.net memory is not reclaimed

Question

I have a 'service' (owin/webapi + odp.net managed) that runs with the 'server gc mode' on x64 machine. After executing test suite memory usage was around 2Gb. I 'generated' memory pressure on that machine (another program that consumed all available memory and more). I was expecting that windows will force .net to release some memory. However it did not. Here is an output from windbg:

0:018> !dumpheap -stat
Statistics:
              MT    Count    TotalSize Class Name
00007ffd10445b90        1           24 System.Collections.Generic.GenericEqualityComparer`1[[System.UInt64, mscorlib]]
...
00007ffd0f904918   100864     11616976 System.Object[]
0000005b19face20       84   1810674044      Free
Total 720544 objects
Fragmented blocks larger than 0.5 MB:
            Addr     Size      Followed by
0000005b1c3c8800  425.6MB 0000005b36d56c50 System.Func`2[[System.IAsyncResult, mscorlib],[System.Net.HttpListenerContext, System]]
0000005c1c3fcf00   28.2MB 0000005c1e02be58 System.Reflection.Emit.MethodBuilder
0000005c1e02e5b0  268.0MB 0000005c2ec313d8 System.Threading.OverlappedData
0000005c2ec31678  142.3MB 0000005c37a830a0 System.Func`2[[System.IAsyncResult, mscorlib],[System.Net.HttpListenerContext, System]]
0000005d1c541aa0  404.8MB 0000005d35a1a958 System.Func`2[[System.IAsyncResult, mscorlib],[System.Net.HttpListenerContext, System]]
0000005d35a1e5a8    3.8MB 0000005d35df0c40 System.Func`2[[System.IAsyncResult, mscorlib],[System.Net.HttpListenerContext, System]]
0000005e1cfe7128  444.1MB 0000005e38c0c3f0 System.Byte[]
0000005e38c8cfb8    4.5MB 0000005e39110988 System.Byte[]
0000005e39111bf8    1.2MB 0000005e3923a570 System.Byte[]
0000005e3923b7e0    0.7MB 0000005e392ed7d0 System.Byte[]
0000005e392eea40    1.1MB 0000005e39401910 System.Byte[]

0:018> !heapstat
Heap             Gen0         Gen1         Gen2          LOH
Heap0       455256040           24           24      3713672
Heap1       464550872           24      4327776       727456
Heap2       428541352     10344768        12616           24
Heap3       474250128           24     21350456           24
Total      1822598392     10344840     25690872      4441176

Free space:                                                 Percentage
Heap0       446429064            0            0      1819552SOH: 98% LOH: 48%
Heap1       459823968            0          144          152SOH: 98% LOH:  0%
Heap2       428520936        34904           24           24SOH: 97% LOH:100%
Heap3       474044952            0          336           24SOH: 95% LOH:100%
Total      1808818920        34904          504      1819752

So there is only around 48Mb really used and 1.8Gb free, most of memory is not in LOH but in Gen 0 heap, but .net would not give that memory back. And meanwhile system was starving for memory. For example IIS failed to start because there were not enough memory.

Why memory was not reclaimed by windows?

Answer 1

Well windows cannot "force" .Net to free memory. While I am no expert in the .Net runtime my experience has been that the .net framework will hold onto memory once your application starts to consume a lot of it. It will slowly give it back as it see's fit. From windows perspective it doesn't really know what .Net will do with it but it .Net has claimed that memory from the OS and so until it (.net) gives it back windows cannot give that memory to another application. There are several 400+mb chunks of System.Byte[] that are being held onto, which could be indicative of a memory leak. From the OS's perspective the Framework has requested the memory and even used that memory so until it is explicitly (from the framework not you) given back windows cannot do anything about it. I would profile your application very carefully because generally the reason .Net isn't releasing memory is because you have a memory leak. Using the memory analysis tools in Visual Studio should show you if you are leaving memory "Pinned" or just holding on to it. Really bad culprites are often static class variables/events.

SO link: .NET Free memory usage (how to prevent overallocation / release memory to the OS)