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Why is the Java I / O drive much slower than the equivalent I / O code written in C?

I have an SSD that must supply at least 10 thousand IOPS for each specification. My test confirms that it can give me 20 KB of IOPS.

Then I create a test like this:

private static final int sector = 4*1024;
private static byte[] buf = new byte[sector];
private static int duration = 10; // seconds to run
private static long[] timings = new long[50000];
public static final void main(String[] args) throws IOException {
    String filename = args[0];
    long size = Long.parseLong(args[1]);
    RandomAccessFile raf = new RandomAccessFile(filename, "r");
    Random rnd = new Random();
    long start = System.currentTimeMillis();
    int ios = 0;
    while (System.currentTimeMillis()-start<duration*1000) {
        long t1 = System.currentTimeMillis();
        long pos = (long)(rnd.nextDouble()*(size>>12));
        raf.seek(pos<<12);
        int count = raf.read(buf);
        timings[ios] = System.currentTimeMillis() - t1;
        ++ios;
    }
    System.out.println("Measured IOPS: " + ios/duration);
    int totalBytes = ios*sector;
    double totalSeconds = (System.currentTimeMillis()-start)/1000.0;
    double speed = totalBytes/totalSeconds/1024/1024;
    System.out.println(totalBytes+" bytes transferred in "+totalSeconds+" secs ("+speed+" MiB/sec)");
    raf.close();
    Arrays.sort(timings);
    int l = timings.length;
    System.out.println("The longest IO = " + timings[l-1]);
    System.out.println("Median duration = " + timings[l-(ios/2)]);
    System.out.println("75% duration = " + timings[l-(ios * 3 / 4)]);
    System.out.println("90% duration = " + timings[l-(ios * 9 / 10)]);
    System.out.println("95% duration = " + timings[l-(ios * 19 / 20)]);
    System.out.println("99% duration = " + timings[l-(ios * 99 / 100)]);
}

And then I run this example and get only 2186 IOPS:

$ sudo java -cp ./classes NioTest /dev/disk0 240057409536
Measured IOPS: 2186
89550848 bytes transferred in 10.0 secs (8.540234375 MiB/sec)
The longest IO = 35
Median duration = 0
75% duration = 0
90% duration = 0
95% duration = 0
99% duration = 0

Why is it running much slower than the same test in C?

Update: here is the Python code that gives 20k IOPS:

def iops(dev, blocksize=4096, t=10):

    fh = open(dev, 'r')
    count = 0
    start = time.time()
    while time.time() < start+t:
        count += 1
        pos = random.randint(0, mediasize(dev) - blocksize) # need at least one block left
        pos &= ~(blocksize-1)   # sector alignment at blocksize
        fh.seek(pos)
        blockdata = fh.read(blocksize)
    end = time.time()
    t = end - start
    fh.close()

Update2 : NIO code (just a piece, will not duplicate the whole method)

...
RandomAccessFile raf = new RandomAccessFile(filename, "r");
InputStream in = Channels.newInputStream(raf.getChannel());
...
int count = in.read(buf);
...
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4 answers

, C-, Java-, , IOMeter. , C Java .

, Java IOMeter, , IOMeter , . SSD, , .

.

+7

, Java 2,5 - 3,5 . pdf, , .

: http://pages.cs.wisc.edu/~guo/projects/736.pdf

- Java , C/++, Java ; - Java, , ; , -, Java , ; - ; Java-, . , C/++.

. , RandomAccessFile, java.nio ?

nio2, C. :)

+3

RandomAccessFile, / Java.

- , BufferedInputStream BufferedOutputStream, , .

, SSD ( SSD ), ; . SSD 1024 , 64 ( , long double s), . (, - , , ).

Now I see that this is not what your code does (or at least it looks like it does), but it’s quite possible that RandomAccessFile . , .

0

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