C++ Memory management

13 Dec 2024

[ c++ memory cache allocation ]

Overview

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Memory Leak

new() without delete
Keep entries that are no longer needed
Missing virtual ~Base()
Circular reference

Memory leaks: Tools

bslma::TestAllocator

Advantages:

fast
small overhead
scoped

Disadvantages:

code changes required
compile and link required

Address Sanitizer

Advantages:

no code change required
fast

Disadvantages:

compile and link required
extra memory cost

Valgrind (memcheck and massif)

Advantages:

no code change required
no compile and link required

Disadvantages:

slow (10-30 times slower)
extra memory cost

KDE massif-visualizer

Memory leaks: Tips

“Static” leaks may hide the real issue - we need enough traffic for profiling
They are all good tools, but for different cases
Catch the problem in earlier stages - Integrate Address Sanitizer in CI
Install the tools so we can start profiling easily
Care about the lifecycle and ownership of what we allocate

Fragmentation

External fragmentation

I have free spaces, but I need to extend the heap
1 - LargestAllocableBlock / TotalFreeMemory
Most allocations can be done with chunks in bins

Internal fragmentation

I allocate a large chunk for a small size
1 - AccessedBytes / TotalAllocatedBytes
Avoid useless and underused allocations

Total bytes
Total in-use allocations
Total free space
Largest allocable block

Valgrind DHAT: a dynamic heap analysis tool

Defragmentation

C++

malloc

OS

Long-running stateless system

start and run for a long time
receive requests and process them, no state

Reference