Improving performance of the IG Publisher: I/O operations

The IG Publisher is the reference tool to build FHIR implementation guides. It is a Java application built upon the FHIR Core library, that reads the implementation guide sources and generates a FHIR package and a web documentation. It is a quite complex application, and new versions are released quite regularly.

I recently had to build a large implementation guide, and it took the IG Publisher around 3 hours to complete. The memory usage was quite high (around 20 GiB at peak) and the result was a 2.57 GiB directory. While waiting for the completion, I was wondering if there was a way to reduce the completion time.

The first step to improve the performance of the IG Publisher is to understand what it does, and which operations are the most time-consuming. To do that, I cloned the IG Publisher code from the GitHub repository, quickly wrote a main(String[]) method, and ran it with the profiler included in Intellij IDEA on the CH Core implementation guide. This IG has a good size, not too big, not too small, and it is a good candidate for a performance analysis.

Analysis of the flame graph#

Flame graph of the IG Publisher before optimization
Flame graph of the IG Publisher building an implementation guide, before I/O optimization

My attention was immediately drawn to the fileToString(String) method, which take a significant amount of time (around 7% of the total time). Opening the class confirmed it was used to read the content of files, an operation that should usually be fast. The code looked like:

public class TextFile {
    public static String fileToString(String src) throws FileNotFoundException, IOException  {
        CSFile f = new CSFile(src);
        if (!f.exists()) {
            throw new IOException("File "+src+" not found");
        FileInputStream fs = new FileInputStream(f);
        try {
            return streamToString(fs);
        } finally {

    public static String streamToString(InputStream input) throws IOException  {
        InputStreamReader sr = new InputStreamReader(input, "UTF-8");
        StringBuilder b = new StringBuilder();
        int i = -1;
        while((i = > -1) {
            String s = Character.toString(i);

        return b.toString().replace("\uFEFF", "");

    // [...]

To read a file, the class did:

  1. create a custom File implementation;
  2. open a FileInputStream to read the file byte content;
  3. create a InputStreamReader to convert bytes to UTF-8 chars;
  4. finally, create a StringBuilder to build a string from the read chars.

One issue with that approach is that we are reading the file byte by byte, which is really inefficient because it produces a lot of disk accesses. A better approach would be to read the file in larger chunks, before converting them to UTF-8 strings. Since we want to retrieve the whole file content as a string, we can also read the whole file content in one go.

Another issue is that the InputStreamReader is creating a lot of temporary strings, that will have to be merged later by the StringBuilder. Processing a byte at a time also prevents the code to optimize compact strings: if a string is composed of only ISO-8859-1/Latin-1 characters, there is no need to decode it as UTF-8, we can directly use the bytes as chars (thanks to JEP 254: Compact Strings). That is a fast path that the reader can't take.

Other methods in that class look implemented in a suboptimal way. Let's check their execution time in the generated profile:

TextFile methods execution time before optimization
TextFile methods execution time, before I/O optimization

A few methods are taking a significant amount of time: 19 seconds for the streamToString(InputStream) method, around 5 seconds for the fileToString() methods and 3.5 seconds for the bytesToString(byte[]) method. I was not expecting such long execution times, especially for the last one. Let's work on optimizing that.

Optimize the code and verify the results#

To optimize the fileToString(String) method, the simplest solution is to use the java.nio.file.Files.readString(Path). But while testing the modified code, errors started to appear when reading Excel spreadsheets. After some investigation, I found that the readString(Path) methods failed while reading invalid UTF-8 byte sequences. An almost equivalent method that does not fail in that case is new String(Files.readAllBytes(Path), Charset). Both methods use the readAllBytes(Path) method, but use different UTF-8 decoders.

For many methods, I avoided using InputStream, OutputStream, InputStreamReader and OutputStreamWriter as much as possible. The Java NIO API is quite helpful to write fewer lines of code, while directly getting a quite optimized implementation.

After those changes, it is time to run the profiler again, and compare the results:

Flame graph of the IG Publisher after optimization
Flame graph of the IG Publisher building an implementation guide, after I/O optimization

The fileToString(String) method is barely visible on the flame graph, and it represents now only 0.36% of the total time. It seems that the optimization was successful. To be sure, let's compare the execution time of the methods in the TextFile class after the optimization:

TextFile methods execution time after optimization
TextFile methods execution time, after I/O optimization

All methods are now under the 0.4 seconds threshold, which is a significant improvement. The streamToString(InputStream) method is 51 times faster, the fileToString(String) method 18 times faster. Not bad for a small patch that also simplifies the code!

Switching from execution time to memory allocation view, the improvement is also significant: the streamToString(InputStream) method allocates 300 MiB instead of 23 GiB , the fileToString(String) method 545 MiB instead of 6 GiB . Avoiding the creation of temporary objects prevents a lot of memory allocation.

I also created unit tests for that class, to ensure that the behavior is the same as before. The patch is now ready to be submitted to the core library.

Further optimizations#

I was surprised to find code that manage UTF-8 byte order mark (BOM) in various methods that read or write files. It is becoming really rare to encounter a UTF-8 BOM in the wild, because they are ultimately useless: the byte order of UTF-8 is fixed, contrary to UTF-16 and UTF-32. While their use in UTF-8 is allowed, the specifications explicitly discourage it. Removing that support would not really optimize the code, but it would surely make it simpler.