How cfcomponent output=true can affect memory consumption


A single, heavily used component with output=true caused ColdFusion to throw Out Of Memory Errors. Setting output=false solved the problem.


Bob Silverberg and I were trying to tackle a memory problem in his unit tests for ValidateThis . Running CF with 1GB of memory, he'd get Out Of Memory Errors and his tests would not complete when run in the browser -- i.e. all tests running in a single page request. When run through Eclipse -- where each test method is run as a separate request -- he had no troubles.

This is our story.

First Line of Defense

I always suggest these as the first culprits in memory problems:

  1. Memory Tracking turned on in the CF Server Monitor
  2. Request output debugging turned on in CFAdmin (I'm NOT talking about the Line Debugger... just regular old debug output)

In and of themselves neither of these are evil; however, when an application, or a particular page, does a tremendous amount of work, both of those can become problems in a hurry.

Bob turned both off, and he was still running out of memory.

My first task...

I needed to replicate the issue. I got all his code and ran his tests. Fortunately, I, too, received an OOME. I then turned off debugging and, sure enough, the tests completed as expected (though slowly).

So, lesson #1 kids: debugging can crash your server

At this point, I no longer have problems, but Bob still does. Thus, we're not done yet

Meanwhile, Back at Bob's Ranch

I suggested to Bob that he start to troubleshoot this by adding the "excludes" attribute onto his DirectoryTestSuite, and initially exclude everything but the first test, then incrementally remove tests from the Excludes until he got the error. Then, he'd probably know which test was causing troubles. 

Normally, this is a fine approach. It turns out that it wouldn't have helped anyway. But hey, this stuff is hit and miss.

Gathering data

My next step was to get a heap dump of ColdFusion as it was running the tests... I needed to see what was consuming all the memory. To do so, I used the java "jmap" command to get the dump and Eclipse MAT to analyze the dump. This is not nearly as complicated as it sounds, and I wrote up full instructions here .

Inspecting data

Narrowing down to "FusionContext"

I used Eclipse MAT to inspect the heap dump. When I first loaded it, I opened the "Dominator Tree" and saw the following two entries sucking up a combined 200+MB of RAM:

  • coldfusion.filter.FusionContext
  • coldfusion.bootstrat.BoostrapClassLoader

Let's look at the first few consumers for each of these:

Now, looking at the BootstrapClassLoader, I see a FontSetBuilder taking up a whole lot of memory. I have no idea what the hell that is, so I ignore it. I figure I can't do anything about it anyway, so....

Let's look at the big old HashMap in the FusionContext. That looks like something I might be able to do something about. Why? Well, I don't know... I just imagine that any variables my app is creating live in there.

Narrowing down to a Code File

When working with Eclipse MAT, keep your eyes on the "Inspector View" as you select different items in the "dominator tree" editor

This was kind of just dumb luck and I'm not sure exactly what it means. Nonetheless, if you look at the image below, you'll see that when I opened up the HashMap in the FusionContext I had a whole mess of entries for "CFDummyComponent". Clicking on one of those showed -- over in the "Inspector" view on the left, that the file was "cfMockRegistry2ecfc" followed by some numbers. This told me that the file associated with that component was "MockRegistry.cfc".

Hmmm... MockRegistry is a component of MightyMock, and Bob's tests make heavy use of MightyMock.

Then, drilling into one of those instances, you see a lot of instances.

Light Bulb

So here's where I put things together:

  1. File named MockRegistry.cfc
  2. Tons of OutputTag instances

What could this mean?

Well, that must mean that something's being output in that file. But what? it's a component that sets up mocking... why would it be outputting anything?

Sure enough, opening the file showed me the answer: <cfcomponent output="true">

Full Disclosure

I left out a step in the lightbulb moment, but the problem is that I cannot for the life of me figure out how to get back to what I saw. Basically, when I was digging through the heap dump in MAT, I saw an instance of "cfsavecontent". That is truly what made me suspicious about the output problem. But, I can't find it again... so be it.


I set output="false" on MockRegistry.cfc, ran all of the unit tests to confirm that didn't break anything. Then I ran Bob's code again. Bingo... no OOME errors. In addition, while the tests were running, I took another heap dump, and Sure enough, memory was down to a completely reasonable level. Rather than consuming 100+MB of memory, FusionContext was down in the teens.

Does this mean output="true" is bad?  No, it doesn't. It means a) don't use it unless you need it, and b) something's going on under the hood in CF where, if you have output="true" on in a component, it seems to keep a reference to  everything in that page's context, such that normal garbage collection can't do its job because the current stack still has a reference to everything in that OutputTag. It appears that when output="false", the data in the component and the component's functions are not being referenced by an OutputTag and as such are free to be reclaimed by the garbage collector


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  1. Sep 16, 2010

    Anonymous says:

    This sound a lot like my memory leak problem I saw on CF8 having to do with the ...

    This sound a lot like my memory leak problem I saw on CF8 having to do with the "output" tag being omitted:

    This begs the question -- which version CF did you test on and what was the updater / hot fixes applied.  Supposedly this issue was fixed in Cumulative Hot fix 4 for ColdFusion 8.0.1. You can download it from here:

  2. Sep 17, 2010

    Marc Esher says:

    Peter, I saw this behavior on CF9.0.1 with the latest hotfixes applied. I'm pre...


    I saw this behavior on CF9.0.1 with the latest hotfixes applied. I'm pretty sure Bob is running the same setup, though on a Mac (I'm on Windows)