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Are cache lines aligned?
With respect to cache memory [Handy98], the most important thing to understand is the cache line. Typically a cache line is 32 bytes long and it is aligned to a 32 byte offset.
How do you align pointers?
Hands down the fastest way to align a pointer is to use 2’s complement math. You need to invert the bits, add one, and mask off the 2 (for 32-bit) or 3 (for 64-bit) least significant bits. The result is an offset that you then add to the pointer value to align it. Works great for 32 and 64-bit numbers.
What is word alignment in memory?
A memory access is said to be aligned when the data being accessed is n bytes long and the datum address is n-byte aligned. When a memory access is not aligned, it is said to be misaligned. Note that by definition byte memory accesses are always aligned.
What is byte alignment?
An object that is “8 bytes aligned” is stored at a memory address that is a multiple of 8. Many CPUs will only load some data types from aligned locations; on other CPUs such access is just faster. There’s also several other possible reasons for using memory alignment – without seeing the code it’s hard to say why.
What is Cacheline size?
The size of these chunks is called the cache line size. Common cache line sizes are 32, 64 and 128 bytes. A cache can only hold a limited number of lines, determined by the cache size. For example, a 64 kilobyte cache with 64-byte lines has 1024 cache lines.
How long is my cache line?
A cache line is the unit of data transfer between the cache and main memory. Typically the cache line is 64 bytes. The processor will read or write an entire cache line when any location in the 64 byte region is read or written.
Is malloc always aligned?
Regular malloc aligns memory suitable for any object type (which, in practice, means that it is aligned to alignof(max_align_t)). This function is useful for over-aligned allocations, such as to SSE, cache line, or VM page boundary.
What is 4 byte aligned address?
For instance, if the address of a data is 12FEECh (1244908 in decimal), then it is 4-byte alignment because the address can be evenly divisible by 4. (You can divide it by 2 or 1, but 4 is the highest number that is divisible evenly.) CPU does not read from or write to memory one byte at a time.
How to align to the cache line size?
You can also use Aligned if you know the cache line size beforehand, or just want to stick with the very common value of 64 (bytes). Thanks for contributing an answer to Stack Overflow!
How big is the cache line on x86?
On x86 cache lines are 64 bytes, however, to prevent false sharing, you need to follow the guidelines of the processor you are targeting (intel has some special notes on its netburst based processors), generally you need to align to 64 bytes for this (intel states that you should also avoid crossing 16 byte boundries).
What is the relation between cache line and memory page?
Relation between cache line and memory page. If a cache line size is 64 byte, and a memory page size is 4KB, then each page has 4KB / 64 bytes == 64 cache lines in it.
Why should code be aligned to even-address boundaries on x86?
Now, since this fetch limitation, if the cpu is forced to fetch values which are located on an odd address (suppose 3), it has to fetch values at 2 and 3, then values at 4 and 5, throw away values 2 and 5 then join 4 and 3 (you are talking about x86, which as a little endian memory layout).