[LOG4J2-1274] Layout improvements to enable avoiding temporary object allocation

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.5
• Fix Version/s: 2.6
• Component/s: Layouts
• Epic Link: GC-free
• Assignee: Remko Popma
• Reporter: Remko Popma
• Created: 14/Feb/16 15:58
• Updated: 04/Apr/16 14:09
• Resolved: 04/Apr/16 14:09

Description

Problem
The current Layout API does not make it easy for implementors to avoid creating temporary objects. Especially these methods:

byte[] toByteArray(LogEvent);
T toSerializable(LogEvent);

The byte array returned from toByteArray(LogEvent) cannot be re-used between log events, since the caller cannot know how many bytes a partially filled array contains.

In practice, all Layout implementations in Log4j 2 except SerializedLayout implement the StringLayout subinterface. This means that the toSerializable() method needs to return a new String object for every log event.

Contrived example of an application that does nothing but log a simple string as often as possible: 94% of GC-ed memory is in the byte[] and char[] arrays.

Forces
I am interested in reducing or even eliminating the allocation of temporary objects for text-based layouts. Many of these use (and re-use) a StringBuilder to build a text representation of the current log event. Once this text representation is built, it needs to be converted to bytes that the Appender can consume. I am aware of two ways in the JDK to convert text to bytes:

• the various String#getBytes methods - these all allocate a new byte array for each invocation
• the underlying java.nio.charset.CharsetEncoder used internally by String - especially method CoderResult encode(CharBuffer in, ByteBuffer out, boolean endOfInput) which converts characters to bytes without object allocation.

The last method is interesting because this gives us an opportunity to also reduce the amount of copying by directly supplying the ByteBuffer buffer used by RandomAccessFileAppender, or the MappedByteBuffer of the MemoryMappedFileAppender.

The resulting API needs to support the fact that implementations may need to call CharsetEncoder#encode multiple times:

• The ByteBuffer may not have enough remaining space to hold all the data; CharsetEncoder#encode returns CoderResult.OVERFLOW to signal this so the caller can consume the contents and reset/clear the buffer before continuing.
• The CharBuffer may not be large enough to hold the full text representation of the log event. Again, CharsetEncoder#encode may need to be invoked multiple times.

Proposal
(Thinking out loud here, I'm open to suggestions.)
It may be sufficient for the layout interface to have a single additional new method:

/**
 * Formats the event suitable for display and writes the result to the specified destination.
 *
 * @param event The Logging Event.
 * @param destination Holds the ByteBuffer to write into.
 */
void writeTo(LogEvent e, ByteBufferDestination destination);

Appenders that want to be allocation-free need to implement the ByteBufferDestination interface:

public interface ByteBufferDestination {
    ByteBuffer getByteBuffer();

    /**
     * Consumes the buffer content and returns a buffer with more available() space
     * (which may or may not be the same instance).
     * <p>
     * Called by the producer when the buffer becomes too full
     * to write more data into it.
     */
    ByteBuffer drain(ByteBuffer buf);
}

Usage: for example RandomAccessFileAppender code can look like this:

// RandomAccessFileAppender
public void append(final LogEvent event) {
    getLayout().writeTo(event, (ByteBufferDestination) manager);
}

Layout implementation of the writeTo method: layouts need to know how to convert LogEvents to text, but writing this text into the ByteBuffer can be delegated to a helper:

// some layout 
public void writeTo(LogEvent event, ByteBufferDestination destination) {
    StringBuilder text = toText(event, getCachedStringBuilder());
	
    TextEncoderHelper helper = getCachedHelper();
    helper.encodeWithoutAllocation(text, destination);
}

/**
 * Creates a text representation of the specified log event
 * and writes it into the specified StringBuilder.
 * <p>
 * Implementations are free to return a new StringBuilder if they can
 * detect in advance that the specified StringBuilder is too small.
 */
StringBuilder toText(LogEvent e, StringBuilder destination) {} // existing logic goes here

public String toSerializable(LogEvent event) { // factored out logic to toText()
    return toText(event, getCachedStringBuilder()).toString();
}

Helper contains utility code for moving the text into a CharBuffer, and for repeatedly calling CharsetEncoder#encode.

public class TextEncoderHelper {
    TextEncoderHelper(Charset charset) {} // create CharsetEncoder

    void encodeWithoutAllocation(StringBuilder text, BinaryDestination destination) {
        ByteBuffer byteBuf = destination.getByteBuffer();
        CharBuffer charBuf = getCachedCharBuffer();
        charBuf.reset();
        int start = 0;
        int todoChars = text.length();
        do {
            int copied = copy(text, start, charBuf);
            start += copied;
            todoChars -= copied;
            boolean endOfInput = todoChars <= 0;
    
            charBuf.flip();
            CodeResult result;
            do {
                result = charsetEncoder.encode(charBuf, byteBuf, endOfInput);
                if (result == CodeResult.OVERFLOW) { // byteBuf full
                    // destination consumes contents
                    // and returns byte buffer with more capacity
                    byteBuf = destination.drain(byteBuf);
                }
            } while (result == CodeResult.OVERFLOW);
        } while (!endOfInput);
    }
    
    /**
     * Copies characters from the StringBuilder into the CharBuffer,
     * starting at the specified offset and ending when either all
     * characters have been copied or when the CharBuffer is full.
     *
     * @return the number of characters that were copied
     */
    int copy(StringBuilder source, int offset, CharBuffer destination) {}
}

Issue Links

• is depended upon by: LOG4J2-1291 Update PatternLayout to utilize gc-free mechanism for LogEvent processing
• is depended upon by: LOG4J2-1292 Update RandomAccessFileAppender to utilize gc-free Layout method
• is related to: LOG4J2-506 Binary logging interface