[jira] [Created] (LUCENE-10605) fix error in 32bit jvm object alignment gap calculation

["sun wuqiang (Jira)" <ji...@apache.org>]

sun wuqiang created LUCENE-10605:
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             Summary: fix error in 32bit jvm object alignment gap calculation
                 Key: LUCENE-10605
                 URL: https://issues.apache.org/jira/browse/LUCENE-10605
             Project: Lucene - Core
          Issue Type: Improvement
          Components: core/other
    Affects Versions: 9.2
         Environment: jdk 7 32-bit

jdk 8 32-bit
            Reporter: sun wuqiang
         Attachments: image-2022-06-08-20-50-27-712.png, image-2022-06-08-21-24-57-674.png

ArrayUtil.{*}oversize{*}(int minTargetSize, int bytesPerElement)
This method is used to calculate the optimal length of an array during expansion.
 
According to current logic,in order to avoid space waste caused by *object alignment gap.* In *32-bit* JVM,the array length will select the numbers(the +current optional+ columns) in the table below. But the results weren't perfect.
See the table below.
!image-2022-06-08-20-50-27-712.png!
 
I used *jol-core* to calculate object alignment gap
{code:java}
<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.16</version>
    <scope>compile</scope>
</dependency> {code}
 
Execute the following code:
{code:java}
System.out.println(ClassLayout.parseInstance(new byte[6]).toPrintable()); {code}
 
!image-2022-06-08-21-24-57-674.png!
 
To further verify that the tool's results are correct, I wrote the following code to infer how much space the array of different lengths actually occupies based on when the OOM occursThe conclusion is consistent with jol-core.
{code:java}
// -Xms16m -Xmx16m
// Used to infer the memory space occupied
// by the length of various arrays
public static void main(String[] args) {
    byte[][] arr = new byte[1024 * 1024][];
    for (int i = 0; i < arr.length; i++) {
        if (i % 100 == 0) {
            System.out.println(i);
        }
        // According to OOM occurrence time
        // in 32-bit JVM,
        // Arrays range in length from 5 to 12,
        // occupying the same amount of memory
        arr[i]=new byte[5];
    }
} {code}
*new byte[5]* and *new byte[12]* use the same amount of memory
----
 

In addition +*- XX: ObjectAlignmentInBytes*+ should also affect the return value of this method. But I don't know whether it is necessary to do this function. If necessary, I will modify it together. Thank you very much!

 



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