Merge pull request #19328 from karalabe/preload

core: prefetch next block state concurrently

cmd/geth/main.go

@@ -102,6 +102,7 @@ var (
 		utils.CacheDatabaseFlag,
 		utils.CacheTrieFlag,
 		utils.CacheGCFlag,
+		utils.CacheNoPrefetchFlag,
 		utils.ListenPortFlag,
 		utils.MaxPeersFlag,
 		utils.MaxPendingPeersFlag,

cmd/geth/usage.go

@@ -139,6 +139,7 @@ var AppHelpFlagGroups = []flagGroup{
 			utils.CacheDatabaseFlag,
 			utils.CacheTrieFlag,
 			utils.CacheGCFlag,
+			utils.CacheNoPrefetchFlag,
 		},
 	},
 	{

cmd/utils/flags.go

@@ -349,6 +349,10 @@ var (
 		Usage: "Percentage of cache memory allowance to use for trie pruning (default = 25% full mode, 0% archive mode)",
 		Value: 25,
 	}
+	CacheNoPrefetchFlag = cli.BoolFlag{
+		Name:  "cache.noprefetch",
+		Usage: "Disable heuristic state prefetch during block import (less CPU and disk IO, more time waiting for data)",
+	}
 	// Miner settings
 	MiningEnabledFlag = cli.BoolFlag{
 		Name:  "mine",
@@ -1336,6 +1340,7 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *eth.Config) {
 		Fatalf("--%s must be either 'full' or 'archive'", GCModeFlag.Name)
 	}
 	cfg.NoPruning = ctx.GlobalString(GCModeFlag.Name) == "archive"
+	cfg.NoPrefetch = ctx.GlobalBool(CacheNoPrefetchFlag.Name)
 
 	if ctx.GlobalIsSet(CacheFlag.Name) || ctx.GlobalIsSet(CacheTrieFlag.Name) {
 		cfg.TrieCleanCache = ctx.GlobalInt(CacheFlag.Name) * ctx.GlobalInt(CacheTrieFlag.Name) / 100
@@ -1595,9 +1600,10 @@ func MakeChain(ctx *cli.Context, stack *node.Node) (chain *core.BlockChain, chai
 		Fatalf("--%s must be either 'full' or 'archive'", GCModeFlag.Name)
 	}
 	cache := &core.CacheConfig{
-		Disabled:       ctx.GlobalString(GCModeFlag.Name) == "archive",
 		TrieCleanLimit:      eth.DefaultConfig.TrieCleanCache,
+		TrieCleanNoPrefetch: ctx.GlobalBool(CacheNoPrefetchFlag.Name),
 		TrieDirtyLimit:      eth.DefaultConfig.TrieDirtyCache,
+		TrieDirtyDisabled:   ctx.GlobalString(GCModeFlag.Name) == "archive",
 		TrieTimeLimit:       eth.DefaultConfig.TrieTimeout,
 	}
 	if ctx.GlobalIsSet(CacheFlag.Name) || ctx.GlobalIsSet(CacheTrieFlag.Name) {

core/blockchain.go

@@ -62,6 +62,9 @@ var (
 	blockExecutionTimer  = metrics.NewRegisteredTimer("chain/execution", nil)
 	blockWriteTimer      = metrics.NewRegisteredTimer("chain/write", nil)
 
+	blockPrefetchExecuteTimer   = metrics.NewRegisteredTimer("chain/prefetch/executes", nil)
+	blockPrefetchInterruptMeter = metrics.NewRegisteredMeter("chain/prefetch/interrupts", nil)
+
 	ErrNoGenesis = errors.New("Genesis not found in chain")
 )
 
@@ -87,9 +90,10 @@ const (
 // CacheConfig contains the configuration values for the trie caching/pruning
 // that's resident in a blockchain.
 type CacheConfig struct {
-	Disabled       bool          // Whether to disable trie write caching (archive node)
 	TrieCleanLimit      int           // Memory allowance (MB) to use for caching trie nodes in memory
+	TrieCleanNoPrefetch bool          // Whether to disable heuristic state prefetching for followup blocks
 	TrieDirtyLimit      int           // Memory limit (MB) at which to start flushing dirty trie nodes to disk
+	TrieDirtyDisabled   bool          // Whether to disable trie write caching and GC altogether (archive node)
 	TrieTimeLimit       time.Duration // Time limit after which to flush the current in-memory trie to disk
 }
 
@@ -126,7 +130,6 @@ type BlockChain struct {
 	genesisBlock  *types.Block
 
 	chainmu sync.RWMutex // blockchain insertion lock
-	procmu  sync.RWMutex // block processor lock
 
 	checkpoint       int          // checkpoint counts towards the new checkpoint
 	currentBlock     atomic.Value // Current head of the block chain
@@ -146,8 +149,9 @@ type BlockChain struct {
 	wg            sync.WaitGroup // chain processing wait group for shutting down
 
 	engine     consensus.Engine
-	processor Processor // block processor interface
+	validator  Validator  // Block and state validator interface
-	validator Validator // block and state validator interface
+	prefetcher Prefetcher // Block state prefetcher interface
+	processor  Processor  // Block transaction processor interface
 	vmConfig   vm.Config
 
 	badBlocks      *lru.Cache              // Bad block cache
@@ -189,8 +193,9 @@ func NewBlockChain(db ethdb.Database, cacheConfig *CacheConfig, chainConfig *par
 		vmConfig:       vmConfig,
 		badBlocks:      badBlocks,
 	}
-	bc.SetValidator(NewBlockValidator(chainConfig, bc, engine))
+	bc.validator = NewBlockValidator(chainConfig, bc, engine)
-	bc.SetProcessor(NewStateProcessor(chainConfig, bc, engine))
+	bc.prefetcher = newStatePrefetcher(chainConfig, bc, engine)
+	bc.processor = NewStateProcessor(chainConfig, bc, engine)
 
 	var err error
 	bc.hc, err = NewHeaderChain(db, chainConfig, engine, bc.getProcInterrupt)
@@ -381,31 +386,13 @@ func (bc *BlockChain) CurrentFastBlock() *types.Block {
 	return bc.currentFastBlock.Load().(*types.Block)
 }
 
-// SetProcessor sets the processor required for making state modifications.
-func (bc *BlockChain) SetProcessor(processor Processor) {
-	bc.procmu.Lock()
-	defer bc.procmu.Unlock()
-	bc.processor = processor
-}
-
-// SetValidator sets the validator which is used to validate incoming blocks.
-func (bc *BlockChain) SetValidator(validator Validator) {
-	bc.procmu.Lock()
-	defer bc.procmu.Unlock()
-	bc.validator = validator
-}
-
 // Validator returns the current validator.
 func (bc *BlockChain) Validator() Validator {
-	bc.procmu.RLock()
-	defer bc.procmu.RUnlock()
 	return bc.validator
 }
 
 // Processor returns the current processor.
 func (bc *BlockChain) Processor() Processor {
-	bc.procmu.RLock()
-	defer bc.procmu.RUnlock()
 	return bc.processor
 }
 
@@ -722,7 +709,7 @@ func (bc *BlockChain) Stop() {
 	//  - HEAD:     So we don't need to reprocess any blocks in the general case
 	//  - HEAD-1:   So we don't do large reorgs if our HEAD becomes an uncle
 	//  - HEAD-127: So we have a hard limit on the number of blocks reexecuted
-	if !bc.cacheConfig.Disabled {
+	if !bc.cacheConfig.TrieDirtyDisabled {
 		triedb := bc.stateCache.TrieDB()
 
 		for _, offset := range []uint64{0, 1, triesInMemory - 1} {
@@ -982,7 +969,7 @@ func (bc *BlockChain) writeBlockWithState(block *types.Block, receipts []*types.
 	triedb := bc.stateCache.TrieDB()
 
 	// If we're running an archive node, always flush
-	if bc.cacheConfig.Disabled {
+	if bc.cacheConfig.TrieDirtyDisabled {
 		if err := triedb.Commit(root, false); err != nil {
 			return NonStatTy, err
 		}
@@ -1147,7 +1134,7 @@ func (bc *BlockChain) InsertChain(chain types.Blocks) (int, error) {
 // is imported, but then new canon-head is added before the actual sidechain
 // completes, then the historic state could be pruned again
 func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, []interface{}, []*types.Log, error) {
-	// If the chain is terminating, don't even bother starting u
+	// If the chain is terminating, don't even bother starting up
 	if atomic.LoadInt32(&bc.procInterrupt) == 1 {
 		return 0, nil, nil, nil
 	}
@@ -1175,7 +1162,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, []
 	defer close(abort)
 
 	// Peek the error for the first block to decide the directing import logic
-	it := newInsertIterator(chain, results, bc.Validator())
+	it := newInsertIterator(chain, results, bc.validator)
 
 	block, err := it.next()
 
@@ -1238,54 +1225,76 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, []
 		if parent == nil {
 			parent = bc.GetHeader(block.ParentHash(), block.NumberU64()-1)
 		}
-		state, err := state.New(parent.Root, bc.stateCache)
+		statedb, err := state.New(parent.Root, bc.stateCache)
 		if err != nil {
 			return it.index, events, coalescedLogs, err
 		}
-		// Process block using the parent state as reference point.
+		// If we have a followup block, run that against the current state to pre-cache
+		// transactions and probabilistically some of the account/storage trie nodes.
+		var followupInterrupt uint32
+
+		if !bc.cacheConfig.TrieCleanNoPrefetch {
+			if followup, err := it.peek(); followup != nil && err == nil {
+				go func(start time.Time) {
+					throwaway, _ := state.New(parent.Root, bc.stateCache)
+					bc.prefetcher.Prefetch(followup, throwaway, bc.vmConfig, &followupInterrupt)
+
+					blockPrefetchExecuteTimer.Update(time.Since(start))
+					if atomic.LoadUint32(&followupInterrupt) == 1 {
+						blockPrefetchInterruptMeter.Mark(1)
+					}
+				}(time.Now())
+			}
+		}
+		// Process block using the parent state as reference point
 		substart := time.Now()
-		receipts, logs, usedGas, err := bc.processor.Process(block, state, bc.vmConfig)
+		receipts, logs, usedGas, err := bc.processor.Process(block, statedb, bc.vmConfig)
 		if err != nil {
 			bc.reportBlock(block, receipts, err)
+			atomic.StoreUint32(&followupInterrupt, 1)
 			return it.index, events, coalescedLogs, err
 		}
 		// Update the metrics touched during block processing
-		accountReadTimer.Update(state.AccountReads)     // Account reads are complete, we can mark them
+		accountReadTimer.Update(statedb.AccountReads)     // Account reads are complete, we can mark them
-		storageReadTimer.Update(state.StorageReads)     // Storage reads are complete, we can mark them
+		storageReadTimer.Update(statedb.StorageReads)     // Storage reads are complete, we can mark them
-		accountUpdateTimer.Update(state.AccountUpdates) // Account updates are complete, we can mark them
+		accountUpdateTimer.Update(statedb.AccountUpdates) // Account updates are complete, we can mark them
-		storageUpdateTimer.Update(state.StorageUpdates) // Storage updates are complete, we can mark them
+		storageUpdateTimer.Update(statedb.StorageUpdates) // Storage updates are complete, we can mark them
 
-		triehash := state.AccountHashes + state.StorageHashes // Save to not double count in validation
+		triehash := statedb.AccountHashes + statedb.StorageHashes // Save to not double count in validation
-		trieproc := state.AccountReads + state.AccountUpdates
+		trieproc := statedb.AccountReads + statedb.AccountUpdates
-		trieproc += state.StorageReads + state.StorageUpdates
+		trieproc += statedb.StorageReads + statedb.StorageUpdates
 
 		blockExecutionTimer.Update(time.Since(substart) - trieproc - triehash)
 
 		// Validate the state using the default validator
 		substart = time.Now()
-		if err := bc.Validator().ValidateState(block, state, receipts, usedGas); err != nil {
+		if err := bc.validator.ValidateState(block, statedb, receipts, usedGas); err != nil {
 			bc.reportBlock(block, receipts, err)
+			atomic.StoreUint32(&followupInterrupt, 1)
 			return it.index, events, coalescedLogs, err
 		}
 		proctime := time.Since(start)
 
 		// Update the metrics touched during block validation
-		accountHashTimer.Update(state.AccountHashes) // Account hashes are complete, we can mark them
+		accountHashTimer.Update(statedb.AccountHashes) // Account hashes are complete, we can mark them
-		storageHashTimer.Update(state.StorageHashes) // Storage hashes are complete, we can mark them
+		storageHashTimer.Update(statedb.StorageHashes) // Storage hashes are complete, we can mark them
 
-		blockValidationTimer.Update(time.Since(substart) - (state.AccountHashes + state.StorageHashes - triehash))
+		blockValidationTimer.Update(time.Since(substart) - (statedb.AccountHashes + statedb.StorageHashes - triehash))
 
 		// Write the block to the chain and get the status.
 		substart = time.Now()
-		status, err := bc.writeBlockWithState(block, receipts, state)
+		status, err := bc.writeBlockWithState(block, receipts, statedb)
 		if err != nil {
+			atomic.StoreUint32(&followupInterrupt, 1)
 			return it.index, events, coalescedLogs, err
 		}
-		// Update the metrics touched during block commit
+		atomic.StoreUint32(&followupInterrupt, 1)
-		accountCommitTimer.Update(state.AccountCommits) // Account commits are complete, we can mark them
-		storageCommitTimer.Update(state.StorageCommits) // Storage commits are complete, we can mark them
 
-		blockWriteTimer.Update(time.Since(substart) - state.AccountCommits - state.StorageCommits)
+		// Update the metrics touched during block commit
+		accountCommitTimer.Update(statedb.AccountCommits) // Account commits are complete, we can mark them
+		storageCommitTimer.Update(statedb.StorageCommits) // Storage commits are complete, we can mark them
+
+		blockWriteTimer.Update(time.Since(substart) - statedb.AccountCommits - statedb.StorageCommits)
 		blockInsertTimer.UpdateSince(start)
 
 		switch status {

core/blockchain_insert.go

@@ -80,10 +80,13 @@ func (st *insertStats) report(chain []*types.Block, index int, dirty common.Stor
 
 // insertIterator is a helper to assist during chain import.
 type insertIterator struct {
-	chain     types.Blocks
+	chain types.Blocks // Chain of blocks being iterated over
-	results   <-chan error
+
-	index     int
+	results <-chan error // Verification result sink from the consensus engine
-	validator Validator
+	errors  []error      // Header verification errors for the blocks
+
+	index     int       // Current offset of the iterator
+	validator Validator // Validator to run if verification succeeds
 }
 
 // newInsertIterator creates a new iterator based on the given blocks, which are
@@ -92,6 +95,7 @@ func newInsertIterator(chain types.Blocks, results <-chan error, validator Valid
 	return &insertIterator{
 		chain:     chain,
 		results:   results,
+		errors:    make([]error, 0, len(chain)),
 		index:     -1,
 		validator: validator,
 	}
@@ -100,17 +104,44 @@ func newInsertIterator(chain types.Blocks, results <-chan error, validator Valid
 // next returns the next block in the iterator, along with any potential validation
 // error for that block. When the end is reached, it will return (nil, nil).
 func (it *insertIterator) next() (*types.Block, error) {
+	// If we reached the end of the chain, abort
 	if it.index+1 >= len(it.chain) {
 		it.index = len(it.chain)
 		return nil, nil
 	}
+	// Advance the iterator and wait for verification result if not yet done
 	it.index++
-	if err := <-it.results; err != nil {
+	if len(it.errors) <= it.index {
-		return it.chain[it.index], err
+		it.errors = append(it.errors, <-it.results)
 	}
+	if it.errors[it.index] != nil {
+		return it.chain[it.index], it.errors[it.index]
+	}
+	// Block header valid, run body validation and return
 	return it.chain[it.index], it.validator.ValidateBody(it.chain[it.index])
 }
 
+// peek returns the next block in the iterator, along with any potential validation
+// error for that block, but does **not** advance the iterator.
+//
+// Both header and body validation errors (nil too) is cached into the iterator
+// to avoid duplicating work on the following next() call.
+func (it *insertIterator) peek() (*types.Block, error) {
+	// If we reached the end of the chain, abort
+	if it.index+1 >= len(it.chain) {
+		return nil, nil
+	}
+	// Wait for verification result if not yet done
+	if len(it.errors) <= it.index+1 {
+		it.errors = append(it.errors, <-it.results)
+	}
+	if it.errors[it.index+1] != nil {
+		return it.chain[it.index+1], it.errors[it.index+1]
+	}
+	// Block header valid, ignore body validation since we don't have a parent anyway
+	return it.chain[it.index+1], nil
+}
+
 // previous returns the previous header that was being processed, or nil.
 func (it *insertIterator) previous() *types.Header {
 	if it.index < 1 {

core/blockchain_test.go

@@ -144,7 +144,7 @@ func testBlockChainImport(chain types.Blocks, blockchain *BlockChain) error {
 		if err != nil {
 			return err
 		}
-		receipts, _, usedGas, err := blockchain.Processor().Process(block, statedb, vm.Config{})
+		receipts, _, usedGas, err := blockchain.processor.Process(block, statedb, vm.Config{})
 		if err != nil {
 			blockchain.reportBlock(block, receipts, err)
 			return err

core/state_prefetcher.go

@@ -0,0 +1,85 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package core
+
+import (
+	"sync/atomic"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/consensus"
+	"github.com/ethereum/go-ethereum/core/state"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/core/vm"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+// statePrefetcher is a basic Prefetcher, which blindly executes a block on top
+// of an arbitrary state with the goal of prefetching potentially useful state
+// data from disk before the main block processor start executing.
+type statePrefetcher struct {
+	config *params.ChainConfig // Chain configuration options
+	bc     *BlockChain         // Canonical block chain
+	engine consensus.Engine    // Consensus engine used for block rewards
+}
+
+// newStatePrefetcher initialises a new statePrefetcher.
+func newStatePrefetcher(config *params.ChainConfig, bc *BlockChain, engine consensus.Engine) *statePrefetcher {
+	return &statePrefetcher{
+		config: config,
+		bc:     bc,
+		engine: engine,
+	}
+}
+
+// Prefetch processes the state changes according to the Ethereum rules by running
+// the transaction messages using the statedb, but any changes are discarded. The
+// only goal is to pre-cache transaction signatures and state trie nodes.
+func (p *statePrefetcher) Prefetch(block *types.Block, statedb *state.StateDB, cfg vm.Config, interrupt *uint32) {
+	var (
+		header  = block.Header()
+		gaspool = new(GasPool).AddGas(block.GasLimit())
+	)
+	// Iterate over and process the individual transactions
+	for i, tx := range block.Transactions() {
+		// If block precaching was interrupted, abort
+		if interrupt != nil && atomic.LoadUint32(interrupt) == 1 {
+			return
+		}
+		// Block precaching permitted to continue, execute the transaction
+		statedb.Prepare(tx.Hash(), block.Hash(), i)
+		if err := precacheTransaction(p.config, p.bc, nil, gaspool, statedb, header, tx, cfg); err != nil {
+			return // Ugh, something went horribly wrong, bail out
+		}
+	}
+}
+
+// precacheTransaction attempts to apply a transaction to the given state database
+// and uses the input parameters for its environment. The goal is not to execute
+// the transaction successfully, rather to warm up touched data slots.
+func precacheTransaction(config *params.ChainConfig, bc ChainContext, author *common.Address, gaspool *GasPool, statedb *state.StateDB, header *types.Header, tx *types.Transaction, cfg vm.Config) error {
+	// Convert the transaction into an executable message and pre-cache its sender
+	msg, err := tx.AsMessage(types.MakeSigner(config, header.Number))
+	if err != nil {
+		return err
+	}
+	// Create the EVM and execute the transaction
+	context := NewEVMContext(msg, header, bc, author)
+	vm := vm.NewEVM(context, statedb, config, cfg)
+
+	_, _, _, err = ApplyMessage(vm, msg, gaspool)
+	return err
+}

core/types.go

@@ -25,7 +25,6 @@ import (
 // Validator is an interface which defines the standard for block validation. It
 // is only responsible for validating block contents, as the header validation is
 // done by the specific consensus engines.
-//
 type Validator interface {
 	// ValidateBody validates the given block's content.
 	ValidateBody(block *types.Block) error
@@ -35,12 +34,18 @@ type Validator interface {
 	ValidateState(block *types.Block, state *state.StateDB, receipts types.Receipts, usedGas uint64) error
 }
 
+// Prefetcher is an interface for pre-caching transaction signatures and state.
+type Prefetcher interface {
+	// Prefetch processes the state changes according to the Ethereum rules by running
+	// the transaction messages using the statedb, but any changes are discarded. The
+	// only goal is to pre-cache transaction signatures and state trie nodes.
+	Prefetch(block *types.Block, statedb *state.StateDB, cfg vm.Config, interrupt *uint32)
+}
+
 // Processor is an interface for processing blocks using a given initial state.
-//
-// Process takes the block to be processed and the statedb upon which the
-// initial state is based. It should return the receipts generated, amount
-// of gas used in the process and return an error if any of the internal rules
-// failed.
 type Processor interface {
+	// Process processes the state changes according to the Ethereum rules by running
+	// the transaction messages using the statedb and applying any rewards to both
+	// the processor (coinbase) and any included uncles.
 	Process(block *types.Block, statedb *state.StateDB, cfg vm.Config) (types.Receipts, []*types.Log, uint64, error)
 }

core/vm/interpreter.go

@@ -28,24 +28,15 @@ import (
 
 // Config are the configuration options for the Interpreter
 type Config struct {
-	// Debug enabled debugging Interpreter options
+	Debug                   bool   // Enables debugging
-	Debug bool
+	Tracer                  Tracer // Opcode logger
-	// Tracer is the op code logger
+	NoRecursion             bool   // Disables call, callcode, delegate call and create
-	Tracer Tracer
+	EnablePreimageRecording bool   // Enables recording of SHA3/keccak preimages
-	// NoRecursion disabled Interpreter call, callcode,
-	// delegate call and create.
-	NoRecursion bool
-	// Enable recording of SHA3/keccak preimages
-	EnablePreimageRecording bool
-	// JumpTable contains the EVM instruction table. This
-	// may be left uninitialised and will be set to the default
-	// table.
-	JumpTable [256]operation
 
-	// Type of the EWASM interpreter
+	JumpTable [256]operation // EVM instruction table, automatically populated if unset
-	EWASMInterpreter string
+
-	// Type of the EVM interpreter
+	EWASMInterpreter string // External EWASM interpreter options
-	EVMInterpreter string
+	EVMInterpreter   string // External EVM interpreter options
 }
 
 // Interpreter is used to run Ethereum based contracts and will utilise the

eth/backend.go

@@ -165,7 +165,13 @@ func New(ctx *node.ServiceContext, config *Config) (*Ethereum, error) {
 			EWASMInterpreter:        config.EWASMInterpreter,
 			EVMInterpreter:          config.EVMInterpreter,
 		}
-		cacheConfig = &core.CacheConfig{Disabled: config.NoPruning, TrieCleanLimit: config.TrieCleanCache, TrieDirtyLimit: config.TrieDirtyCache, TrieTimeLimit: config.TrieTimeout}
+		cacheConfig = &core.CacheConfig{
+			TrieCleanLimit:      config.TrieCleanCache,
+			TrieCleanNoPrefetch: config.NoPrefetch,
+			TrieDirtyLimit:      config.TrieDirtyCache,
+			TrieDirtyDisabled:   config.NoPruning,
+			TrieTimeLimit:       config.TrieTimeout,
+		}
 	)
 	eth.blockchain, err = core.NewBlockChain(chainDb, cacheConfig, chainConfig, eth.engine, vmConfig, eth.shouldPreserve)
 	if err != nil {

eth/config.go

@@ -92,7 +92,9 @@ type Config struct {
 	// Protocol options
 	NetworkId uint64 // Network ID to use for selecting peers to connect to
 	SyncMode  downloader.SyncMode
-	NoPruning bool
+
+	NoPruning  bool // Whether to disable pruning and flush everything to disk
+	NoPrefetch bool // Whether to disable prefetching and only load state on demand
 
 	// Whitelist of required block number -> hash values to accept
 	Whitelist map[uint64]common.Hash `toml:"-"`