DDR4 is effectively a block device and not 'random access'.
Pretty much only cache is RAM proper these days (aka: all locations have equal access time... that is, you can access it randomly with little performance loss).
I’m confused. What’s the difference between a cache line and a row in RAM? They’re both multiples of bytes. You have data sharing per chunk in either case.
The distinction seems to be how big the chunk is not uniformity of access time (is a symmetrical read disk not a block device?)
Hard disk chunks are 512 bytes classically, and smaller than the DDR4 row of 1024 bytes !!
So yes. DDR4 has surprising similarities to a 512byte sector hard drive (modern hard drives have 4k blocks)
>> What’s the difference between a cache line and a row in RAM?
Well DDR4 doesn't have a cache line. It has a burst length of 8, so the smallest data transfer is 64 bytes. This happens to coincide with L1 cache lines.
The row is 1024 bytes long. Its pretty much the L1 cache on the other side, so to speak. When your CPU talks to DDR4, it needs to load a row (RAS all 1024 bytes) before it can CAS read a 64 byte burst length 8 chunk.
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DDR4, hard drives, and Flash are all block devices.
The main issue for Flash technologies, is that the erase size is even larger than the read/write block size. That's why we TRIM for NVMe devices.
Thanks, I see what you mean at the interface level.
In terms of performance analogy though, hard drives do not get random access to blocks, but RAM does. The practical block size of hard drives is sequential reads of 100kiB+ due to seeks.
Pretty much only cache is RAM proper these days (aka: all locations have equal access time... that is, you can access it randomly with little performance loss).