Handling EventBlocks and roundtripping about ghc-events

[Mikolaj]

That sounds good to me as a past Threadscope contributor, but we'd probably need feedback from current heavy eventlog users, e.g., @mpickering. Who would be the main consumer of the new feature?

[TeofilC]

I think the main consumer would be tools that want to figure out mutator time. Currently we expose information about GC pauses but don't expose information about event log flush pauses. This information could also be added to Threadscope for instance.

There's also quite an old GHC ticket asking for this https://gitlab.haskell.org/ghc/ghc/-/issues/11950. Surprisingly this feature was already implemented in the eventlog when this ticket was opened (!) but just not exposed by GHC-events.

The other appeal is that it makes it a bit easier to process event logs in a streaming way. For instance, currently the API doesn't expose a way to write an eventlog without sorting all the events (in order to create dummy eventblocks). I recently ran into this when trying to filter out a small time range from a very large eventlog. It would also make it a bit easier to process events in order without sorting the eventlog, though it could be done without this too.