Improve benchmarks, apply measurements to code

2 files changed, 64 insertions, 3 deletions

diff --git a/benches/benchmark.rs b/benches/benchmark.rs
index c999811..43d53f7 100644
--- a/benches/benchmark.rs
+++ b/benches/benchmark.rs
@@ -2,6 +2,8 @@ mod utils;
 
 use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion};
 use log_db::*;
+use std::fs::OpenOptions;
+use std::path::Path;
 use tempfile;
 use utils::*;
 
@@ -156,6 +158,58 @@ pub fn get_various_memtable_capacities(c: &mut Criterion) {
     }
 }
 
+fn reverse_read_file_with_various_buffer_sizes(c: &mut Criterion) {
+    let mut group = c.benchmark_group("reverse_read_file_with_various_buffer_sizes");
+    group.sample_size(50);
+
+    // odd powers of 2
+    let buffer_sizes = [128, 512, 2048, 8192, 32768, 131_072, 524_288];
+    const PREFILL_N: usize = 100_000;
+
+    let data_dir_obj = tempfile::tempdir().expect("Failed to get tmpdir");
+    let data_dir = &data_dir_obj
+        .path()
+        .to_str()
+        .expect("Failed to convert tmpdir path to str");
+
+    // Create a db instance for prefilling
+    let mut db = DB::configure()
+        .data_dir(&data_dir)
+        .fields(&vec![
+            (Field::Id, RecordFieldType::Int),
+            (Field::Name, RecordFieldType::String),
+            (Field::Data, RecordFieldType::Bytes),
+        ])
+        .primary_key(Field::Id)
+        .initialize()
+        .expect("Failed to initialize DB");
+
+    prefill_db(&mut db, PREFILL_N).expect("Failed to prefill DB");
+    drop(db);
+
+    for size in buffer_sizes {
+        group.bench_with_input(BenchmarkId::from_parameter(size), &size, |b, &_size| {
+            let mut file = OpenOptions::new()
+                .read(true)
+                .open(Path::new(data_dir).join("db"))
+                .expect("Failed to open log file");
+
+            b.iter(|| {
+                let mut rev_reader = ReverseLogReader::new_with_size(&mut file, size)
+                    .expect("Failed to create ReverseLogReader");
+
+                // This is to avoid optimizing out the loop
+                let mut i = 0;
+                for _ in &mut rev_reader {
+                    i += 1;
+                }
+
+                i
+            });
+        });
+    }
+}
+
 // Register the benchmark group
 criterion_group!(
     benches,
@@ -163,5 +217,6 @@ criterion_group!(
     upsert_write_durability,
     get_from_disk_various_initial_sizes,
     get_various_memtable_capacities,
+    reverse_read_file_with_various_buffer_sizes,
 );
 criterion_main!(benches);
diff --git a/src/reverse_log_reader.rs b/src/reverse_log_reader.rs
index 8a2126f..5d80dba 100644
--- a/src/reverse_log_reader.rs
+++ b/src/reverse_log_reader.rs
@@ -3,6 +3,7 @@ use std::fs::{self};
 use std::io::{self, Read, Seek, SeekFrom};
 
 pub struct ReverseLogReader<'a> {
+    /// The file to read from end to beginning.
     file: &'a mut fs::File,
     /// The internal buffer used to read from the file.
     /// It is populated with the last INTERNAL_BUF_SIZE bytes read from the file
@@ -13,16 +14,21 @@ pub struct ReverseLogReader<'a> {
     /// is populated with the next (= closer to the start of the file) INTERNAL_BUF_SIZE bytes from the file.
     /// Note: This is the index of the next byte to be read from the internal buffer + 1
     internal_pos: usize,
+    /// A flag indicating whether the record separator at the cursor position has been consumed.
+    /// Useful to avoid consuming the separator once when reading until an escape character, and
+    /// a second time when reading a new record and validating it ends in a separator.
     consumed_record_sep: bool,
 }
 
-const INTERNAL_BUF_SIZE: usize = 4096;
+// This value is based on the reverse_read_file_with_various_buffer_sizes benchmark.
+// Greater values yield little to no performance improvement.
+const DEFAULT_INTERNAL_BUF_SIZE: usize = 32768;
 impl<'a> ReverseLogReader<'a> {
     pub fn new(file: &mut fs::File) -> Result<ReverseLogReader, io::Error> {
         file.seek(SeekFrom::End(0))?;
         Ok(ReverseLogReader {
             file,
-            internal_buf: vec![0; INTERNAL_BUF_SIZE],
+            internal_buf: vec![0; DEFAULT_INTERNAL_BUF_SIZE],
             internal_pos: 0,
             consumed_record_sep: false,
         })
@@ -61,7 +67,7 @@ impl<'a> ReverseLogReader<'a> {
         self.consumed_record_sep = false;
 
         let mut result_buf: Vec<u8> = vec![];
-        let mut read_buf = Vec::with_capacity(INTERNAL_BUF_SIZE);
+        let mut read_buf = Vec::with_capacity(self.internal_buf.len());
         loop {
             read_buf.clear();
             let read = self.read_until(ESCAPE_CHARACTER, &mut read_buf)?;