Is it better to allocate memory in the power of two?

Question

When we use malloc() to allocate memory, should we give the size which is in power of two? Or we just give the exact size that we need?
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Answer 1

If you're allocating some sort of expandable buffer where you need to pick some number for initial allocations, then yes, powers of 2 are good numbers to choose. If you need to allocate memory for struct foo, then just malloc(sizeof(struct foo)). The recommendation for power-of-2 allocations stems from the inefficiency of internal fragmentation, but modern malloc implementations intended for multiprocessor systems are starting to use CPU-local pools for allocations small enough for this to matter, which prevents the lock contention that used to result when multiple threads would attempt to malloc at the same time, and spend more time blocked due to fragmentation.

By allocating only what you need, you ensure that data structures are packed more densely in memory, which improves cache hit rate, which has a much bigger impact on performance than internal fragmentation. There exist scenarios with very old malloc implementations and very high-end multiprocessor systems where explicitly padding allocations can provide a speedup, but your resources in that case would be better spent getting a better malloc implementation up and running on that system. Pre-padding also makes your code less portable, and prevents the user or the system selecting the malloc behavior at run-time, either programmatically or with environment variables.

Premature optimization is the root of all evil.