Re: NIO not so hot

Rupert Smith <>
Mon, 2 Jun 2014 07:46:39 -0700 (PDT)
On Sunday, June 1, 2014 11:15:01 AM UTC+1, Robert Klemme wrote:

Can you point me to writing about that internal byte buffer pool in the
JRE? I could not find anything.

Take a look here:

Line 179.

You can see:

179 static int read(FileDescriptor fd, ByteBuffer dst, long position,
180 NativeDispatcher nd, Object lock)
181 throws IOException
182 {
183 if (dst.isReadOnly())
184 throw new IllegalArgumentException("Read-only buffer");
185 if (dst instanceof DirectBuffer)
186 return readIntoNativeBuffer(fd, dst, position, nd, lock);
188 // Substitute a native buffer
189 ByteBuffer bb = Util.getTemporaryDirectBuffer(dst.remaining());
190 try {
191 int n = readIntoNativeBuffer(fd, bb, position, nd, lock);
192 bb.flip();
193 if (n > 0)
194 dst.put(bb);
195 return n;
196 } finally {
197 Util.offerFirstTemporaryDirectBuffer(bb);
198 }
199 }

So when using a heap buffer, a temporary direct buffer is taken from a pool, read into, then the data is copied into the heap buffer.

Many benchmarks will do:

time this {
// Read some data into the buffer

time this {
// Read some data into the buffer

And come to the conclusion that heap buffers are faster. But now we know that every heap buffer IO operation uses a direct buffer under the covers, how can heap buffer IO operations be faster?

If we do the pooling ourselves, we can find that direct buffers are faster.


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