我有一块固态硬盘，根据规格书应该提供不少于10k IOPS。我的基准测试证实它可以给我20k IOPS。
然后我创建了如下测试：
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)]);
}
然后我运行这个示例，只获得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
为什么它的速度比同样的C测试慢那么多？
更新：这里是Python代码，可以提供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()
更新2：NIO 代码（只是一部分，不会重复所有方法）
...
RandomAccessFile raf = new RandomAccessFile(filename, "r");
InputStream in = Channels.newInputStream(raf.getChannel());
...
int count = in.read(buf);
...
System.nanoTime()而不是currentTimeMillis()可以让您更精确地测量各个时间。 - Dan Getz