Hi again every body. New day – new thing to write about. today we will talk about memory allocation, and effect it has on your website performance. With .NET, memory allocations are usually overlooked because CLR handles that for you. Except in rare cases that you need to handle unmanaged resources, that you have to be conscious about releasing that said resources yourself, it’s usually fire and forget approach.
Truth is, it is more complicated than that. The more objects you created, the more memory you need, and the more time CLR needs to clean it up after you. When you might have written code that is executed blazing fast in your benchmarks, in reality, your website might still struggle to perform well in long run – and that’s because of Garbage collection. Occasional GC is not of a concern – because it’s nature of .NET CLR, but frequent GC, especially Gen 2 GC, is definitely something you should look into and should fix, because it natively affects your website performance.
The follow up question – how do you fix that.
Of course, the first step is always measuring the memory allocations of your app. Locally you can use something like Jetbrains dotMemory to profile your website, but that has a big caveat – you can’t really mimic the actual traffic to your website. Sure, it is very helpful to profile something like a scheduled job, but it is less than optimal to see how your website performs in reality. To do that, we need another tool, and I’ve found nothing better than Application Insights Profiler trace on Azure. It will sample your website periodically, taking ETL ( event trace log) files in 220 seconds (Note, depends on your .NET version, you might download a .diagsession or a .netperf.zip file from Application Insights, but they are essentially the same inside (zipped .ETL)). Those files are extremely informative, they contains whole load of information which might be overwhelming if you’re new, but take small steps, you’ll be there.
To open a ETL file, common tool is Perfview (microsoft/perfview: PerfView is a CPU and memory performance-analysis tool (github.com)). Yes it has certain 2000 look like other analysis tool (remember Windbg), but it is fast, efficient, and gets the job done
Note that once extracted ETL can be very big – in 1GB or more range often. Perfview has to go through all that event log so it’s extremely memory hungry as well, especially if you open multiple ETL files at once. My perfview kept crashing when I had a 16GB RAM machine (I had several Visual Studio instances open), and that was solved when I switched to 32GB RAM
The first step is to confirm the allocation problems with GCStats (this is one of the extreme ones, but it does happen)
Two main things to look into – Total Allocs, i.e. the total size of objects allocated, and then the time spent in Garbage collection. They are naturally closely related, but not always. Total allocation might not be high but time for GC might be – in case of large objects allocation (we will talk about it in a later post). Then for the purpose of memory allocation analysis, this is where you should look at
What you find in there, might surprise you. And that’s the purpose of this series, point out possible unexpected allocations that are easy – or fairly easy – to fix.
In this first post, we will talk about a somewhat popular feature – Injected<T>.
We all know that in Optimizely Content/Commerce, the preferred way of dependency injection is constructor injection. I.e. if your class has a dependency on a certain type, that dependency should be declared as a parameter of the constructor. That’s nice and all, but not always possible. For example you might have a static class (used for extension methods) so no constructor is available. Or in some rare cases, that you can’t added a new parameter to the constructor because it is a breaking change.
Adding Injected<T> as a hidden dependency in your class is at least working, so can you forget about it?
Not quite!
This is how the uses of Injected<T> result in allocation of Structuremap objects – yes every time you call Injected<T>.Service the whole dependency tree must be built again.
And that’s not everything, during that process, other objects need to be created as well. You can right click on a path and select “Include item”. The allocations below are for anything that were created by `module episerver.framework episerver.framework!EPiServer.ServiceLocation.Injected1[System.__Canon].get_Service() i.e. all object allocations, related to Injected<T>
You can expand further to see what Injected<T>(s) have the most allocations, and therefore, are the ones should be fixed.
How can one fix a Injected<T> then? The best fix is to make it constructor dependency, but that might not always be possible. Alternative fix is to use ServiceLocator.GetInstance, but to make that variable static if possible. That way you won’t have to call Injected<T>.Service every time you need the instance.
There are cases that you indeed need a new instance every time, then the fix might be more complicated, and you might want to check if you need the whole dependency tree, or just a data object.
Moral of the story
- Performance can’t be guessed, it must be measured
- Injected<T> is not your good friend. You can use it if you have no other choice, but definitely avoid it in hot paths.
