cryptocore: prefetch nonces in 512-byte blocks

On my machine, reading 512-byte blocks from /dev/urandom
(same via getentropy syscall) is a lot faster in terms of
throughput:

Blocksize    Throughput
 16          28.18 MB/s
512          83.75 MB/s

For a single-threaded streaming write, this drops the CPU usage of
nonceGenerator.Get to almost 1/3:

        flat  flat%   sum%        cum   cum%
Before     0     0% 95.08%      0.35s  2.92%  github.com/rfjakob/gocryptfs/internal/cryptocore.(*nonceGenerator).Get
After  0.01s 0.092% 92.34%      0.13s  1.20%  github.com/rfjakob/gocryptfs/internal/cryptocore.(*nonceGenerator).Get

This change makes the nonce reading single-threaded, which may
hurt massively-parallel writes.

1 files changed, 50 insertions, 0 deletions

diff --git a/internal/cryptocore/randprefetch.go b/internal/cryptocore/randprefetch.go
new file mode 100644
index 0000000..8825a05
--- /dev/null
+++ b/internal/cryptocore/randprefetch.go
@@ -0,0 +1,50 @@
+package cryptocore
+
+import (
+	"bytes"
+	"log"
+	"sync"
+)
+
+/*
+Number of bytes to prefetch.
+
+512 looks like a good compromise between throughput and latency:
+Benchmark16-2      	 3000000	       567 ns/op	  28.18 MB/s
+Benchmark64-2      	 5000000	       293 ns/op	  54.51 MB/s
+Benchmark128-2     	10000000	       220 ns/op	  72.48 MB/s
+Benchmark256-2     	10000000	       210 ns/op	  76.17 MB/s
+Benchmark512-2     	10000000	       191 ns/op	  83.75 MB/s
+Benchmark1024-2    	10000000	       171 ns/op	  93.48 MB/s
+Benchmark2048-2    	10000000	       165 ns/op	  96.45 MB/s
+Benchmark4096-2    	10000000	       165 ns/op	  96.58 MB/s
+Benchmark40960-2   	10000000	       147 ns/op	 108.82 MB/s
+*/
+const prefetchN = 512
+
+type randPrefetcherT struct {
+	sync.Mutex
+	buf bytes.Buffer
+}
+
+func (r *randPrefetcherT) read(want int) (out []byte) {
+	out = make([]byte, want)
+	r.Lock()
+	// Note: don't use defer, it slows us down!
+	have, err := r.buf.Read(out)
+	if have == want && err == nil {
+		r.Unlock()
+		return out
+	}
+	// Buffer was empty -> re-fill
+	r.buf.Reset()
+	r.buf.Write(RandBytes(prefetchN))
+	have, err = r.buf.Read(out)
+	if have != want || err != nil {
+		log.Panicf("randPrefetcher could not satisfy read: have=%d want=%d err=%v", have, want, err)
+	}
+	r.Unlock()
+	return out
+}
+
+var randPrefetcher randPrefetcherT