Change to scaling overestimation burn algorithm

Change the treshold to 1.1

Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

chain/vm/burn.go

@@ -1,18 +1,44 @@
 package vm
 
+import (
+	"math/big"
+)
+
 const (
-	gasOveruseNum   = 3
+	gasOveruseNum   = 11
 	gasOveruseDenom = 10
 )
 
 // ComputeGasOutputs computes amount of gas to be refunded and amount of gas to be burned
 // Result is (refund, burn)
 func ComputeGasOutputs(gasUsed, gasLimit int64) (int64, int64) {
-	allowedGasOverUsed := gasUsed + (gasUsed*gasOveruseNum)/gasOveruseDenom
-	gasToBurn := gasLimit - allowedGasOverUsed
-	if gasToBurn < 0 {
-		gasToBurn = 0
+	if gasUsed == 0 {
+		return 0, gasLimit
 	}
 
-	return gasLimit - gasUsed - gasToBurn, gasToBurn
+	// over = gasLimit/gasUsed - 1 - 0.3
+	// over = min(over, 1)
+	// gasToBurn = (gasLimit - gasUsed) * over
+
+	// so to factor out division from `over`
+	// over*gasUsed = min(gasLimit - (13*gasUsed)/10, gasUsed)
+	// gasToBurn = ((gasLimit - gasUsed)*over*gasUsed) / gasUsed
+	over := gasLimit - (gasOveruseNum*gasUsed)/gasOveruseDenom
+	if over < 0 {
+		return gasLimit - gasUsed, 0
+	}
+
+	// if we want sharper scaling it goes here:
+	// over *= 2
+
+	if over > gasUsed {
+		over = gasUsed
+	}
+
+	// needs bigint, as it overflows in pathological case gasLimit > 2^32 gasUsed = gasLimit / 2
+	gasToBurn := big.NewInt(gasLimit - gasUsed)
+	gasToBurn = gasToBurn.Mul(gasToBurn, big.NewInt(over))
+	gasToBurn = gasToBurn.Div(gasToBurn, big.NewInt(gasUsed))
+
+	return gasLimit - gasUsed - gasToBurn.Int64(), gasToBurn.Int64()
 }

chain/vm/burn_test.go

@@ -14,21 +14,26 @@ func TestGasBurn(t *testing.T) {
 		refund int64
 		burn   int64
 	}{
-		{100, 200, 30, 70},
-		{1000, 1300, 300, 0},
-		{500, 700, 150, 50},
+		{100, 200, 10, 90},
+		{100, 150, 30, 20},
+		{1000, 1300, 240, 60},
+		{500, 700, 140, 60},
 		{200, 200, 0, 0},
 		{20000, 21000, 1000, 0},
 		{0, 2000, 0, 2000},
-		{500, 651, 150, 1},
+		{500, 651, 121, 30},
+		{500, 5000, 0, 4500},
+		{7499e6, 7500e6, 1000000, 0},
+		{7500e6 / 2, 7500e6, 375000000, 3375000000},
+		{1, 7500e6, 0, 7499999999},
 	}
 
 	for _, test := range tests {
 		test := test
 		t.Run(fmt.Sprintf("%v", test), func(t *testing.T) {
 			refund, toBurn := ComputeGasOutputs(test.used, test.limit)
-			assert.Equal(t, test.burn, toBurn)
-			assert.Equal(t, test.refund, refund)
+			assert.Equal(t, test.refund, refund, "refund")
+			assert.Equal(t, test.burn, toBurn, "burned")
 		})
 	}
 }

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/drand/drand v1.0.3-0.20200714175734-29705eaf09d4
 	github.com/drand/kyber v1.1.1
 	github.com/fatih/color v1.8.0
-	github.com/filecoin-project/chain-validation v0.0.6-0.20200804050537-d19ebfb80735
+	github.com/filecoin-project/chain-validation v0.0.6-0.20200805000849-cee42fef3885
 	github.com/filecoin-project/filecoin-ffi v0.30.4-0.20200716204036-cddc56607e1d
 	github.com/filecoin-project/go-address v0.0.2-0.20200504173055-8b6f2fb2b3ef
 	github.com/filecoin-project/go-amt-ipld/v2 v2.1.1-0.20200731171407-e559a0579161 // indirect