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Tech talk / Re: Signals
« on: May 12, 2013, 09:53:15 am »
I don't think the pathfinding itself is the problem. If you use presignals instead, they will also trigger pathfinding if the chosen path has a red light.
The biggest difference is that all PBS (path based signals) will reserve the track up to the next light and later need to free this. This is fine in front of stations or splits where it's actually useful.
The problem is that most people also use them on their straight tracks without splits/joins. This means that the signal not only checks if there is a train on the block in front of it (so on those 1 to 5 pieces of track between lights) but also checks if these are reserved (so double the work already). Then if a train runs over these tracks in needs to reserve those tiles + free them afterwards (rough estimate, triple the work?).
Depending on the implementation, these extra checks are mediocre to very CPU intensive. (where normal signals are very light on CPU)
My experience:
playing on openttdcoop server with 2000 trains running and 10 or more people connected to that server. No lag.
Playing on #7 here with 3 players. 20 trains on full PBS tracks causing constant reconnect taking a few seconds.
So TL;DR:
PBS is really CPU heavy and should only be used in front of stations or other shared pieces of track (although pre-signals are still preferred if you know how to use them).
PS: In some rare cases I have even seen trains stop before an empty track just because the PBS couldn't reserve the track in time.
The biggest difference is that all PBS (path based signals) will reserve the track up to the next light and later need to free this. This is fine in front of stations or splits where it's actually useful.
The problem is that most people also use them on their straight tracks without splits/joins. This means that the signal not only checks if there is a train on the block in front of it (so on those 1 to 5 pieces of track between lights) but also checks if these are reserved (so double the work already). Then if a train runs over these tracks in needs to reserve those tiles + free them afterwards (rough estimate, triple the work?).
Depending on the implementation, these extra checks are mediocre to very CPU intensive. (where normal signals are very light on CPU)
My experience:
playing on openttdcoop server with 2000 trains running and 10 or more people connected to that server. No lag.
Playing on #7 here with 3 players. 20 trains on full PBS tracks causing constant reconnect taking a few seconds.
So TL;DR:
PBS is really CPU heavy and should only be used in front of stations or other shared pieces of track (although pre-signals are still preferred if you know how to use them).
PS: In some rare cases I have even seen trains stop before an empty track just because the PBS couldn't reserve the track in time.