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I have a service which intermittently starts gobbling up server memory over time and needs to be restarted to free it. I turned +ust with gflags, restarted the service, and started taking scheduled UMDH snapshots. When the problem reoccurred, resource manager reported multiple GB under Working set and Private bytes, but the UMDH snapshots account only for a few MB allocations in the process' heaps.

At the top of UMDH snapshot files, it mentions "Only allocations for which the heap manager collected a stack are dumped".
How can an allocation in a process be without a trace when +ust flags were specified?

How can I find out where/how these GBs were allocated?

Thomas Weller
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adaj21
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1 Answers1

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UMDH is short for User Mode Dump Heap. The term Heap is a key term here: it refers to the C++ heap manager only. This means that all memory which is allocated by other means than the C++ heap manager is not tracked by UMDH.

This can be

  • direct calls to VirtualAlloc()
  • memory used by .NET, since .NET has its own heap manager

But even for C++, there is the case that allocations larger than 512 kB are not efficiently manageable by the C++ heap manager, so it just redirects it to VirtualAlloc() and does not create a heap segment of such large allocations.

How can I find out where/how these GBs were allocated?

For direct calls to VirtualAlloc(), the WinDbg command !address -summary may give an answer. For .NET, the SOS extension and the !dumpheap -stat can give an answer.

Thomas Weller
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