From 0475d3893a4295c81ff6e999b6abfbc61d320fcd Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Thu, 21 Dec 2023 11:49:56 +0000
Subject: [PATCH] squashfs: cache the last block decompressed in the file for
 10x speedup

Before this change, we would spend an awful lot of CPU time reading
and decompressing the same block to only read a small part from it.

This change caches the last block used in the file handle and for
typical block sizes used in reads this makes a 10x difference in
throughput.
---
 filesystem/squashfs/file.go | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/filesystem/squashfs/file.go b/filesystem/squashfs/file.go
index e7c5790c..1b77d969 100644
--- a/filesystem/squashfs/file.go
+++ b/filesystem/squashfs/file.go
@@ -13,10 +13,12 @@ import (
 //	include all of the data
 type File struct {
 	*extendedFile
-	isReadWrite bool
-	isAppend    bool
-	offset      int64
-	filesystem  *FileSystem
+	isReadWrite   bool
+	isAppend      bool
+	offset        int64
+	filesystem    *FileSystem
+	blockLocation int64  // the position of the last block decompressed
+	block         []byte // the actual last block decompressed
 }
 
 // Read reads up to len(b) bytes from the File.
@@ -99,9 +101,19 @@ func (fl *File) Read(b []byte) (int, error) {
 			if int64(block.size) > fs.blocksize {
 				return read, fmt.Errorf("unexpected block.size=%d > fs.blocksize=%d", block.size, fs.blocksize)
 			}
-			input, err := fs.readBlock(location, block.compressed, block.size)
-			if err != nil {
-				return read, fmt.Errorf("error reading data block %d from squashfs: %v", i, err)
+			var input []byte
+			if fl.blockLocation == location && fl.block != nil {
+				// Read last block from cache
+				input = fl.block
+			} else {
+				var err error
+				input, err = fs.readBlock(location, block.compressed, block.size)
+				if err != nil {
+					return read, fmt.Errorf("error reading data block %d from squashfs: %v", i, err)
+				}
+				// Cache the last block
+				fl.blockLocation = location
+				fl.block = input
 			}
 			outputBlock(input)
 		}