Question

Can anyone provide exhaustive explanation, please? I’m diving into concurrent programming and met those registers while trying to understand consensus.

From Lamport’s “On interprocess communication”: …a regular register is atomic if two successive reads that overlap the same write cannot obtain the new then the old value….

Assume, that first comes thread0.write(0) – with no overlapping. Basically, one can say using Lamports definition that thread1 can read first 1 and then 0 again, if both reads are consequent and overlap with thread0.write(1). But how is that possible?

Answer 1

Reads and writes to a shared memory location take a finite period of time, so they may either overlap, or be completely distinct.

e.g.

Thread 1:      wwwww     wwwww
Thread 2:   rrrrr              rrrrr
Thread 3:   rrrrr rrrrr

The first read from thread 2 overlaps with the first write from thread 1, whilst the second read and second write do not overlap. In thread 3, both reads overlap the first write.

A safe register is only safe as far as reads that do not overlap writes. If a read does not overlap any writes then it must read the value written by the most recent write. Otherwise it may return any value that the register may hold. So, in thread 2, the second read must return the value written by the second write, but the first read can return any valid value.

A regular register adds the additional guarantee that if a read overlaps with a write then it will either read the old value or the new one, but multiple reads that overlap the write do not have to agree on which, and the value may appear to “flicker” back and forth. This means that two reads from the same thread (such as in thread 3 above) that both overlap the write may appear “out of order”: the earlier read returning the new value, and the later returning the old value.

An atomic register guarantees that the reads and writes appears to happen at a single point in time. Readers that act at a point before that point will all read the old value and readers that act after that point will all read the new value. In particular, if two reads from the same thread overlap a write then the later read cannot return the old value if the earlier read returns the new one. Atomic registers are linearizable.

The Art of Multiprocessor Programming by Maurice Herlihy and Nir Shavit gives a good description, along with examples and use cases.