On Wed, 7 Oct 2020 13:30:14 GMT, Andrew Haley <a...@openjdk.org> wrote:
> DataOutputStream is very slow post-disabling of Biased Locking. This > was discovered when benchmarking a transaction library, which showed > significant performance loss when moving to JDK 15. WIth some small > changes to DataOutputStream we can get the performance back. There's a > JMH benchmark at > http://cr.openjdk.java.net/~aph/JDK-8254078/jmh-tests.tar > > Some Stream classes use very fine-grained locking. > > In particular, writeInt is defined like this: > > out.write((v >>> 24) & 0xFF); > out.write((v >>> 16) & 0xFF); > out.write((v >>> 8) & 0xFF); > out.write((v >>> 0) & 0xFF); > incCount(4); > > Unfortunately, ByteArrayOutputStream.write(byte) is defined like this: > > public synchronized void write(int b) { > ensureCapacity(count + 1); > buf[count] = (byte) b; > count += 1; > } > > so we acquire and release a lock for every byte that is output. > > For example, writing 4kb of ints goes from 17.3 us/op to 53.9 us/op when > biased locking is disabled: > > > +UseBiasedLocking DataOutputStreamTest.dataOutputStreamOverByteArray avgt 6 > 53.895 ± 5.126 us/op > -UseBiasedLocking DataOutputStreamTest.dataOutputStreamOverByteArray avgt 6 > 17.291 ± 4.430 us/op > > There are refactorings of DataOutputStream we can do to mitigate this. This pull request has now been integrated. Changeset: 17f04fc9 Author: Andrew Haley <a...@openjdk.org> URL: https://git.openjdk.java.net/jdk/commit/17f04fc9 Stats: 143 lines in 3 files changed: 132 ins; 0 del; 11 mod 8254078: DataOutputStream is very slow post-disabling of Biased Locking Reviewed-by: rriggs, shade, alanb ------------- PR: https://git.openjdk.java.net/jdk/pull/542
