Enter VMMap, which is “ a process virtual and physical memory analysis utility. But why only allocate barely close to 3 GB – to be more precise 2.6 GB, as the profiler shows above -, when it was supposed to go up to 6 GB ? Anyone Got A Bigger Light ?Ĭlearly we need a tool that can shed more light on the problem, and allow us to see the memory usage in better detail. Each array takes roughly 40 million bytes, as expected. When we first ran the code, it could only allocate 66 arrays.ĭrilling down into the objects for which memory is allocated, we see the same number of arrays successfully allocated (67) as in the console output.
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Oddly enough, this time the code can successfully allocate 67 arrays (the code fails just after displaying error for the 0-based array no 66). This will run the code once more, and allow us to take a snapshot after the exception is thrown, so that we understand where the usage goes: Figure 3 – Memory profiling using Visual Studio Luckily, Visual Studio has a built-in memory profiler we can use right away. Visual Studio’s built-in profiling dashboard (top right) shows the memory used by our process going close to 3 GB – just as the exception hits. The exception thrown is our familiar System.OutOfMemoryException. Not only that it doesn’t complete successfully, but the code doesn’t even make it till the 100th 10-million int array. Let’s hit F5 and see how the sample code performs: Figure 2 – The sample code in action We’re also catching any exception that might come up, and write it on the screen directly. However, this all will come in handy when we’ll analyze the memory usage in detail. Prevent the GC from destroying the objects created, byĪside from allocating the arrays themselves, most of the code is fluff, and deals with writing output messages, waiting for a key to be pressed to get to the next section or delaying allocating the subsequent array. This will end up on the heapĬonsole.WriteLine("Allocated (but not touched) for array ", k, BlockSIZE) Int intArray = new int Ĭonsole.WriteLine("Press a key to start") Now that the numbers are clear, let’s write the code:Ĭonst int BlockSIZE = 10000000 // 10 million This should be in theory the number of 40 mil arrays required to fill 6 GB of memory. If we divide the 6 GB (6.442.450.944 bytes) to 40 million bytes, we’ll get roughly 162. This will actually be the same on either a 32-bit platform or a 64-bit one. Thus, an array of 10 million int elements would take 40 million bytes of memory. As it turns out, an int will always take 4 bytes of memory. What we need to know is how much an int takes in memory. As for how many of these arrays should be: our target for now is to replicate the initial scenario that started this blog post – that is consuming 6 GB of memory – so we should choose the number of arrays accordingly. Let’s make each array contain 10 million int values. We’ll pick a simple method of allocating lots of memory – creating several large int arrays. Let’s replicate the issue you’ve encountered first – the out-of-memory thing.
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First we’ll look at a simple example of code that consumes a lot of memory fast, then uncover interesting facts about our problem, hit a “Wait, what ?” moment, learn the fundamentals of virtual memory, find the root cause of our problem then finish with a series of Q&A. Not so fast ! Tell me more about what goes on under the hood: Buckle up, since we’re going for a ride. Build your solution again and you’re done. Go into your project’s Properties, go to Build, and choose Platform target as 圆4. TL DR (small scroll bar => therefore I deduct a lot of text => I’ve got no time for that, and need the answer now): The default build settings for Visual Studio limit your app’s virtual address space to 4 GB.
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The question that this post will set out to answer thus becomes simply: “ Why do I get a System.OutOfMemoryException when running my recently created C# app on an idle, 64-bit Windows machine with lots of RAM ?“. 6 GB surely should have been in reach for your code. What’s going on here ? You’re not running some other memory-consuming app. Figure 1 – Visual Studio breaking into an exception