Preserve EventBlock events #126
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TeofilC
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Dec 29, 2025
The eventlog format consists a stream of blocks. Each of these blocks starts with an EventBlock event, which tells us the capability the block belongs to, the size and the end time. Note that the difference between the time of the last block and the start of the current one denotes the time spent writing the eventlog to disk (neat!). The events in each block are sorted since each is exclusively owned by own capability. Currently, ghc-events just discards these events. When we want to write stuff to disk again, we group all events from each capability together and create an artificial EventBlock. This creates a valid eventlog but we lose a great deal of information, and lose some nice properties. Chunks are guaranteed to be smaller than GHC's eventlog buffer size, and when using eventlog-flush-interval they are also limited in time. If we have a chunk from each capability loaded into memory then we can efficiently sort our events without having to load the entire eventlog. In practice, we are merging a sorted stream for each capability. Unfortunately this change on its own is not enough to allow roundtripping eventlogs faithfully, but it is a step towards doing so. We still need to preserve information about the order of blocks in the eventlog. Resolves #99
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The eventlog format consists a stream of blocks. Each of these blocks starts with an
EventBlockevent, which tells us the capability the block belongs to, the size and the end time. Note that the difference between the time of the last block and the start of the current one denotes the time spent writing the eventlog to disk (neat!). The events in each block are sorted since each is exclusively owned by own capability.Currently,
ghc-eventsjust discards these events. When we want to write stuff to disk again, we group all events from each capability together and create an artificialEventBlock. This creates a valid eventlog but we lose a great deal of information, and lose some nice properties.Chunks are guaranteed to be smaller than GHC's eventlog buffer size, and when using
eventlog-flush-intervalthey are also limited in time. If we have a chunk from each capability loaded into memory then we can efficiently sort our events without having to load the entire eventlog. In practice, we are merging a sorted stream for each capability.Unfortunately this change on its own is not enough to allow roundtripping eventlogs faithfully, but it is a step towards doing so. We still need to preserve information about the order of blocks in the eventlog.
Resolves #99