Ben J. Christensen

I’ve spent the past day playing with Ruby and decided to test some basic performance – iterating and string parsing – to get an idea of what the performance is really like.

Ruby is not doing so well in my tests.

Note to Ruby Experts: If anyone can demonstrate what I’m doing wrong in my code or testing, I would love to be corrected.

My approach was to write the exact same code in Java and Ruby that loads up a file, reads each lines, tokenizes it into words using whitespace as the delimiter and counts up the tokens.

This avoids network or database IO and other external resources – except the filesystem which I don’t consider a significant variable in the test.

Further testing I’m planning on doing will test Rails, multi-threading and other common things I do in my apps.

On my laptop, a MacBook Pro 2.53Ghz Core 2 Duo with 4GB memory, the average times are:

• Ruby 1.8.6 app: 8022ms
• Java 5 app: 2986ms
• Java 6 app: 1443ms

Source Code

• FileReadParse.java
• file_read_parse.rb

After downloading, strip the .doc ending off of the files.

Program Output

macbook-pro:src benjc$ /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/bin/java FileReadParse

Starting to read file…

The number of tokens is: 7764115

It took 1502 ms

macbook-pro:performance benjc$ ruby file_read_parse.rb

Starting to read file …

The number of tokens is: 7764115

It took 7999.955 ms

A simple test of performance for ConcurrentHashMap.

The insert is only a little slower, but the retrieval is 3x slower in this quick and dirty single-threaded test.

TimeUtil timer = new TimeUtil();

        timer.start();

        HashMap map = new HashMap();

        for (int i = 0; i < 1000000; i++) {

            map.put(i, i);

        }

        timer.printCurrent();

        timer.start();

        ConcurrentHashMap cmap = new ConcurrentHashMap();

        for (int i = 0; i < 1000000; i++) {

            cmap.put(i, i);

        }

        timer.printCurrent();

        timer = new TimeUtil();

        timer.start();

        map = new HashMap();

        for (int i = 0; i < 1000000; i++) {

            map.get(i);

        }

        timer.printCurrent();

        timer.start();

        cmap = new ConcurrentHashMap();

        for (int i = 0; i < 1000000; i++) {

            cmap.get(i);

        }

        timer.printCurrent();

Action completed in: 458 ms 0.458 seconds

Action completed in: 574 ms 0.574 seconds

Action completed in: 30 ms 0.03 seconds

Action completed in: 113 ms 0.113 seconds

I recently came across the problem of large resultsets being pulled into a java app via MySQL JDBC. I had dealt with this years ago but forgotten about it.

The test case below shows how the entire ResultSet is buffered in memory by default — which can be a “very bad thing” when dealing with hundreds or thousands of megabytes of data when it’s intended to be processed row by row.

Using mysql-connector-java-3.1.12-bin.jar and a JDK 5 with 32MB heap:

ET-COMMONS INFO: JVM MEMORY MONITOR => Total: 33  Used: 1  Free: 32

Retrieving data …

Ran out of memory at row: 0

java.lang.OutOfMemoryError: Java heap space

at com.mysql.jdbc.ByteArrayBuffer.getBytes(ByteArrayBuffer.java:128)

at com.mysql.jdbc.ByteArrayBuffer.readLenByteArray(ByteArrayBuffer.java:248)

at com.mysql.jdbc.MysqlIO.nextRow(MysqlIO.java:1304)

at com.mysql.jdbc.MysqlIO.readSingleRowSet(MysqlIO.java:2272)

at com.mysql.jdbc.MysqlIO.getResultSet(MysqlIO.java:423)

at com.mysql.jdbc.MysqlIO.readResultsForQueryOrUpdate(MysqlIO.java:1962)

at com.mysql.jdbc.MysqlIO.readAllResults(MysqlIO.java:1385)

at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1728)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2988)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2917)

at com.mysql.jdbc.Statement.executeQuery(Statement.java:824)

at JDBCTest.main(JDBCTest.java:26)

Using mysql-connector-java-5.1.6-bin.jar and the same JDK 5 with 32MB heap:

ET-COMMONS INFO: JVM MEMORY MONITOR => Total: 33  Used: 1  Free: 32

Retrieving data …

Ran out of memory at row: 0

java.lang.OutOfMemoryError: Java heap space

at com.mysql.jdbc.ByteArrayBuffer.getBytes(ByteArrayBuffer.java:128)

at com.mysql.jdbc.ByteArrayBuffer.readLenByteArray(ByteArrayBuffer.java:248)

at com.mysql.jdbc.MysqlIO.nextRow(MysqlIO.java:1304)

at com.mysql.jdbc.MysqlIO.readSingleRowSet(MysqlIO.java:2272)

at com.mysql.jdbc.MysqlIO.getResultSet(MysqlIO.java:423)

at com.mysql.jdbc.MysqlIO.readResultsForQueryOrUpdate(MysqlIO.java:1962)

at com.mysql.jdbc.MysqlIO.readAllResults(MysqlIO.java:1385)

at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1728)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2988)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2917)

at com.mysql.jdbc.Statement.executeQuery(Statement.java:824)

at JDBCTest.main(JDBCTest.java:26)

Thus we see that both the old and new versions of the MySQL JDBC driver by default attempt to load the entire resultset into memory.

I now increase the heap to 1GB to allow it to grow and find that the test query uses up > 500MB of heap before it even starts the rs.next() loop.

ET-COMMONS INFO: JVM MEMORY MONITOR => Total: 33  Used: 1  Free: 32

Retrieving data …

ET-COMMONS INFO: JVM MEMORY MONITOR => Total: 298  Used: 183  Free: 115

ET-COMMONS INFO: JVM MEMORY MONITOR => Total: 527  Used: 381  Free: 146

Starting to retrieve data. Memory Used: 517

Done retrieving data => 2318284   Memory Used: 551

Here is the code for this:

            ResultSet rs = conn.createStatement().executeQuery(“<sql query that returns lots of data>”);

            System.out.println(“Starting to retrieve data. Memory Used: “ + getUsedMemory());

            while (rs.next()) {

                rs.getString(1);

                rowsReturned++;

            }

            System.out.println(“Done retrieving data => “ + rowsReturned + ”   Memory Used: “  

+ getUsedMemory());

Thus you can see that the “executeQuery()” method loads up 500MB of data before it passes on the “rs.next()” loop. The full ResultSet is being buffered in memory.

Solution

To make the JDBC driver stream the results instead of buffer them all first we do the following:

            stmt.setFetchSize(Integer.MIN_VALUE);

Now it behaves like we expect it to … only 2MB used instead of > 500MB.

There are some caveats:

• http://javaquirks.blogspot.com/2007/12/mysql-streaming-result-set.html
• http://dev.mysql.com/doc/refman/5.0/en/connector-j-reference-implementation-notes.html

stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY,
              java.sql.ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);

As I work on another set of indexes with large numbers of ints I’ve revisited how different storage mechanisms behave.

I’ve set the JVM to just over 64MB (70MB to be precise).

An int being 4 bytes means that 64MB of ints is as follows:

       int sixtyFourMB = 64 * 1024 * 1024 / 4;

That is 16,777,216 ints.

An int array can successfully store 64MB in the JVM meaning it is properly assigning each int to 4 bytes without overhead.

      int[] vals = new int[sixtyFourMB];

However, if you use ArrayList<Integer> you run out of memory after 3,392,918 ints being added.

That is 1/5 of what can be stored in the int[]!

Now, assigning the space to ArrayList is fine:

                ArrayList<Integer> ints = new ArrayList<Integer>(sixtyFourMB);

It’s when you add the Integer objects (int is cast to Integer behind the scenes in JDK 5) that it dies 1/5 in.

So the overhead is obviously in the Integer object.

I next try the Trove library and use the TIntArrayList.

If I leave it to assign its memory space by default, it runs out of memory at 10,485,760 (62%).

If however I tell it how many ints to expect it sizes itself properly and does not try to grow too large and fits the entire 64MB.

     TIntArrayList ints = new TIntArrayList(sixtyFourMB);

Thus, when backed by an int[] the memory can be efficiently stored, but when using an Object[] memory is very inefficient when attempting to store primitive ints.

Out of curiosity for what an Object() really does with memory I did the following:

            ArrayList<Object> ints = new ArrayList<Object>(sixtyFourMB);

            for (i = 0; i < sixtyFourMB; i++) {

                ints.add(null);

            }

That works, it all fits in memory.

If however I change the “null” to “new Object()” such as:

                ints.add(new Object());

It fails at 702065 … which means that the object[] has already taken up the space and there is no room for the objects on the heap.

Thus, for storage of simple data primitive arrays is MUCH more efficient.

If the flexibility of the Collections API is needed for dynamic manipulation of those arrays then use GNU Trove … but carefully or the dynamic sizing of the arrays in the background can cause a lot of waste.

An old but interesting article specifies memory usage by normal objects and classes:

http://www.roseindia.net/javatutorials/determining_memory_usage_in_java.shtml

Here are two points from that article of note:

1. The class takes up at least 8 bytes. So, if you say new Object(); you will allocate 8 bytes on the heap.
2. Each data member takes up 4 bytes, except for long and double which take up 8 bytes. Even if the data member is a byte, it will still take up 4 bytes! In addition, the amount of memory used is increased in 8 byte blocks. So, if you have a class that contains one byte it will take up 8 bytes for the class and 8 bytes for the data, totalling 16 bytes (groan!).