[PATCH] EnableTracing: output from multiple threads may be mixed together

[David Holmes]

Hi Yunda,

There is a potential problem with using a ttyLocker here, depending on 
exactly what events are being traced and from where - you must always be 
in code where it is both safe to acquire the lock, and safe to block 
waiting for the lock if it is not available.

I think I would prefer to see unlocked tracing by default with a flag to 
use locked-tracing if requested.

David

On 19/04/2013 6:26 PM, 云达(Yunda) wrote:
> Hi all,
>
> I found that the output from multiple threads may be mixed together when
> using EnableTracing. It happens many times in my test case like this:
>
> Allocation outside TLAB: [Allocation in new TLAB: [Allocation in new
> TLAB: [Allocation in new TLAB: [Class = [C, Allocation in new TLAB:
> [Class = [I, Allocation in new TLAB: [Java Monitor Wait: [Class =
> java/lang/String, Allocation Size = 24, Allocation Size = 24, Class =
> java/lang/String, Class = [I, Allocation in new TLAB: [Class = [C,
> Allocation in new TLAB: [Allocation Size = 192]
>
> Class = com/sun/org/apache/xerces/internal/dom/DeferredElementNSImpl,
> Allocation Size = 24, Allocation Size = 24, TLAB Size = 23712280]
>
> TLAB Size = 24607080]
>
> Allocation Size = 24, Monitor Class = java/lang/ref/Reference$Lock, TLAB
> Size = 25054480]
>
> TLAB Size = 25054480]
>
> Allocation in new TLAB: [Class = [CTLAB Size = 24607080]
>
> Allocation Size = 72, Class = [C, TLAB Size = 24159728]
>
> , Allocation Size = 32, TLAB Size = 23712288]
>
> It's very confusing and it's even not easy to tell how many events there
> are. I think the reason is that the writeEvent() method of each Event*
> class output the fields of event one by one without using any lock. So I
> made a small patch which add ttyLocker to writeEvent() method and after
> applying this patch there's no output mixed together in my test
> case(against http://hg.openjdk.java.net/hsx/hsx24/hotspot/):
>
> diff -r edd1619a3ae4 src/share/vm/trace/traceEventClasses.xsl
>
> --- a/src/share/vm/trace/traceEventClasses.xsl    Thu Apr 18 13:50:58
> 2013 -0700
>
> +++ b/src/share/vm/trace/traceEventClasses.xsl Fri Apr 19 16:12:38 2013
> +0800
>
> @@ -132,6 +132,7 @@
>
>     void writeEvent(void) {
>
>       ResourceMark rm;
>
>       HandleMark hm;
>
> +    ttyLocker ttyl;
>
>       TraceStream ts(*tty);
>
>       ts.print("<xsl:value-of select="@label"/>: [");
>
> <xsl:apply-templates select="value|structvalue" mode="write-data"/>
>
> I searched before sending this mail I didn't find anyone who covering thisJ
>
> Regards,
>
> Yunda
>
>
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