RFR: 8254078: DataOutputStream is very slow post-disabling of Biased Locking [v7]

[Roger Riggs]

On Thu, 22 Oct 2020 09:08:31 GMT, Andrew Haley <aph at 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.
>
> Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8254078: DataOutputStream is very slow post-disabling of Biased Locking

Marked as reviewed by rriggs (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/542