8179527: Ineffective use of volatile hurts performance of Charset.atBugLevel()

[Langer, Christoph]

Hi Ogata,

I'm redirecting this RFR to core-libs-dev and nio-dev as it potentially affects all platforms and should be discussed there.

As for your proposal: I can generally understand that using volatile at this place hurts performance on the power platform and this should be improved. I however don't fully oversee if just removing the volatile qualifier is the only thing to do here. Maybe one should implement some double checked locking pattern around the static field bugLevel and its initialization from system properties? I guess during VM initialization multiple threads can got to this place and the volatile qualifier was added for synchronizing access to "bugLevel".

So, waiting for comments from other experts...

Best regards
Christoph

> -----Original Message-----
> From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-
> bounces at openjdk.java.net] On Behalf Of Kazunori Ogata
> Sent: Dienstag, 27. Juni 2017 07:13
> To: jdk10-dev at openjdk.java.net; ppc-aix-port-dev at openjdk.java.net
> Subject: RFR: 8179527: Ineffective use of volatile hurts performance of
> Charset.atBugLevel()
> 
> Hi all, please review a change for JDK-8182743.
> 
> Bug report: https://bugs.openjdk.java.net/browse/JDK-8182743
> Webrev: http://cr.openjdk.java.net/~horii/8179527/webrev.00/
> 
> This change removes a "volatile" qualifier of a static variable used by
> Charset.atBugLevel() because it does not improve thread-safety of the code
> and it hurts performance on PPC (and should be the same in ARM, too).
> Removing the "volatile" improved performance by 26% in a POWER8 machine
> using following micro benchmark:
> 
> ------
> import java.io.*;
> import java.nio.ByteBuffer;
> import java.nio.charset.Charset;
> import java.util.ArrayList;
> 
> class ConvertTest {
>     static String str;
> 
>     public static void main(String[] args) {
>         byte buf[] = { 0x41 };
>         Charset fromCharset = Charset.forName("iso-8859-1");
> 
>         long start = System.currentTimeMillis();
> 
>         for(long i = 0; i < 100000000; i++)
>             str = new String(buf, 0, 1, fromCharset);
> 
>         long end = System.currentTimeMillis();
> 
>         System.out.println("Elapsed: "+(end-start)+" ms");
>     }
> }
> ------
> 
> 
> Regards,
> Ogata