OpenEdge Memory Management Anti-pattern
Hello all,
I’ve recently been reviewing some Progress 4GL and have found an all too common anti-pattern related to memory management.
When a variable is defined, the Progress runtime client (Virtual Machine) allocates memory at runtime for that variable. Once the variable is out of scope, the memory is released and everyone is happy. Progress programmers have grown comfortable with this design and obliviously define variables whenever they are needed knowing that they will be de-allocated automagically by the Progress VM.
Then came dynamic objects.
Progress programmers were overjoyed! They could now create temp-tables, buttons, queries all on the fly at runtime. No more convoluted if-then statements or .i’s or having to code a different “for each” for every combination of where clause.
However, as with any power bestowing feature, there is a dark side to this wonderful new world of dynamic 4gl: memory management. Most programmers never really stopped to consider the fact that if something is created dynamically at runtime, the VM has no way of knowing the scope. It cannot tell when to release the memory required for the dynamic object. REMEMBER: the scope of the variable you happen to assign the object to HAS NO BEARING on the scope of the OBJECT since it can be passed around. In other words, the scope of the variable holding the HANDLE to the object is NOT bound to the OBJECT itself. The scope of ALL OBJECTS are always at the SESSION. This applies to GUI widgets, dynamic queries, temp-tables, etc.
Java (and other VM’s) solve this through the use of a separate execution thread running concurrently called a Garbage Collector. Its job is to scan memory and find dynamic objects that are no longer “reachable” and release their memory. Unfortunately, the Progress VM has no such thread/concept.
To add insult to injury, not only does this leak memory but it also causes progressively worse performance: The more widgets in memory, the more time it takes to create another widget. Here are three examples (run on a 2.1ghz processor):
Button Handles
Minimum memory required/lost per Button Handle: 512 bytes
Query Handles
Minimum memory required/lost per Query Handle: 1024 bytes
Temp-Table Handles
Min. memory required/lost per Temp-Table Handle: 512 bytes
So, as you can see, from both a memory and CPU footprint standpoints, it is very important to be sure to clean up your objects.
This may seem like a large number of handles, but remember two important points:
1. This is at the session level. This means that if a.p calls b.p which creates objects then those will exist for the life of the session: THERE IS NO SCOPE other than SESSION FOR DYNAMIC OBJECTS and they are NEVER automatically reclaimed!
2. If the programs are running as part of a long-running session such as AppServer, EagleIQ server or Webspeed, then you have to consider the cumulative affect over days, weeks or months.
Also note that if it is a temp-table, it could potentially have a much larger memory footprint.
So, what must be done?
It is up to the Progress programmer to clean up each and every dynamic object created using the “delete object” command.
It may be appropriate to create a widget-pool in which to assign your objects so you can just delete the pool and all the objects within will be released as well. In fact, if you create a non-persistent widget-pool, it will be automatically deleted when it goes out of scope. Creating the object into a non-persistent pool will make it behave as if it were scoped at the level that the widget-pool is created: in effect, making it behave as if it were statically defined.
If you don’t use a non-persistent widget-pool, then It is also important to be sure the “clean up” code is executed even when there is an error. For example, the following will bleed memory if an error condition is raised within the blah blah:
procedure doQuery: def var qh as handle. create query qh. /* so some business logic here */ do while true: blah blah end. delete object qh. end.
However, if you create a non-persistent widget pool, then it is automatically deleted when it goes out of scope. So the following will not leak memory even if an error condition happens:
procedure doQuery:
def var qh as handle.
create widget-pool "wp".
create query qh in widget-pool "wp".
/* so some business logic here */
do while true:
blah blah
end.
delete object qh.
end.
The widget-pool may be defined at the .p level as well. In this case the pool is deleted when the .p is exited.
Oh, by the way, persistent procedures and memptr’s are two other constructs that have a session level scope. However, they cannot be part of a widget-pool and therefore must be handled individually.
OE Architect Editor Corruption Bug Fix in SP3
An editor corruption issue that causes OpenEdge Architect to crash or stop responding has been fixed in the just released OpenEdge Service Pack 3.
This issue was cropping up frequently enough during heads down coding sessions to cause use of OE Architect to be a hit or miss proposition for some of us.
While more prevelant on multi-core and HT processors single-core processors are not immune.
At Solvepoint we’re long-time Eclipse users so we’re eager to see OpenEdge Architect (based on Eclipse) to be stabilized and advanced.
Reference Kbase ID P124071 and Service Pack 3 Reference ID OE00150179.
Designing Systems That Work
“Design is not cleaning up the mess, or adding ornamentation at the end. It’s a process of thinking, organizing, trying, testing, reworking, creating anew, refining, honing, and more. Successful systems work because they are well conceived, and responsive to user needs, styles, wishes, and habits. They continue to work because they are well structured, and can be easily maintained and enhanced.” -Arthur Fink
Premier Worldwide Progress Event – Exchange, 2008 is at Disneyland Resort Paris, France
Details for Exchange, 2008 (June 8-11 at Disneyland Resort Paris, France) are posted at Progress Exchange, ‘08. Exchange is an annual event sponsored by Progress Software drawing over 1,000 users, and technical and business experts from around the world. Exchange events include several days of general and technical sessions, workshops and hands-on demonstrations and training sessions on the latest Progress technologies.
Need more info on OpenEdge and Progress? Join a Progress User Group.
Progress User Groups (PUGs) can be found all over the world. PUGs enable you to attend local technical presentations on current Progress topics, exchange ideas with other Progress users, and discuss the technology that is impacting today’s businesses. If you live in the Delaware Valley/Greater Philadelphia Area consider joining us for lunch at DVPUG. If you live elsewhere visit PSDN’s list of Progress User Groups to find one nearest you.
What are “Spectrum” and “RDL”?
“Spectrum” and “RDL” are product names that preceded “Progress 4GL”, “OpenEdge” and “ABL”.
“Spectrum” dates back to 1980.
10.1B Changes Integer Math Wrapping Behavior
An encryption component built into one of Solvepoint’s products came to our attention in a regression test on 10.1B. The root cause was a change in integer math wrapping behavior. Notice, I said “change”. I did not say “improvement”.
Some background, first. In most major computing languages integers wrap when an operation overflows the maximum allowed integer.
- In C# int i = int.MaxValue + 1; wraps.
- In ANSI C unsigned integers wrap.
- In Java integers wrap.
Integer wrapping behavior is an embedded, predictable and necessary part of many applications including a number of encryption algorithms, communications stacks, and data verification algorithms.
What about Progress applications? Well,…
All versions of Progress prior to 10.1B wrapped integers.
With the coming of 64-bit integers someone at Progress realized that it was possible to do 32-bit arithmetic in 64-bits and actually know whether the result went beyond the maximum (or minimum) integer. But just because something could be done doesn’t mean it should be done. So even though the Decimal data-type exists as an easy alternative to those wanting to avoid classic integer behavior, Progress changed integer math in 10.1B to throw an error instead of wrapping. If you’re incredulous, see Solution ID P119716.
In 10.1B 32-bit integer wraps throw an error “Value integer too large to fit in INTEGER datatype. (13682)“. Be prepared for some code written long ago to break.
If we multiply an integer less than the maximum 32-bit integer by a multiplicand that will result in a value greater than the maximum 32-bit integer, we can see the issue played out. In the following example we will use 1234567891 (a large easy to remember number less than max 32-bit int, +2,147,483,647) and multiply it by 10. In Pre 10.1B the result is -539222978, but as of 10.1B it returns an error.
Looking at the binary underneath this, see where 10.1B detects the overflow within 64-bits and throws the 32-bit exception #13682:
But can we now declare all Progress integer math safe? Not really. Since 64-bit integers in 10.1B are as blind to the >64 bits as 32-bit integers were to the >32 bits before 10.1B, 64-bit integers in 10.1B wrap around the way 32-bit integers did before 10.1B. (read that sentence several time if need be ) To help you understand why, Progress says “Checking for 64bit overflow would be too expensive. To do it would require putting everything in 128 bits to do calculations and then to copy it all back, since >64bits are required to do 64bit overflow checking. This would cause all arithmetic in the 4gl to grind to a halt. There are therefore no plans to do anything about this.”
So you’ve been warned. When this bites you’ll be able to say, “Darn!, and I read something about that somewhere!”
PS: And yes, for all the hard-core C jocks, signed integer wrapping is implementation dependent, not guaranteed to demonstrate modulus behavior, but usually wraps. Use of a signed integers rather than unsigned integers when classic integer wrapping is needed in C is a no-no.
Dataserver for Oracle 8i De-supported.
According to Progress the “Next Major Release after OpenEdge 10.1A” will not have support for Dataserver for Oracle 8i. Progress Software lists Dataserver for Oracle 10g as the Replacement Feature. Extended Support ends December 2007.
Progress defines “De-supported” usage as: “De-support is used where changes in technology or standards have made a feature obsolete and it is removed from the OpenEdge product. De-supported features have replacement equivalents and have zero impact on backwards compatibility.”
If you are stuck with an older embedded version of Oracle and need to move beyond OpenEdge 10.1A it may be time to explore available alternatives.
Most Dataserver users are not affected by the decision to De-Support Oracle 8i as they are able to upgrade Oracle.
OpenEdge ASSIGN Statement – More than just performance.
Back in the early ’90s when I broke the news of the ASSIGN statement in a Profiles in Progress article, I had no idea what silliness would follow. I also had no idea that twenty years later we would be seeing newly written code that still gets it wrong.
For the few who are as yet unaware, wrapping consecutive assignments with an ASSIGN has multiple benefits.
a = 4.
b = 3.
c = 7.
isn’t as good as
ASSIGN a = 4
b = 3
c = 7.
Shortly after the Profiles in Progress article was published there were signs that some in the Progress community were getting it wrong. Even though the article clearly graphed variations in execution time, some performance pundits started running around saying the ASSIGN statement was 2.7163639281 times faster than not using the ASSIGN. I’m exaggerating the precision of the number, of course, to make a point. None of those digits (including the initial “2″) are significant. None.
The ASSIGN statement varies for many reasons, not the least of which are whether what’s being assigned are components of an index in common, or part of a key that fully qualify a unique index. Let’s demonstrate one of these two factors using the example above.
Let’s suppose that a, b, and c are components of an index a-b-c. And let’s start, for simplicity’s sake, with a, b and c all having the value of “1″. In the ASSIGN-less code snippet, the index a-b-c will move through two transitional values, 4-1-1 and 4-3-1, before it gets to 4-3-7. Using ASSIGN, index a-b-c becomes 4-3-7 directly. Not using ASSIGN causes three-fold the activity:
Further, if index a-b-c is unique and the transitional values 4-1-1 or 4-3-1 already exist for another row, then the code without the ASSIGN statement will fail. Yes, fail.
Here’s a diagram that illustrates this index key collision for the above example:
The ASSIGN statement is not just an optional performance improvement as some believe. In some contexts, likely those least considered, lack of ASSIGN will affect reliability. Reliability is more important than Performance. Performance is also impacted as, in the example above, the application database must now perform triple the number of index lookups, inserts, and removes. Not good.
So everyone is using the ASSIGN statement, right? Sadly, no. One would think that we should find proper use of the ASSIGN statement it in all code written since the 80’s. Sadly, this isn’t the case –even in newly written 2007 code. We’ve seen it with our own eyes. As a community of Progress users, I know we can do better.
While this article isn’t an exhaustive presentation of the reasons why using ASSIGN is better, I’m hoping that the reliability and performance example above is compelling enough. It should be.
Hope this helps.
lkrela – What OpenEdge 10.1B users need to know and why it matters.
Progress users have come to expect extreme reliability from the Progress DB. This is no accident and has taken much consistent work over many years by the Progress Engine Crew. It is amazing how the Progress DB keeps ticking given what’s thrown at it in the field.
If you aren’t already aware, you should know that something bad has slipped through Progress’ regression tests.
If you are a 10.1B user, especially if you aren’t yet on Service Pack 3, your transactions may be exposed to data corruption. Tom Harris, OpenEdge RDBMS Development, calls it “a bad bug”. In an e-mail to the PEG DBA Forum, Tom has asked for users to “please” use the lkrela startup parameter “if you are running 10.1B prior to service pack 3″.
On OpenEdge versions 10.1B through Service Pack 2 inclusive there are critical bugs in locking that affect how locks are released, which in turn affect transaction consistency.
For your reference, the link to Solution ID is P126982.
While there is a fix in Service Pack 3, until Progress updates its regression tests for this and possibly related conditions, we are recommending to our clients that they keep using the lkrela startup parameter on 10.1B.
Sponsored by Solvepoint
This blog is sponsored by Solvepoint, a leader in Progress performance tuning, database administration, software development, maintenance and integration. More Information