuring the time it takes to perform certain tasks. I've found that, of all the tasks performed (downloading, XML Parsing, sending the request, handeling the request, parsing objects from XML), downloading the actual XML takes the longest.

Now, my XML files are very, very, very easy and very, very, very small. I use them primarily to read RSS-like data and show them in a UITableView.

My app works very well, and there is nothing that feels really slow, but there is one application in the App Store right now which does something very similar to my application, but is way faster and feels more 'snappy', if you know what I mean. It also has the great feature to load the headlines one by one from the RSS-feed.

Currently I'm experimenting with gzip compression of my data, but the compression only makes my data half the size and it doesn't seem to do any real good for performance. The main thing is, that the data has to be downloaded, before it gets parsed. It would be very cool to have a 'stream' of data, which is parsed as it comes in. That way, I can do two jobs almost simultaneous and load headlines one by one (making user interactivity more attractive).

Anyone has an idea of how to improve my performance? Either by great compression tips or entirely different ways to communicate with the server.. All is welcome!

UPDATE: putting the latency and responsiveness of the server aside; how could I get a source of XML to be 'streamed' to my iPhone (downloaded byte for byte) and at the same time get parsed? Right now it is a linear process of downloading -> parsing -> showing, but it could become semi-parallel by downloading & parsing at the same time (and show each item when it is done downloading, instead of loading it all at the same time in a UITableView)

Answer 1

How are you parsing the XML data. The need to parse XML as you receive it is the whole reason pull parsing was invented, and they have the NSXMLParser event driven parser that operates on a stream of data...

That also means compression of the data is counterproductive.

For very small amounts of XML, the issue of speed is probably about latency of connection.

Thus, other big factor could be the DNS lookup. You could do that yourself once beforehand and cache the IP, perhaps rechecking only on failure to connect to your server...

Answer 2

Since your xml files are very small, (just 8k or so, I imagine?) I don't think you'd see a big performance boost trying to parse them as they come in. You can use an asynchronous NSURLConnection to do that, but I can't see it helping with a small file very much. Have you considered the time it takes to generate the XML file on the server? Are you using PHP to create the XML, or accessing a static file? For testing purposes, it might be interesting to access static files and see how that compares.

I ran into a problem like this a while ago, and it turned out the mySQL database on my server was being slow, and it really had nothing to do with my app.

Also, use Instruments to look at the amount of memory that is allocated while you're downloading and parsing an XML document. Some XML parsers load the entire XML document into memory and then allow you to index randomly into the document's elements, while others provide step by step parsing only. The step-by-step method is much better for limited devices like the iPhone and it's probably what you're using, but a quick look at memory consumption should tell you.

Answer 3

Assuming you're using NSXMLParser's initWithContentsOfURL:, that's probably part of the problem. It seems like that downloads the entire contents of the URL, then hands it off to the parser to parse all at once.

Despite the fact that the NSXMLParser is an event-driven parser, it doesn't seem to support streaming data to the parser in an incremental manner. You could, of course, replace NSXMLParser with some other parsing library that handles incremental data in a more sensible way.

An alternative would be to use NSURLConnection, and create a new NSXMLParser and re-parse the data each time some data comes in, in the connection:didReceiveData: method of your NSURLConnection's delegate. You''d have to write some extra code to ignore the extra events from re-parsing the beginning of the file more than once.

This seems like it'd be more work than just grabbing some other library and adapting it, but maybe not, depending on how you're handling the downstream creation of your table data.

Answer 4

If NSMutableArray is the underlying data structure of your UITableView you should try using initWithContentsOfURL. The format on the server needs to be in apples "plist" xml which is easy to generate. I'm guessing that if cocoa already has resources acquired for processing xml it would be quicker to use them instead of creating your own xml parser instance.