ASP.NET Quiz Answers: Does Page.Cache leak memory?

ASP.NET Caching

writerus drivelus — Fri, 11 Aug 2006 13:55:43 GMT

Interesting Finds: August 10 part 2 and August 11, 2006

Jason Haley — Sat, 12 Aug 2006 06:24:43 GMT

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Blake05 — Mon, 14 Aug 2006 16:34:11 GMT

Very nice Follow up. Keep em coming.

-blake

Link Listing - August 14, 2006

Christopher Steen — Tue, 15 Aug 2006 07:45:25 GMT

Announcing
the Windows Mobile Virtual User Group Meeting [Via: trobbins ]
Refactoring
...

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

JA — Mon, 04 Sep 2006 13:03:33 GMT

Excellent. Thanks for this.

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Tu Hoang — Tue, 24 Oct 2006 04:52:38 GMT

the 2.0 Framework is also facing the same issue.

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Tess — Wed, 25 Oct 2006 08:54:05 GMT

Any version of the framework will. It is not really a problem in the framework as such but a problem with hooking up eventhandlers and delegate to static/cache objects.

ASP.NET Crash: Bad CacheItemRemovedCallback - Part II

If broken it is, fix it you should — Mon, 27 Nov 2006 13:47:11 GMT

In August I wrote about how you could cause a nasty high memory situation by using CacheItemRemovedCallbacks

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

dasveed — Fri, 02 Mar 2007 17:30:21 GMT

Can you advise what objects are stored in the asp.net "internal cache". I.e. the performance counter "Cache Total Entries" is a total of the counter "Cache API entries" and some other entries - "internal asp.net" cache entries.

In our application, we have about 1000 entries in the API cache - some compiled xslt transforms (with 3 hour expiries) and some other smaller stuff, but our "Cache Total Entries" is peaking at about 15000.

If we turn off "disableMemoryCollection" in machine/web.config then scavenging decides to cull everything in the cache.

If we turn it on, privatebytes bloats up until it runs out of memory and we have to recycle our web app's worker process.

It's all pointing to a bug in .net 2's implementation of cache scavenging.

Any ideas what .net uses the cache for internally?

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Todd Powers — Tue, 29 Apr 2008 01:38:12 GMT

I was having a similar issue and stumbled across your quiz and consequently these answers. The articles and comments were all a great read and I did learn a lot from this. So thank you.

My issue was that I was storing objects in cache, which, when expired, still consumed memory.

I was under the assumption that the framework would automatically call the Dispose() method on any object which implemented IDisposable, when the object was expired from cache.

This seems like a legitimate expectation, doesn't it? Is there some reason why this isn't done?

I ended up solving my problem by implementing a Shared sub in a global class, and passing AddressOf -sub- into the Cache.Insert, as the onRemove callback function.

I would have assumed that we didn't have to go to all this trouble, if the object implemented the IDisposable interface. I would have assumed that the framework would have been responsible enough to dispose of the object itself when the object was expired from cache.

The work-around did the trick, but don't you think the framework should have handled this?

I'd be interested to know if not, why?

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Tess — Tue, 29 Apr 2008 11:22:00 GMT

I dont have enough information about why it was implemented this way to make any type of judgement on that, and I can see how people might jump to the conclusion that IDisposable objects are disposed/closed when thrown out of cache.

However, if you think about it from a broader perspective, there really is no difference between an object going out of scope when a method ends, or when it gets thrown out of cache. For example if you use a connection in a method you would have to manually close it or use a using statement to get it auto disposed/closed. If you don't it will stick around until finalized...

I could see one problem with autodisposing items when they are no longer cached, if for example someone else has a reference to it you would end up with a situation where you would have a reference to an already disposed object, without knowing it... Something like that might be possible to check for, but there is always a tradeoff between perf and getting all the frills.

Don't get me wrong, I see your point but I think that very careful consideration would have to be taken before making such a decision. Since off the top of my head I can already come up with one issue with it, I personally think the current solution, where the developer is given a choice to do something with the object when it goes out of scope may just be the cleanest one...

Anyways, just my 2 cent, in reality I don't know enough about the decisions around this to talk intelligently about it.

ASP.NET Memory Issues - High Memory Usage with AjaxPro (fixed in current version)

If broken it is, fix it you should — Sun, 14 Sep 2008 18:05:13 GMT

I was helping a colleague out with an OOM (OutOfMemory) situation he was dealing with. Problem description:

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Mike — Wed, 02 Sep 2009 22:08:08 GMT

Can you please reply to the comment "Friday, March 02, 2007 9:30 AM by dasveed"? I have noticed the same types of numbers in my application.

re: ASP.NET Quiz Answers: Does Page.Cache leak memory?

Tess — Thu, 03 Sep 2009 12:29:52 GMT

Regarding dasveed's comment.

The Cache Total entries includes both Cache API entries i.e. Cache.Add(...) or Cache["..."] = ... , output cache, sessions etc.

There are two counters that you can use to determine when the cache will be scavenged.

Cache % Machine Memory Limit used = "The amount of physical memory used by the machine divided by the physical memory limit for the cache, as a percentage. When this reaches 100%, half of the cache entries will be forcibly removed. The __Total__ instance is the average of all instances, and therefore cannot be used to determine when cache entries will be forcibly removed."

and

Cache % Process Memory Limit Used = "The value of private bytes for the worker process divided by the private bytes memory limit for the cache, as a percentage. When this reaches 100%, half of the cache entries will be forcibly removed. The __Total__ instance is the average of all instances, and therefore cannot be used to determine when cache entries will be forcibly removed."

You can partially decide when cache will be scavenged by setting the Memory limit settings on the application pool. These values are used to determine if the cache should be scavenged or not. At that point it will scavenge as many cached items as it needs to recover memory.

To make sure a particluar cached object stays cached until it expires you can set it to non-removable (which is done for the session for example). All other cached objects should be seen as items that could potentially be removed if needed and should be tested for their existence when used.

Re. the 2nd question about how the cache is stored internally. Simplified, the cache is a static variable with a list of cacheitems referencing the actual cached items.

In-proc session state uses the cache by caching a session item per session with a dictionary of session vars. The cached session is set to non-removable and has the expiration time set to a sliding expiration of for example 20 mins if that is the session timeout.

hope that helps