In every release of RSS Bandit, I spend some time working on performance. Making the application use less memory and less CPU time while adding more features is a challenge. Recently I read a post by Mitch Denny entitled RSS Bandit Performance Problems where after some investigation he found a key culprit for some of our memory consumption issues in the last release. Mitch wrote

Last weekend I was subscribed to over about 1000 RSS feeds and conicidentally last weekend RSSBandit also became unusable. Obviously I had reached some kind of threshold that the architecture of RSSBandit wasn’t designed to cope with.

My first instinct was to ditch and go and find something a bit faster – after all it is a .NET application and we know how much of a memory hog those things are! Errr – hang on. Don’t I encourage our customers to go out and use .NET to build mission critical enterprise applications every day? I really needed to take a closer look at what was going on.

In idle RSSBandit takes up around 120–170MB of RAM on my laptop. Thats more than Outlook and SQL Server, and often more than Visual Studio (except when its in full flight) but to be honest I’m not that surprised because in order for it to give me the unread items count it has to process quite a few files containing lots of unique strings – that means relatively large chunks of being allocated just for data.

I decided to look a bit deeper and run the CLR Allocation Profiler over the code to see where all the memory (and by extension good performance was going). I remembered this article by Rico Mariani which included the sage words that “space is king” and while I waited for the profiler to download tried to guess what the problem would be based on my previous experience.

What I imagined was buckets of string allocations to store posts in their entirety and a significant number of XML related object allocations but when I looked at the allocation graph I saw something interesting.

... [see http://notgartner.com/Downloads/AllocationGraph3.GIF]

As you can see there is a huge amount of traffic between this native function and the NativeWindow class. It was at this point that I started to suspect what the actual problem was and had to giggle at how many times this same problem pops up in smart client applications.

From what I can tell the problem is an excessive amount of marshalling to the UI thread is going on. This is causing threads to synchronise (tell tale DynamicInvoke calls are in there) and quite a bit of short term memory to be allocated over the lifetime of the application. Notice that there is 610MB of traffic between the native function and NativeWindow so obviously that memory isn’t hanging around.

The fix? I don’t know - but I suspect if I went in to the RSSBandit source and unplugged the UI udpates from the UpdatedFeed event the UI responsiveness would increase significantly (the background thread isn’t continually breaking into the main loop to update an unread count on a tree node).

It seems most of the memory usage while running RSS Bandit on Mitch's computer came from callbacks in the RSS parsing code that update the unread item count in the tree view within the GUI whenever a request to fetch an RSS feed is completed. Wow!!!

This is the last place I would have considered looking for somewhere to optimize our code and yet another example of why  "measurement is king" when it comes to figuring out how to improve the performance of an application. Given that a lot of the time a feed is requested there is no need to update the UI since no new items are fetched, there is a lot of improvement that we can gain here.

Yet again I am reminded that writing a rich client application like RSS Bandit using the .NET Framework means that you have to be familiar with a bunch of Win32/COM interop crap even if all you want to do is code in C#. I guess programming wouldn't be so challenging if not for gotchas like this lying around all over the place. :)