Java Virtual Machine and page swaping ..

Question

Hi,
Could you please tell me if the JVM based on page swapping or not?
I am asking this question because of the following:
1- on my laptop I have only 256MB and I set the heap size to -Xms=512MB
-Xmx=1024MB and the heap starts working.
2- I note that there is many many access to my hard disk while running my application and this process degrade the total performance.
Does this mean that JVM use page swapping in some how to cover the amount of memory set by -Xms,-Xmx.
in other experiment on other machine with 2GB RAM, I tried to keep hashtable of size approximately = 100MB (this is the size of all keys + all values only without the skeleton of the hashtable i.e. the pointers). I note that the system becomes very slow writing to the hashtable and there was a great access to the harddisk!?
Having two GB RAM means at lease we can store a hashtable of total size =1GB in memory?!!!!
Please tell me what is the problem.
Many thanks,
Regards,
Ahmad.
Message was edited by:
user525205

Answer 1

Hi Ahmad,
With a 32-bit JVM you could potentially initialize a maximum heap size of 4GB. Though, in most of the cases this amount is little under 4GB or a little under 2GB. In the case of your system, which has 256MB if you set a maximum heap size of 1024MB/1GB, then the JVM will swap to disk pages to account for the extra space needed.
Now, for the other case, the machine with 2GB of RAM, you probably are seeing disk I/O due to flushing the buffer caches when syncing, checkpointing, trickling etc. What are the exact operations you're performing when you see the application going to disk? You can use the output of "db_stat -m -h <env_path>" to display the cache statistics, and "db_stat -d <db_file> -h <env_path>" to obtain statistics for a particular database within the environment.
Regards,
Andrei