why is data structure alignment important for performance?

Question

Can someone give me a short and plausible explanation for why the compiler adds padding to data structures in order to align its members? I know that it's done so that the CPU can access the data more efficiently, but I don't understand why this is so.

And if this is only CPU related, why is a double 4 byte aligned in Linux and 8 byte aligned in Windows?

Answer 1

Alignment helps the CPU fetch data from memory in an efficient manner: less cache miss/flush, less bus transactions etc.

Some memory types (e.g. RDRAM, DRAM etc.) need to be accessed in a structured manner (aligned "words" and in "burst transactions" i.e. many words at one time) in order to yield efficient results. This is due to many things amongst which:

1. setup time: time it takes for the memory devices to access the memory locations
2. bus arbitration overhead i.e. many devices might want access to the memory device

"Padding" is used to correct the alignment of data structures in order to optimize transfer efficiency.

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In other words, accessing a "mis-aligned" structure will yield lower overall performance. A good example of such pitfall: suppose a data structure is mis-aligned and requires the CPU/Memory Controller to perform 2 bus transactions (instead of 1) in order to fetch the said structure, the performance is thus consequently lower.