Calculate default mempool capacity as 2x the ledger max block size

[intricate]

Closes #1467

[mrBliss]

What if the max block size changes in the ledger? (Will this happen?) If that happens, we'll only change the capacity when restarting the node.

cc: @edsko @dcoutts

[intricate]

Should we support a dynamically changing mempool capacity? I wasn't sure if we wanted that additional complexity.

[mrBliss]

I would indeed avoid it. But basing its static size on a dynamically changing thing might be confusing (needs a comment). The main reason I'm commenting on this is to ask whether the max block size will actually change in practice.

[dcoutts]

The max block size will change. I presume the question is "if using the default size (no local override), is it acceptable to require a restart before picking up a change in the default block size?".

Consider what happens when syncing. We'll start with the size from the genesis and not change it. The mainnet genesis specifies a 2Mb block size, that was later reduced down to 64k (and will probably be bumped up again after the Shelley hard fork).

[edsko]

Seems to me that if operators override the max block size, they will want to keep an eye on the max block size on the chain anyway, and requiring a restart when this happens (which will be rare) doesn't seem like a huge deal.

[mrBliss]

bors r+

[iohk-bors]

Timed out

[edsko]

bors r+

[iohk-bors]

Build succeeded

• buildkite/ouroboros-network
• ci/hydra:Cardano:ouroboros-network:required