In the code snippet below, I was expecting a defer_tick to be required at <code class=

Source available <a href="https://github.com/hydro-project/hydroflow/blob/rohit_hydrof

Expected a defer_tick to be required, but isn't about hydroflow HOT 4 OPEN

rohitkulshreshtha commented on July 17, 2024

Expected a defer_tick to be required, but isn't

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Comments (4)

rohitkulshreshtha commented on July 17, 2024

Source available here

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MingweiSamuel commented on July 17, 2024

Option: require defer_tick() on all "backwards" edges (higher -> lower stratum number)

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MingweiSamuel commented on July 17, 2024

hydroflow/hydroflow_lang/src/graph/flat_to_partitioned.rs

Lines 384 to 390 in 77f3e5a

 DelayType::Stratum => { 

 // Any negative edges which go onto the same or previous stratum are bad. 

 // Indicates an unbroken negative cycle. 

 if dst_stratum <= src_stratum { 

 return Err(Diagnostic::spanned(dst_port.span(), Level::Error, "Negative edge creates a negative cycle which must be broken with a `defer_tick()` operator.")); 

 } 

 }

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jhellerstein commented on July 17, 2024

Current status: if we have a dataflow edge from a higher-numbered stratum to a lower one, the runtime effectively defers the dataflow until next tick. Unfortunately this hides correctness in our operational semantics and may be confusing for users.

Simple (inadequate) solution: require users to insert a defer_tick operator between the output of stratum x and the input of stratum k < x. The problem with this solution is that they may have a defer_tick elsewhere in the cycle (say from k+i to k+i+1) that already enforces the desired semantics.

Ideal solution: any cycle that spans two strata should be tested for having a defer_tick anywhere along that cycle. Errors should be omitted only when there is no defer_tick on the cycle. It makes sense to suggest that the defer_tick be inserted on the dataflow edge that goes from a higher stratum to a lower one.

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Expected a defer_tick to be required, but isn't about hydroflow HOT 4 OPEN

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	DelayType::Stratum => {
	// Any negative edges which go onto the same or previous stratum are bad.
	// Indicates an unbroken negative cycle.
	if dst_stratum <= src_stratum {
	return Err(Diagnostic::spanned(dst_port.span(), Level::Error, "Negative edge creates a negative cycle which must be broken with a `defer_tick()` operator."));
	}
	}