Is it better to allocate memory in the power of tw

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?
Like

//char *ptr= malloc( 200 ); 
char *ptr= malloc( 256 );//instead of 200 we use 256

If it is better to give size which is in the power of two, what is the reason for that? Why is it better?

Thanks

Edit

The reason of my confusion is following quote from Joel's blog Back to Basics

Smart programmers minimize the potential distruption of malloc by always allocating blocks of memory that are powers of 2 in size. You know, 4 bytes, 8 bytes, 16 bytes, 18446744073709551616 bytes, etc. For reasons that should be intuitive to anyone who plays with Lego, this minimizes the amount of weird fragmentation that goes on in the free chain. Although it may seem like this wastes space, it is also easy to see how it never wastes more than 50% of the space. So your program uses no more than twice as much memory as it needs to, which is not that big a deal.

Sorry, I should have posted the above quote earlier. My apologies!

Most replies, so far, say that allocating memory in the power of two is a bad idea, then in which scenario its better to follow Joel's point about malloc()? Why did he say that? Is the above quoted suggestion obsolete now?

Kindly explain it.
Thanks

Answer 1

You may want to allocate memory in terms of the processor's word size; not any old power of 2 will do.

If the processor has a 32-bit word (4 bytes), then allocate in units of 4 bytes. Allocating in terms of 2 bytes may not be helpful since the processor prefers data to start on a 4 byte boundary.

On the other hand, this may be a micro-optimization. Most memory allocation libraries are set up to return memory that is aligned at the correct position and will leave the least amount of fragmentation. If you allocate 15 bytes, the library may pad out and allocate 16 bytes. Some memory allocators have different pools based on the allocation size.

In summary, allocate the amount of memory that you need. Let the allocation library / manager handle the actual amount for you. Put more energy into correctness and robustness than worry about these trivial issues.

Answer 2

With today's amount of memory and its speed I don't think it's relevant anymore.

Furthermore, if you're gonna allocate memory frequently you better consider custom memory pooling / pre-allocation.

Answer 3

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.

Answer 4

You should use realloc() instead of malloc() when reallocating. http://www.cplusplus.com/reference/clibrary/cstdlib/realloc/

Always use a power of two? It depends on what your program is doing. If you need to reprocess your whole data structure when it grows to a power of two, yeah it makes sense. Otherwise, just allocate what you need and don't hog memory.

Answer 5

When I'm allocating a buffer that may need to keep growing to accommodate as-yet-unknown-size data, I start with a power of 2 minus 1, and every time it runs out of space, I realloc with twice the previous size plus 1. This makes it so I never have to worry about integer overflows; the size can only overflow when the previous size was SIZE_MAX, at which point the allocation would already have failed, and 2*SIZE_MAX+1 == SIZE_MAX anyway.

In contrast, if I just used a power of 2 and doubled it each time, I might successfully get a 2^31 byte buffer and then reallocate to a 0 byte buffer next time I doubled the size.

As some people have commented about power-of-2-minus-12 being good for certain malloc implementations, one could equally start with a power of 2 minus 12, then double it and add 12 at each step...

On the other hand if you're just allocating small buffers that won't need to grow, request exactly the size you need. Don't try to second-guess what's good for malloc.

Answer 6

This is totally dependent on the given libc implementation of malloc(3). It's up to that implementation to reserve heap chunks in whatever order it sees fit.

To answer the question - no, it's not "better" (here by "better" you mean ...?). If the size you ask for is too small, malloc(3) will reserve bigger chunk internally, so just stick with your exact size.