Quick things to check when you experience high memory levels in ASP.NET

This article describes the quick things to check when you experience high memory in Microsoft ASP.NET.

Original product version:   ASP.NET
Original KB number:   893660

Introduction

This article will start with some common issues, actions to remedy these issues, and a brief explanation of why these situations can cause problems.

ASP.NET Support Voice column

In the April 2005 Support Voice column, we inadvertently provided a link to the wrong file. Instead of linking to a download for the Web service, we linked to the XML file returned by the Web service. That link has been corrected. If you'd like to review the article with the correct file attached, see Dynamic page updates using XMLHTTP.

What is considered high memory

Obviously, this question is going to be dependent on volume and activity of specific applications. But, in general, high memory is when your Aspnet_wp.exe process (Internet Information Services (IIS)) or W3wp process (IIS) memory is consistently increasing and isn't returning to a comfortable level.

In general terms, a comfortable level would be under 600 MB in the default 2-GB user memory address space. Once the memory level is higher than that comfortable level, we're doing less than we should be. This behavior may affect other applications that are running on the system.

The key is to understand some applications require more memory than others. If you're exceeding these limits, you may add more memory or add another server to your Web farm (or consider a Web farm). Profiling is also recommended in these cases, which can enable developers to create leaner applications. In this article, we're looking at a situation where you consistently see memory rise until the server stops performing.

Application set up for debugging

One reason for high memory that we see here in Support a lot is when you have debugging, tracing, or both enabled for your application. Enabling debugging and tracing is a necessity when you develop your application. By default, when you create your application in Visual Studio .NET, you'll see the following attribute set in your Web.config file:

<compilation
 ...
 debug="true"
 />

and/or

 <trace
 enabled="true"
 ...
 />

Also, when you do a final build of your application, make sure that you do it in Release mode, not Debug mode. Once you're in production, debugging should no longer be necessary. It can really slow down your performance and eat up your memory. Setting this attribute means you change a few things about how you handle your application.

First, batch compile will be disabled, even if it's set in this compilation element. What this means is that you create an assembly for every page in your application so that you can break into it. These assemblies can be scattered randomly across your memory space, making it more and more difficult for you to find the contiguous space to allocate memory.

Second, the executionTimeout attribute (<httpRuntime> Element) is set to a high number, overriding the default of 90 seconds. It's fine when debugging, because you can't have the application time out while you patiently step through the code to find your blunders. However, it's a significant risk in production. It means that should you have a rogue request for whatever reason, it will hold on to a thread and continue any detrimental behavior for days rather than just a minute and a half.

Finally, you'll be creating more files in your Temporary ASP.NET files folder, and the System.Diagnostics.DebuggableAttribute (System.Diagnostics Namespace gets added to all generated code, which can cause performance degradation.

If you get nothing else from this article, I do hope you get this information. Leaving debugging enabled is bad. We see this behavior all too often, and it's so easy to change. Remember it can be set at the page level. Make sure all of your pages aren't setting it.

String concatenation

There are applications that build HTML output by using server-side code and by just building one large HTML string to send to the browser. It's fine, but if you're building the string by using + and & concatenation, you may not be aware of how many large strings you're building. For example:

string mystring = "<html>";
mystring = mystring + "<table><tr><td>";
mystring = mystring + "First Cell";
mystring = mystring + "</td></tr></table>";
mystring = mystring + "</html>";

This code seems harmless enough, but here's what you're storing in memory:

<html>
<html><table><tr><td>
<html><table><tr><td>First Cell
<html><table><tr><td>First Cell</td></tr></table>
<html><table><tr><td>First Cell</td></tr></table></html>

You may think that you're just storing the last line, but you're storing all of these lines. You can see how it could get out of hand, especially when you're building a large table, perhaps by looping through a large recordset. If it's what you're doing, use our System.Text.StringBuilder class, so that you just store the one large string. See Use Visual C# to improve string concatenation performance

.NET Framework Service Pack 1 (SP1)

If you aren't running the .NET Framework SP1 yet, install this SP if you're experiencing memory issues. I won't go into great detail, but basically, with SP1 we're now allocating memory in a much more efficient manner. Basically, we're allocating 16 MB at a time for large objects rather than 64 MB at a time. We've all moved, and we all know that we can pack a lot more into a car or truck if we're using many small boxes rather than a few large boxes. It's the idea here.

Don't be afraid to recycle periodically

We recycle application pools in IIS every 29 hours by default. The Aspnet_wp.exe process will keep going until you end the task, restart IIS, or restart the computer. This behavior means that this process could be running for months. It's a good idea for some applications to just restart the worker process every couple of days or so, at a convenient time.

Questions to ask

The previous were all things that you can fix quickly. However, if you're experiencing memory issues, ask yourself these questions:

  • Am I using many large objects? More than 85,000 KB are stored in a large object heap.

  • Am I storing objects in Session state? These objects are going to stay in memory for much longer than if you use and dispose them.

  • Am I using the Cache object? When it's used wisely, it's a great benefit to performance. But when it's used unwisely, you wind up with much memory used that is never released.

  • Am I returning recordsets too large for a Web application? No one wants to look at 1,000 records on a Web page. You should be designing your application so that you never get more than 50 to 100 records in one trip.

Debugging

I won't get into setting up WinDbg. But you can use the following commands to see what exactly is in your memory, if you wish to troubleshoot more complicated issues.

!eeheap -gc

This command will show you how much managed memory you have. If this value is high, there's something that your managed code is building.

!dumpheap -stat

This command will take quite a while to run, even hours if your memory is large. But this command will give you a list of all of your objects, how many of each type, and how much memory each is using. (For example, for the StringBuilder class, you'll see many System.String objects)

Once you have found an object taking much memory, you can dig further by using the following command:

!do <addr>

You can get the address of the object you're looking for in the dumpheap command.

To learn more about how to set up and use these commands, see Troubleshooting ASP.NET using WinDbg and the SOS extension.

We'll be trying to incorporate more ways to use this diagnostic tool for specific situations in these columns. Let us know if we're doing a good job!

Memory and performance articles

Garbage Collector Basics and Performance Hints

Developing High-Performance ASP.NET Applications

ASP.NET Performance Monitoring, and When to Alert Administrators

Improving .NET Application Performance and Scalability