Why am I getting an Out Of Memory Exception in my C# application?

Question

My memory is 4G physical, but why I got out of memory exception even if I create just 1.5G memory object. Any ideas why? (I saw at the same time, in the performance tab of task manager the memory is not full occupied, and I could also type here -- so memory is not actually low, so I think I hit some other memory limitations)?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TestBigMemoryv1
{
    class MemoryHolderFoo
    {
        static Random seed = new Random();
        public Int32 holder1;
        public Int32 holder2;
        public Int64 holder3;

        public MemoryHolderFoo()
        {
            // prevent from optimized out
            holder1 = (Int32)seed.NextDouble();
            holder2 = (Int32)seed.NextDouble();
            holder3 = (Int64)seed.NextDouble();
        }
    }

    class Program
    {
        static int MemoryThreshold = 1500; //M
        static void Main(string[] args)
        {
            int persize = 16;
            int number = MemoryThreshold * 1000 * 1000/ persize;
            MemoryHolderFoo[] pool = new MemoryHolderFoo[number];
            for (int i = 0; i < number; i++)
            {
                pool[i] = new MemoryHolderFoo();
                if (i % 10000 == 0)
                {
                    Console.Write(".");
                }
            }

            return;
        }
    }
}

Answer 1

On a 32-bit Windows Operating system the maximum 'user-mode' memory that a single application can access is 2GB... assuming that you have 4GB of memory on the box.

http://stackoverflow.com/questions/588882/unmanaged-vc-applications-memory-consumption-on-windows-server

http://blogs.technet.com/markrussinovich/archive/2008/07/21/3092070.aspx

(It's funny you asked this because I asked almost the same thing yesterday...)

Answer 2

In a normal 32 bit windows app, the process only has 2GB of addressable memory. This is irrelevant to the amount of physical memory that is available.

So 2GB available but 1.5 is the max you can allocate. The key is that your code is not the only code running in the process. The other .5 GB is probably the CLR plus fragmentation in the process.

Answer 3

You've got a max of 2Gb addressable memory as a 32bit app, as the other posters mentioned. Dont forget about overhead. You're creating an array of 93 million objects - if there happens to be 4 bytes of overhead per object that's an extra 350Mb of memory.

Answer 4

Just to add to the previous replies: you can go beyond the 2Gb limit on systems booted with the /3Gb [and optionally userva] boot flags.

Answer 5

Just additional to the other points; if you want access to a dirty amount of memory, consider x64 - but be aware that the maximum single object size is still 2GB. And because references are larger in x64, this means that you actually get a smaller maximum array/list size for reference-types. Of course, by the time you hit that limit you are probably doing things wrong anyway!

Other options:

• use files
• use a database

(obviously both has a performance difference compared to in-process memory)

Answer 6

One more thing to be aware of; some .NET objects require 'contiguous' memory. i.e if you are trying to allocate a large array the system may need not only sufficient free memory in your process but also for all that free memory to be in one big chunk... and unfortunately the process memory gets fragmented over time so this may not be available.

Some objects/data types have this requirement and some don't... I can't remember which ones do, but I seem to recall StringBuilder and MemoryStream have different requirements.