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Memory allocations may return "Out of Memory" errors even when there are large free blocks of memory available within the virtual address space of the process.
Heap allocations within a process may result in the memory manager having to extend the heap using virtual memory to satisfy the request. The memory manager extends the heap by allocating contiguous virtual memory from the process address space. By default, the memory manager requests double the previous successfully allocated heap segment. If it fails allocating virtual memory for a larger heap segment, it does not try to allocate a segment the size of the previous or smaller segments.
For example, if the previous successfully allocated heap segment was 64 MB, the heap manager will attempt to extend the heap by 128 MB if it needs to extend the heap. If there is not 128 MB of contiguous virtual memory, the allocation to extend the heap will fail and the heap allocation will subsequently fail even though there may have been another 64-MB free block.
To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:
Microsoft has confirmed that this is a problem in Microsoft Windows NT version 4.0. This problem was first corrected in Windows NT version 4.0 Service Pack 5.
Ntdll was fixed so that, if Ntdll.dll cannot extend the heap because of an "Out of Memory" error on a first attempt, it divides the requested virtual memory allocation size by 2 and will continue to attempt to extend the heap in this way until the size requested falls below what is needed to satify the heap allocation.
Article ID: 216383 - Last Review: November 2, 2013 - Revision: 4.0
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