Add benchmark for plotting (paritytech#292)

* Add benchmark for plotting as an example

* Change arguments from megabytes to pieces

* Make benchmark a cargo benchmark without harness

Co-authored-by: Nazar Mokrynskyi <nazar@mokrynskyi.com>

crates/subspace-farmer/Cargo.toml

@@ -62,3 +62,7 @@ default = []
 cuda = [
     "subspace-solving/cuda",
 ]
+
+[[bench]]
+name = "plot-write"
+harness = false

crates/subspace-farmer/benches/plot-write.rs

@@ -0,0 +1,34 @@
+use std::sync::Arc;
+
+use rand::prelude::*;
+use subspace_core_primitives::PIECE_SIZE;
+use subspace_farmer::Plot;
+use tempfile::TempDir;
+
+#[tokio::main]
+async fn main() {
+    let batch_size = 4096; // 16M
+    let piece_count = 2u64.pow(30); // 1G
+    let base_directory = TempDir::new_in(std::env::current_dir().unwrap()).unwrap();
+
+    let mut pieces = Vec::with_capacity(batch_size as usize * PIECE_SIZE);
+    pieces.resize(batch_size as usize * PIECE_SIZE, 0u8);
+    rand::thread_rng().fill(&mut pieces[..]);
+    let pieces = Arc::new(pieces.try_into().unwrap());
+
+    let plot = Plot::open_or_create(&base_directory).unwrap();
+
+    let start = std::time::Instant::now();
+
+    for index in (0..piece_count / batch_size).map(|i| i * batch_size) {
+        plot.write_many(Arc::clone(&pieces), index as u64).unwrap();
+    }
+    drop(plot);
+
+    let took = start.elapsed();
+    let write_size = piece_count * PIECE_SIZE as u64 / 1024 / 1024;
+    eprintln!(
+        "Writing {write_size}M to disk took {took:?}. Speed is around {:.2} M/s",
+        write_size as f64 / took.as_secs_f64()
+    );
+}

crates/subspace-farmer/src/plot.rs

@@ -184,7 +184,7 @@ impl Plot {
     }
 
     /// Writes a piece/s to the plot by index, will overwrite if piece exists (updates)
-    pub(crate) fn write_many(
+    pub fn write_many(
         &self,
         encodings: Arc<FlatPieces>,
         first_index: PieceIndex,