perf: Speedup to params simulation (#9481)

## Description

In Osmosis' simulation, we were observing that a large percent of the time was spent within SimulateParamChangeProposalContent, in particular within the composed key function. This is from a trace of our simulator running for 10 minutes:

<img width="931" alt="Screenshot 2021-06-08 at 2 32 28 PM" src="https://user-images.githubusercontent.com/6440154/121238788-16cdd000-c85e-11eb-8251-94537e69f8ce.png">

~This change lowers the number of calls to ComposedKey() by a factor of 3, and speeds up the underlying ComposedKey function. (sprintf is not particularly efficient for string concatenation, https://dev.to/pmalhaire/concatenate-strings-in-golang-a-quick-benchmark-4ahh ). After these change, the effect of ComposedKey() can no longer be seen in the profiler outputs.~

~This change does not change the result of the simulator on a given seed.~

See https://github.com/cosmos/cosmos-sdk/pull/9481#issuecomment-857119210 for description of changes.

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -132,6 +132,7 @@ if input key is empty, or input data contains empty key.
 ### Improvements
 
 * (gRPC-Web) [\#9493](https://github.com/cosmos/cosmos-sdk/pull/9493) Add `EnableUnsafeCORS` flag to grpc-web config.
+* (x/params) [\#9481](https://github.com/cosmos/cosmos-sdk/issues/9481) Speedup simulator for parameter change proposals.
 * (store) [\#9403](https://github.com/cosmos/cosmos-sdk/pull/9403) Add `RefundGas` function to `GasMeter` interface
 * (baseapp, types) [\#9390](https://github.com/cosmos/cosmos-sdk/pull/9390) Add current block header hash to `Context`
 * (x/staking) [\#9423](https://github.com/cosmos/cosmos-sdk/pull/9423) Staking delegations now returns empty list instead of rpc error when no records found.

x/params/simulation/operations.go

@@ -1,6 +1,7 @@
 package simulation
 
 import (
+	"fmt"
 	"math/rand"
 
 	sdk "github.com/cosmos/cosmos-sdk/types"
@@ -8,44 +9,45 @@ import (
 	"github.com/cosmos/cosmos-sdk/x/params/types/proposal"
 )
 
+func min(a int, b int) int {
+	if a <= b {
+		return a
+	}
+	return b
+}
+
 // SimulateParamChangeProposalContent returns random parameter change content.
 // It will generate a ParameterChangeProposal object with anywhere between 1 and
 // the total amount of defined parameters changes, all of which have random valid values.
 func SimulateParamChangeProposalContent(paramChangePool []simulation.ParamChange) simulation.ContentSimulatorFn {
+	numProposals := 0
+	// Bound the maximum number of simultaneous parameter changes
+	maxSimultaneousParamChanges := min(len(paramChangePool), 1000)
+	if maxSimultaneousParamChanges == 0 {
+		panic("param changes array is empty")
+	}
+
 	return func(r *rand.Rand, _ sdk.Context, _ []simulation.Account) simulation.Content {
-
-		lenParamChange := len(paramChangePool)
-		if lenParamChange == 0 {
-			panic("param changes array is empty")
-		}
-
-		numChanges := simulation.RandIntBetween(r, 1, lenParamChange)
+		numChanges := simulation.RandIntBetween(r, 1, maxSimultaneousParamChanges)
 		paramChanges := make([]proposal.ParamChange, numChanges)
 
-		// map from key to empty struct; used only for look-up of the keys of the
-		// parameters that are already in the random set of changes.
-		paramChangesKeys := make(map[string]struct{})
+		// perm here takes at most len(paramChangePool) calls to random
+		paramChoices := r.Perm(len(paramChangePool))
 
 		for i := 0; i < numChanges; i++ {
-			spc := paramChangePool[r.Intn(len(paramChangePool))]
-
-			// do not include duplicate parameter changes for a given subspace/key
-			_, ok := paramChangesKeys[spc.ComposedKey()]
-			for ok {
-				spc = paramChangePool[r.Intn(len(paramChangePool))]
-				_, ok = paramChangesKeys[spc.ComposedKey()]
-			}
-
-			// add a new distinct parameter to the set of changes and register the key
-			// to avoid further duplicates
-			paramChangesKeys[spc.ComposedKey()] = struct{}{}
+			spc := paramChangePool[paramChoices[i]]
+			// add a new distinct parameter to the set of changes
 			paramChanges[i] = proposal.NewParamChange(spc.Subspace(), spc.Key(), spc.SimValue()(r))
 		}
 
+		title := fmt.Sprintf("title from SimulateParamChangeProposalContent-%d", numProposals)
+		desc := fmt.Sprintf("desc from SimulateParamChangeProposalContent-%d. Random short desc: %s",
+			numProposals, simulation.RandStringOfLength(r, 20))
+		numProposals++
 		return proposal.NewParameterChangeProposal(
-			simulation.RandStringOfLength(r, 140),  // title
-			simulation.RandStringOfLength(r, 5000), // description
-			paramChanges,                           // set of changes
+			title,        // title
+			desc,         // description
+			paramChanges, // set of changes
 		)
 	}
 }

x/simulation/params.go

@@ -119,7 +119,7 @@ func NewSimParamChange(subspace, key string, simVal simulation.SimValFn) simulat
 
 // ComposedKey creates a new composed key for the param change proposal
 func (spc ParamChange) ComposedKey() string {
-	return fmt.Sprintf("%s/%s", spc.Subspace(), spc.Key())
+	return spc.Subspace() + "/" + spc.Key()
 }
 
 // Proposal Contents