I think Martin explained the reasoning quite well in his talk High
Performance Managed Languages
<https://www.infoq.com/presentations/performance-managed-languages>
On Wed, Jul 19, 2017 at 10:28 AM, Avi Kivity <a...@scylladb.com> wrote:
> Out of curiosity, if you're doing heavy duty processing, why did you
> choose a garbage-collected language?
>
> On 07/19/2017 04:21 AM, David Ryan wrote:
>
>
> Hi all,
>
> My first post here. Not sure it's the best place for it, but hoping
> someone here might be able to assist. We're developers of a streaming
> application that does 100k+ messages per second processing, so anything
> that allocates to the heap can cause GC pressure. We've been targeting
> removing allocations recently and one difficult ones is conversion from
> String to UTF-8 to OutputStream. The advised methods for String to UTF-8
> all create byte[] as intermediary objects.
>
> We've been able to read Strings from streams using a ThreadLocal
> ByteBuffer and CharBuffer which will allocate the char[] and String object
> only. For reference, we've done the following. In a 1minute JMC Flight
> Recorder char[] and String make up about 1 gig or allocations which is
> unavoidable because we're processing a lot of string data:
>
> public static final class StringBuffers
> {
> ByteBuffer buffer = ByteBuffer.allocate(512);
> CharBuffer charBuffer = CharBuffer.allocate(300);
> CharsetDecoder decoder = Charset.forName("UTF8").newDecoder();
> }
>
> public static final class U8Utf8MethodHandleReader extends
> AbstractReader
> {
> private final ThreadLocal<StringBuffers> buffers = new
> ThreadLocal<StringBuffers>()
> {
> @Override
> public StringBuffers initialValue()
> {
> return new StringBuffers();
> }
> };
>
> public U8Utf8MethodHandleReader(final MethodHandle setHandle)
> {
> super(setHandle);
> }
>
> @Override
> public void read(final Object o, final TypeInputStream in) throws
> Throwable
> {
> final int len = in.read();
>
> // Grab a thread local set of buffers to use temporarily.
> final StringBuffers buf = buffers.get();
>
> // get a reference to the buffers.
> final ByteBuffer b = buf.buffer;
> final CharBuffer c = buf.charBuffer;
>
> b.clear();
> c.clear();
>
> // read the stream into the byte buffer.
> in.getStream().read(b.array(), 0, len);
> b.limit(len);
>
> // decode the bytes into the char buffer.
> final CharsetDecoder decoder = buf.decoder;
> decoder.reset();
> decoder.decode(b, c, true);
>
> // flip the char buffer.
> c.flip();
>
> // get a copy of
> final String str = c.toString();
>
> // finally set the string value via method handle.
> setHandle.invoke(o, str);
> }
> }
>
> For writing Strings we've tried a similar method:
>
> public static final class StringBuffers
> {
> ByteBuffer buffer = ByteBuffer.allocate(512);
> CharsetEncoder encoder = Charset.forName("UTF8").newEncoder();
> }
>
> public static final class U8Utf8MethodHandleWriter extends
> AbstractWriter
> {
> private final ThreadLocal<StringBuffers> buffers = new
> ThreadLocal<StringBuffers>()
> {
> @Override
> public StringBuffers initialValue()
> {
> return new StringBuffers();
> }
> };
>
> public U8Utf8MethodHandleWriter(final MethodHandle getHandle)
> {
> super(getHandle);
> }
>
> @Override
> public void write(final Object o, final TypeOutputStream out)
> throws Throwable
> {
> // finally set the string value.
> final String str = (String) getHandle.invoke(o);
>
> final OutputStream os = out.getStream();
>
> // empty strings just write 0 for length.
> if (str == null)
> {
> os.write(0);
> return;
> }
>
> // Grab a thread local set of buffers to use temporarily.
> final StringBuffers buf = buffers.get();
>
> // get a reference to the buffers.
> final ByteBuffer b = buf.buffer;
>
> // this does allocate an object, but at least it isn't copying
> the buffer!
> final CharBuffer c = CharBuffer.wrap(str);
>
> // clear the byte buffer.
> b.clear();
>
> // decode the bytes into the char buffer.
> final CharsetEncoder encoder = buf.encoder;
> encoder.reset();
> encoder.encode(c, b, true);
>
> // flip the char buffer.
> b.flip();
>
> final int size = b.limit();
>
> if (size > 255)
> {
> throw new TypeException("u8ascii: String length exceeded
> max length of 255. len =" + size);
> }
>
> if (writeNotNull)
> {
> os.write(1);
> }
>
> os.write(size);
> os.write(b.array(), 0, size);
> }
> }
>
> The offending CharBuffer.wrap(str) currently allocates 766MB in a one
> minute period and has the largest allocation profile for the application.
>
> Interested if anyone else has found a better solution for this or can
> suggest alternative solutions.
>
> Thanks,
> David.
>
>
>
>
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