Quick things to check when you experience high memory levels in ASP.NET (893660)

INTRODUCTION

Hello, this is Michael Graham from the ASP.NET team, and this will be my first of hopefully many entries to this column. Just a quick note about myself: I started with the ASP team in 1997 and was there not long after the introduction of Visual InterDev 1.0, Microsoft's first foray into the world of server-side applications in the Internet environment. So I have been at this for quite a long time! Today, I'm going to go over some reasonably simple things that you can check for when you find yourself running into memory issues in ASP.NET. We'll start with some common issues, actions you can take to remedy these issues, and a brief explanation of why these situations can cause problems. Let's get to it!

What is considered high memory?

Obviously, this is going to be dependent on volume and activity of specific applications. But, in general, high memory is when see that your Aspnet_wp.exe process (Internet Information Services (IIS) 5.0) or W3wp process (IIS 6.0) memory is consistently increasing and is not returning to a comfortable level. In very 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, we are performing less than we should be, and this may affect other applications running on the system. The key here is to understand that some applications require more memory than others, and if you are exceeding these limits, you may want to add more memory or add another server to your Web farm (or consider a Web farm). Also, profiling is recommended in these cases, which can enable developers to create leaner applications. In this article, we are looking at a situation where you consistently see memory rise until the server stops performing.

Common reasons for high memory

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. While you are developing your application, this is a necessity. By default, when you create your application in Visual Studio .NET, you will 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 this in "Release" mode, not "Debug" mode. Once you are in production, this should no longer be necessary. It can really slow down your performance and eat up your memory. Why? Well, 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 specifically 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 somewhat randomly across your memory space, making it more and more difficult for you to find the contiguous space to allocate memory for when you need it. Second, the executionTimeout attribute (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpgenref/html/gngrfhttpruntimesection.asp) is set to a very high number, overriding the default of 90 seconds. This is 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 is a big risk in production. This 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 will be creating more files in your Temporary ASP.NET files folder, and the System.Diagnostics.DebuggableAttribute (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemdiagnostics.asp) 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. Leaving debugging enabled is bad. We see this all too often, and it is so easy to change. Also, remember that this can be set at the page level, so make sure that all of your pages are not setting this.

String concatenation

There are applications that build HTML output by using server-side code and by just building one big HTML string to send to the browser. This is fine, but if you are building the string by using "+" and "&" concatenation, you may not be aware of how many large strings you are 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 are 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 are just storing the last line, but you are storing all of these lines. You can see how this could get out of hand, especially when you are building a large table, perhaps by looping through a large recordset. If this is what you are doing, use our System.Text.StringBuilder class, so that you just store the one large string. Here's how:

.NET Framework 1.1 Service Pack 1 (SP1)

If you are not running the .NET Framework 1.1 SP1 yet, install this if you are experiencing memory issues. I won't go into great detail, but basically, with SP1 we are now allocating memory in a much more efficient manner. Basically, we are 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 are using a lot of small boxes rather than a few large boxes. That is the idea here.

Don't be afraid to recycle periodically

In IIS 6.0, by default, we recycle application pools every 29 hours. In IIS 5.0, the Aspnet_wp.exe process will keep going until you end the task, restart IIS, or restart the computer. This means that this process could be running for months. For some applications, it's a pretty good idea 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 are experiencing memory issues, ask yourself these questions:

• Am I using a lot of 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 is used wisely, this is a great benefit to performance. But when it is used unwisely, you wind up with a lot of 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 here are some commands you can use 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 is something that your managed code is building.

!dumpheap -stat

This command will take quite a while to run, even hours if your memory is very 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 will see a lot of System.String objects)

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

!do <addr>

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

To learn more about how to set up and use these commands, take a look at this previous Support Voice column:We'll be trying to incorporate more ways to use this wonderful diagnostic tool for specific situations in these columns. Please let us know if we are doing a good job with this!

Resources

Memory articlesPerformance articlesAs always, feel free to submit ideas on topics you want addressed in future columns or in the Knowledge Base using the Ask For It form.