Hashmap slower after deserialization - Why?

Question

I have a pretty large Hashmap (~250MB). Creating it takes about 50-55 seconds, so I decided to serialize it and save it to a file. Reading from the file takes about 16-17 se

Answer 1

This question was interesting, so I wrote my own test case to verify it. I found no difference in speed for a live lookup Vs one that was loaded from a serialized file. The program is available at the end of the post for anyone interested in running it.

• The methods were monitored using JProfiler.
• The serialized file is comparable to yours. ~ 230 MB.
• Lookups in memory cost 1210 ms without any serialization

• After serializing the map and reading them again, the cost of lookups remained the same (well almost - 1224 ms)

• The profiler was tweaked to add minimal overhead in both scenarios.
• This was measured on Java(TM) SE Runtime Environment (build 1.6.0_25-b06) / 4 CPUs running at 1.7 Ghz / 4GB Ram 800 Mhz

Measuring is tricky. I myself noticed the 8 second lookup time that you described, but guess what else I noticed when that happened.

GC activity

Your measurements are probably picking that up too. If you isolate the measurements of Map.get() alone you'll see that the results are comparable.

---

public class GenericTest
{
    public static void main(String... args)
    {
        // Call the methods as you please for a live Vs ser <-> de_ser run
    }

    private static Map generateHashMap()
    {
        Map map = new HashMap();
        final Random random = new Random();
        for(int counter = 0 ; counter < 10000000 ; counter++)
        {
            final int value = random.nextInt();
            final long key = random.nextLong();
            map.put(key, value);
        }
        return map;
    }

    private static void lookupItems(int n, Map map)
    {
        final Random random = new Random();
        for(int counter = 0 ; counter < n ; counter++)
        {
            final long key = random.nextLong();
            final Integer value = map.get(key);
        }
    }

    private static void serialize(Map map)
    {
        try
        {
            File file = new File("temp/omaha.ser");
            FileOutputStream f = new FileOutputStream(file);
            ObjectOutputStream s = new ObjectOutputStream(new BufferedOutputStream(f));
            s.writeObject(map);
            s.close();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    private static Map deserialize()
    {
        try
        {
            File file = new File("temp/omaha.ser");
            FileInputStream f = new FileInputStream(file);
            ObjectInputStream s = new ObjectInputStream(new BufferedInputStream(f));
            HashMap map = (HashMap) s.readObject();
            s.close();
            return map;
        }
        catch (Exception e)
        {
            throw new RuntimeException(e);
        }
    }
}