consensus/ethash: use DAGs for remote mining, generate async

consensus/ethash/consensus.go

@@ -461,6 +461,13 @@ func calcDifficultyFrontier(time uint64, parent *types.Header) *big.Int {
 // VerifySeal implements consensus.Engine, checking whether the given block satisfies
 // the PoW difficulty requirements.
 func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Header) error {
+	return ethash.verifySeal(chain, header, false)
+}
+
+// verifySeal checks whether a block satisfies the PoW difficulty requirements,
+// either using the usual ethash cache for it, or alternatively using a full DAG
+// to make remote mining fast.
+func (ethash *Ethash) verifySeal(chain consensus.ChainReader, header *types.Header, fulldag bool) error {
 	// If we're running a fake PoW, accept any seal as valid
 	if ethash.config.PowMode == ModeFake || ethash.config.PowMode == ModeFullFake {
 		time.Sleep(ethash.fakeDelay)
@@ -471,25 +478,48 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	}
 	// If we're running a shared PoW, delegate verification to it
 	if ethash.shared != nil {
-		return ethash.shared.VerifySeal(chain, header)
+		return ethash.shared.verifySeal(chain, header, fulldag)
 	}
 	// Ensure that we have a valid difficulty for the block
 	if header.Difficulty.Sign() <= 0 {
 		return errInvalidDifficulty
 	}
-	// Recompute the digest and PoW value and verify against the header
+	// Recompute the digest and PoW values
 	number := header.Number.Uint64()
 
-	cache := ethash.cache(number)
-	size := datasetSize(number)
-	if ethash.config.PowMode == ModeTest {
-		size = 32 * 1024
+	var (
+		digest []byte
+		result []byte
+	)
+	// If fast-but-heavy PoW verification was requested, use an ethash dataset
+	if fulldag {
+		dataset := ethash.dataset(number, true)
+		if dataset.generated() {
+			digest, result = hashimotoFull(dataset.dataset, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
+
+			// Datasets are unmapped in a finalizer. Ensure that the dataset stays alive
+			// until after the call to hashimotoFull so it's not unmapped while being used.
+			runtime.KeepAlive(dataset)
+		} else {
+			// Dataset not yet generated, don't hang, use a cache instead
+			fulldag = false
+		}
 	}
-	digest, result := hashimotoLight(size, cache.cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
-	// Caches are unmapped in a finalizer. Ensure that the cache stays live
-	// until after the call to hashimotoLight so it's not unmapped while being used.
-	runtime.KeepAlive(cache)
+	// If slow-but-light PoW verification was requested (or DAG not yet ready), use an ethash cache
+	if !fulldag {
+		cache := ethash.cache(number)
+
+		size := datasetSize(number)
+		if ethash.config.PowMode == ModeTest {
+			size = 32 * 1024
+		}
+		digest, result = hashimotoLight(size, cache.cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
 
+		// Caches are unmapped in a finalizer. Ensure that the cache stays alive
+		// until after the call to hashimotoLight so it's not unmapped while being used.
+		runtime.KeepAlive(cache)
+	}
+	// Verify the calculated values against the ones provided in the header
 	if !bytes.Equal(header.MixDigest[:], digest) {
 		return errInvalidMixDigest
 	}

consensus/ethash/ethash.go

@@ -29,6 +29,7 @@ import (
 	"runtime"
 	"strconv"
 	"sync"
+	"sync/atomic"
 	"time"
 	"unsafe"
 
@@ -281,6 +282,7 @@ type dataset struct {
 	mmap    mmap.MMap // Memory map itself to unmap before releasing
 	dataset []uint32  // The actual cache data content
 	once    sync.Once // Ensures the cache is generated only once
+	done    uint32    // Atomic flag to determine generation status
 }
 
 // newDataset creates a new ethash mining dataset and returns it as a plain Go
@@ -292,6 +294,9 @@ func newDataset(epoch uint64) interface{} {
 // generate ensures that the dataset content is generated before use.
 func (d *dataset) generate(dir string, limit int, test bool) {
 	d.once.Do(func() {
+		// Mark the dataset generated after we're done. This is needed for remote
+		defer atomic.StoreUint32(&d.done, 1)
+
 		csize := cacheSize(d.epoch*epochLength + 1)
 		dsize := datasetSize(d.epoch*epochLength + 1)
 		seed := seedHash(d.epoch*epochLength + 1)
@@ -306,6 +311,8 @@ func (d *dataset) generate(dir string, limit int, test bool) {
 
 			d.dataset = make([]uint32, dsize/4)
 			generateDataset(d.dataset, d.epoch, cache)
+
+			return
 		}
 		// Disk storage is needed, this will get fancy
 		var endian string
@@ -348,6 +355,13 @@ func (d *dataset) generate(dir string, limit int, test bool) {
 	})
 }
 
+// generated returns whether this particular dataset finished generating already
+// or not (it may not have been started at all). This is useful for remote miners
+// to default to verification caches instead of blocking on DAG generations.
+func (d *dataset) generated() bool {
+	return atomic.LoadUint32(&d.done) == 1
+}
+
 // finalizer closes any file handlers and memory maps open.
 func (d *dataset) finalizer() {
 	if d.mmap != nil {
@@ -589,20 +603,34 @@ func (ethash *Ethash) cache(block uint64) *cache {
 // dataset tries to retrieve a mining dataset for the specified block number
 // by first checking against a list of in-memory datasets, then against DAGs
 // stored on disk, and finally generating one if none can be found.
-func (ethash *Ethash) dataset(block uint64) *dataset {
+//
+// If async is specified, not only the future but the current DAG is also
+// generates on a background thread.
+func (ethash *Ethash) dataset(block uint64, async bool) *dataset {
+	// Retrieve the requested ethash dataset
 	epoch := block / epochLength
 	currentI, futureI := ethash.datasets.get(epoch)
 	current := currentI.(*dataset)
 
-	// Wait for generation finish.
-	current.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
-
-	// If we need a new future dataset, now's a good time to regenerate it.
-	if futureI != nil {
-		future := futureI.(*dataset)
-		go future.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
+	// If async is specified, generate everything in a background thread
+	if async && !current.generated() {
+		go func() {
+			current.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
+
+			if futureI != nil {
+				future := futureI.(*dataset)
+				future.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
+			}
+		}()
+	} else {
+		// Either blocking generation was requested, or already done
+		current.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
+
+		if futureI != nil {
+			future := futureI.(*dataset)
+			go future.generate(ethash.config.DatasetDir, ethash.config.DatasetsOnDisk, ethash.config.PowMode == ModeTest)
+		}
 	}
-
 	return current
 }
 

consensus/ethash/sealer.go

@@ -114,7 +114,7 @@ func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan s
 		hash    = header.HashNoNonce().Bytes()
 		target  = new(big.Int).Div(two256, header.Difficulty)
 		number  = header.Number.Uint64()
-		dataset = ethash.dataset(number)
+		dataset = ethash.dataset(number, false)
 	)
 	// Start generating random nonces until we abort or find a good one
 	var (
@@ -233,21 +233,22 @@ func (ethash *Ethash) remote(notify []string) {
 			log.Info("Work submitted but none pending", "hash", hash)
 			return false
 		}
-
 		// Verify the correctness of submitted result.
 		header := block.Header()
 		header.Nonce = nonce
 		header.MixDigest = mixDigest
-		if err := ethash.VerifySeal(nil, header); err != nil {
-			log.Warn("Invalid proof-of-work submitted", "hash", hash, "err", err)
+
+		start := time.Now()
+		if err := ethash.verifySeal(nil, header, true); err != nil {
+			log.Warn("Invalid proof-of-work submitted", "hash", hash, "elapsed", time.Since(start), "err", err)
 			return false
 		}
-
 		// Make sure the result channel is created.
 		if ethash.resultCh == nil {
 			log.Warn("Ethash result channel is empty, submitted mining result is rejected")
 			return false
 		}
+		log.Trace("Verified correct proof-of-work", "hash", hash, "elapsed", time.Since(start))
 
 		// Solutions seems to be valid, return to the miner and notify acceptance.
 		select {