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Map CPUs to Scylla Shards

Due to its thread-per-core architecture, many things within Scylla can be better understood when you look at it on a per-CPU basis. There are Linux tools such as top and perf that can give information about what is happening within a CPU, given a CPU number.

A common mistake users make is to assume that there is a direct and predictable relationship between the Scylla Shard ID and the CPU ID, which is not true.

Starting in version 3.0, Scylla ships with a script to let users know about the mapping between CPUs and Scylla Shards. For users of older versions, a copy of the script can be downloaded from the Seastar git tree.

Examples of usage

To list the mapping of a specific shard:

$ -n scylla -s 0
shard: 0, cpu: 1

To list the mapping of all shards:

$ -n scylla
shard: 0, cpu: 1
shard: 1, cpu: 2
shard: 3, cpu: 4
shard: 2, cpu: 3