RFR 8041565: JMX ObjectName could be refactored to save memory

[Roger Riggs]

Hi Jaroslav,

Minor comments:

1488+:  In forms like:

  _pattern_flag &= (~PROPLIST_PATTERN & 0xff);"

The &0xff seems unnecessary since the store is to a byte field.

1644:  the ? and : operators should be surrounded by spaces.

There are other style issues, such as then statements on the same line 
as the 'if' but that
may be beyond the scope of this change.

Otherwise looks fine  (as a 9 reviewer, but not specifically a 
serviceability reviewer).

Thanks, Roger


On 4/13/2015 5:43 AM, Jaroslav Bachorik wrote:
> Please, review the following change
>
> Issue : https://bugs.openjdk.java.net/browse/JDK-8041565
> Webrev: http://cr.openjdk.java.net/~jbachorik/8041565/webrev.00
>
> In situations when there are 10s of thousands ObjectNname instances 
> around (enterprise setups etc.) the 3 separate internal boolean fields 
> can lead to a noticeable memory waste. Adding insult to the injury, 
> with the current field layout it is necessary to align the instances 
> by 4 bytes.
>
> When using JOL (http://openjdk.java.net/projects/code-tools/jol/) to 
> inspect the object layout we can see this:
>
> Before optimization (JDK8u40):
> ---
> javax.management.ObjectName object internals:
> OFFSET SIZE TYPE DESCRIPTION VALUE
>       0 12 (object header)| N/A
>      12 4 int ObjectName._domain_length N/A
>      16 1 boolean ObjectName._domain_pattern N/A
>      17 1 boolean ObjectName._property_list_pattern N/A
>      18 1 boolean ObjectName._property_value_pattern N/A
>      19 1 (alignment/padding gap) N/A
>      20 4 String ObjectName._canonicalName N/A
>      24 4 Property[] ObjectName._kp_array N/A
>      28 4 Property[] ObjectName._ca_array N/A
>      32 4 Map ObjectName._propertyList N/A
>      36 4 (loss due to the next object alignment)
> Instance size: 40 bytes (estimated, the sample instance is not available)
> Space losses: 1 bytes internal + 4 bytes external = 5 bytes total
> {noformat}
>
> After optimization (JDK9 internal build):
> ---
>
> javax.management.ObjectName object internals:
>  OFFSET SIZE TYPE DESCRIPTION VALUE
>       0 12 (object header) N/A
>      12 2 short ObjectName._domain_length N/A
>      14 1 byte ObjectName._pattern_flag N/A
>      15 1 (alignment/padding gap) N/A
>      16 4 String ObjectName._canonicalName N/A
>      20 4 Property[] ObjectName._kp_array N/A
>      24 4 Property[] ObjectName._ca_array N/A
>      28 4 Map ObjectName._propertyList N/A
> Instance size: 32 bytes (estimated, the sample instance is not available)
> Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
>
>
> After optimization we can save 8 bytes per instance which can 
> translate to very interesting numbers on large installations.
>
>
> To achieve this the domain name length is set to be *short* instead of 
> *int* and the three booleans kept for the performance purposes are 
> encoded into one byte value (as proposed by the reporter, 
> Jean-Francois Denise).
>
> All the regression and JCK tests are passing after this change.
>
>
> Thanks,
>
> -JB-