Improve Default Sieve Workers and Prime Table Limit Guesses.

Optimal Sieve Workers appear to be well modeled by Exponential Functions.
Increase Prime Table Limit Cap to take in account the general increase of Ram Size.

Signed-off-by: Pttn <28868425+Pttn@users.noreply.github.com>

Author: Pttn

Stella.cpp

@@ -157,14 +157,16 @@ void Instance::init(const Configuration &configuration) {
 
 	const auto initialBits(configuration.initialBits);
 	_sieveWorkers = configuration.sieveWorkers;
+	const double primorialLog2Approx(static_cast<double>(configuration.initialTargetBits) - 40.);
 	if (_sieveWorkers == 0) {
-		double proportion;
-		if (_pattern.size() >= 7) proportion = 0.85 - initialBits/1920.;
-		else if (_pattern.size() == 6) proportion = 0.75 - initialBits/1792.;
-		else if (_pattern.size() == 5) proportion = 0.7 - initialBits/1280.;
-		else if (_pattern.size() == 4) proportion = 0.5 - initialBits/1280.;
-		else proportion = 0.;
-		if (proportion < 0.) proportion = 0.;
+		double proportion(0.);
+		if (_pattern.size() >= 10) proportion = std::exp(-0.00053*primorialLog2Approx);
+		else if (_pattern.size() == 9) proportion = std::exp(-0.00065*primorialLog2Approx);
+		else if (_pattern.size() == 8) proportion = std::exp(-0.00088*primorialLog2Approx);
+		else if (_pattern.size() == 7) proportion = std::exp(-0.0013*primorialLog2Approx);
+		else if (_pattern.size() == 6) proportion = std::exp(-0.0021*primorialLog2Approx);
+		else if (_pattern.size() == 5) proportion = std::exp(-0.0037*primorialLog2Approx);
+		else if (_pattern.size() == 4) proportion = std::exp(-0.0059*primorialLog2Approx);
 		if (proportion > 1.) proportion = 1.;
 		_sieveWorkers = std::ceil(proportion*static_cast<double>(_threads));
 	}
@@ -175,14 +177,16 @@ void Instance::init(const Configuration &configuration) {
 
 	_primeTableLimit = configuration.primeTableLimit;
 	if (_primeTableLimit == 0) {
-		uint64_t primeTableLimitMax(2147483648ULL);
-		if (sysInfo.getPhysicalMemory() < 536870912ULL)
+		uint64_t primeTableLimitMax(4294967296ULL);
+		if (sysInfo.getPhysicalMemory() == 0ULL) // Could not detect Ram, reasonable default for a machine with 4 GiB.
+			primeTableLimitMax = 1073741824ULL;
+		else if (sysInfo.getPhysicalMemory() < 536870912ULL) // A Vintage Computer, maybe...
 			primeTableLimitMax = 67108864ULL;
-		else if (sysInfo.getPhysicalMemory() < 17179869184ULL)
+		else if (sysInfo.getPhysicalMemory() < 34359738368ULL)
 			primeTableLimitMax = sysInfo.getPhysicalMemory()/8ULL;
 		_primeTableLimit = std::pow(initialBits, 6.)/std::pow(2., 3.*static_cast<double>(_pattern.size()) + 7.);
-		if (_threads > 16) {
-			_primeTableLimit *= 16;
+		if (_threads > 32) {
+			_primeTableLimit *= 32;
 			_primeTableLimit /= static_cast<double>(_threads);
 		}
 		_primeTableLimit = std::min(_primeTableLimit, primeTableLimitMax);