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Analysis of ISM core dump on E420R.

When using the "+XX:+UseISM" parameter, the user no longer has control of how large the allocation of the heap is. When we attempted to use this parameter at Sun's recommendation, the JVM core dumped immediately (April 2001). We reported the problem to the Sun representative who asked us to use it and also to the Java Service group. We received numerous "we're working on it" notes, then a long period of silence. As of about 8 months later (Jan 2002), we were informed that this would be fixed in Version 1.4.0 which is to be released "shortly".

The net of the core dump is this: This pair of options is designed to grab as much memory as it can for the heap, and then lock it down. It seems, this was only tested on machine with a memory size larger than my 4GB. There are no Sun SPECjbb2000 disclosures where this ISM parameter is used where the real memory of the system is less than 16GB of real memory.

The information below will show that the reason the JVM code is failing on our system is that the "+XX:+UseISM" parameter used with the "+XX:+AggressiveHeap" is trying to allocate more memory than the system has to allocate. Hence, an immediate core dump.

Our belief is that this pair of options was specially formulated and only intended for high performance benchmarking. Your "actual performance" may be lower, as the saying goes.

The truss command shows there to be 4,156,555,264 bytes available

output from truss:

shmget(0, 4156555264, 0600|IPC_CREAT) Err#22 EINVAL

The only problem is that "available memory" is 4,136,960,000 as shown in the system log entries below:


May 7 23:53:11 itamae unix: [ID 389951 kern.info]
mem = 4194304K (0x100000000)
May 7 23:53:11 itamae unix: [ID 930857 kern.info]
avail mem = 4136960000

If you do the math, the allocation is 19MB more than we have, and the Java command core dumps. 19MB is actually not that much memory, but you can't be just a "little bit" out of memory and still run!

Trying to Allocate = 4,156,555,264 (from truss - shmget)
Available Memory = 4,136,960,000 (from /var/adm/messages)
Overallocation = 19,595,264 bytes

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