Only send sigverify to GPU if batch size is >64

Seems to be a decent crossover point for Xeon E5-2620 v4 8c,16t vs. nvidia 1080ti

benches/sigverify.rs

@@ -0,0 +1,36 @@
+#[macro_use]
+extern crate criterion;
+extern crate bincode;
+extern crate rayon;
+extern crate solana;
+
+use criterion::{Bencher, Criterion};
+use solana::packet::{to_packets, PacketRecycler};
+use solana::sigverify;
+use solana::transaction::test_tx;
+
+fn bench_sig_verify(bencher: &mut Bencher) {
+    let tx = test_tx();
+
+    // generate packet vector
+    let packet_recycler = PacketRecycler::default();
+    let batches = to_packets(&packet_recycler, &vec![tx; 128]);
+
+    // verify packets
+    bencher.iter(|| {
+        let _ans = sigverify::ed25519_verify(&batches);
+    })
+}
+
+fn bench(criterion: &mut Criterion) {
+    criterion.bench_function("bench_sig_verify", |bencher| {
+        bench_sig_verify(bencher);
+    });
+}
+
+criterion_group!(
+    name = benches;
+    config = Criterion::default().sample_size(2);
+    targets = bench
+);
+criterion_main!(benches);