Thursday, January 13, 2011

Memory Leaks, part 2

While keeping up on the OTN forums, I saw one post that was working with DOMDocuments and used .freeNode. Having a system in production that doesn't use .freeNode, I wondered whether this was part of the known small memory leak that an OS session has over time. So I went back to my previous post of Memory Leaks and revisited and expanded it to see whether not using .freeNode was causing memory leaks.

The following was tested on two different machines. Both were running the same version of Redhat Linux. The older machine was and 4 Gigs of memory. The newer machine has and 8 Gigs of memory. The processor is better of course too.

Test procedure:
I ran the following updated script three times on both systems. On the older system, I ran it 300,000 times. On the newer system, I ran it 600,000 times (which still ran faster than the older system). You can see by the comments what two lines of code I added in run #2 and the one line of code I added in run #3. After doing each run, I would close that window/session and start a new one. I'm using PL/SQL Developer and have it setup so each window runs as a separate database session.

l_dom_doc dbms_xmldom.DOMDocument;
l_node dbms_xmldom.domnode; -- added run 2

PROCEDURE p_local_open_close IS
l_dom_doc := dbms_xmldom.newDOMDocument;
l_node := dbms_xmldom.makenode(dbms_xmldom.createelement(l_domdoc, 'root')); -- added run 2
dbms_xmldom.freeNode(l_node); -- added run 3
END p_local_open_close;

DBMS_APPLICATION_INFO.SET_MODULE(module_name => 'user_domDocument_mem_leak_test',
action_name => 'SLEEPING');
/* Sleeps so have time to find OS PID via
select s.sid, s.process, s.serial#, s.module, s.action, p.spid os_pid
from gv$session s,
gv$process p
where s.module = 'user_domDocument_mem_leak_test'
and s.paddr = p.addr;
FOR i IN 1..300000 -- adjust as needed for your system


To determine memory usage, I was using top to see what the VIRT (total virtual memory) for the OS session was. The SQL in the script shows how to get the OS session based on the Oracle session that was running. If your version of top doesn't support top -p , another way as my coworker turned up is ps -opid,vsz -p . This shows the PID and VSZ (Virtual Size) columns. I'm sure there are other ways too.

Run results:
Runs #2 and #3 were identical in memory usage to run #1. For the system, VIRT would start out at/around 438m and increase to 470m after the run. For the system, memory would start at 1181m and stay there.

It would appear that using dbms_xmldom.freeNode is completely optional as Oracle handles the memory from this correctly. This is determined by the fact that memory usage from the first run, which had no dbms_xmldom.domnode in it, was exactly the same as the second and third runs.

Additional observation:
I was expecting that memory usage on the system to remain a constant like it did on the system. As my previous post said, the fix was applied to a system and has since been upgraded to As was released sometime in early 2008 (or so it appears) it is highly possible the patch applied to was not part of the release. Looks like I will have to verify this on the client's system and see about getting this fixed if it still occurs on their system as well.

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