So Spanner uses TrueTime as an integral part of it's concurrency control algorithm. To preserve external consistency, Spanner sometimes has to wait out an uncer

So Spanner uses TrueTime as an integral part of it's concurrency control algorithm. To preserve external consistency, Spanner sometimes has to wait out an uncertainty interval at transaction commit. The tighter the bounds on timestamp intervals from TrueTime, the less this waiting impacts performance. I've heard through the grape vine that TrueTime currently operates much better than the numbers in the original paper, but can't confirm if that's true. They use GPS for global sync and a local atomic clock as reference. From conversations over the last few years this seems to be the common setup at big DCs, as the cost for a couple atomic clocks is minuscule compared to the scale of everything else.