Eventlog should include delimiters showing when the process writes to the .eventlog file
Quoting a comment I just added at https://ghc.haskell.org/trac/ghc/wiki/EventLog\#Limitations :
Event logging can produce "observer effects". In particular, programs can pause while writing to the
.eventlogfile. These display in Threadscope as periods of 100% CPU use with no obvious cause. This effect is exacerbated by heavy use of debug tools which expand the eventlog file, such asTrace.traceMarkerIO. (This effect was at least seen on OS X. In an extreme case, a program doing nothing but writing 200,000 trace markers, which took 120 ms to run, showed a single 10ms pause in the middle to write them to the.eventlogfile. These periods were shown to be due to writing the.eventlogfile by usingdtruss -a, which shows long periods ofwrite_nocancelsyscalls, writing to the.eventlogfile opened earlier withopen_nocancel.)
This behavior caused a few hours of investigation for us. It would make it much more obvious what is happening if this behavior were logged in the eventlog. Specifically, I would like for the eventlog to include a "starting writing eventlog file" event at the start of each of these periods, and a "stopped" event at the end of each period.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.3 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Runtime System |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | simonmar |
| Operating system | |
| Architecture |