2 minute read

Yesterday I received an email from a blog reader about caching and memory leaks…

Paraphrasing freely it went something like this:

We use Page.Cache to store temporary data, but we have recently discovered that it causes high memory consumption. The bad thing is that the memory never goes down even though the cache items have expired, and we suspect a possible memory leak in its implementation.

We have created this simple page

protected void Page_Load(object sender, EventArgs e)
    this.Page.Cache.Add(Guid.NewGuid().ToString(), Guid.NewGuid().ToString(), null, DateTime.MaxValue, TimeSpan.FromMinutes(1), CacheItemPriority.NotRemovable, new CacheItemRemovedCallback(this.OnRemoved));

public void OnRemoved(string key, object value, CacheItemRemovedReason r)
    value = null;

Which we stress with ACT (Application Center Test) for 5 minutes. Memory usage peaks at 450 MB, and after some time it decreases to 253 MB, but never goes down completely even though we waited for 10 minutes after the stress test. Our expectation is that the memory should go down to about 50-60 MB.

The question is does the above scenario fall into the category of memory leaks?

The short answer to the question is No (or at least not that I currently know of), however with a few minor changes to the code and the stress test you will see memory go down significantly.

So then why does memory not go down using this sample? And also, the test (when I ran it on my machine) generated a total of 15 161 requests, but the memory usage seems to indicate that a lot more than just the actual cached items stick around, why?

I will answer the question more thoroughly but before I do (later this week) I wanted to give all of you a chance to dissect this and see what things you come up with because I think it can bring up some nice discussion items, so feel free to comment away.

Hint: With a subtle (but not too obvious change) I can get the memory usage to peak at a maximum of 48 MB on my machine, now that is a huge difference:).

Some questions we want answered are

  1. Why does memory usage not go down to about 50-60 MB?
  2. Why does it seem like we are using more memory than the actual items we store in cache?
  3. What makes the stress test “invalid”?
  4. What is the difference between Page.Cache and Cache?

I will summarize the comments and add any additional things I can think of on Thursday or Friday.


Short note on reader emails

Since I get quite a bit of emails I don’t have time to answer them privately, so if you email me and I don’t answer, and you really need urgent help, please contact Microsoft Support or comment on the blog.

If I think the question is common enough that the answer would benefit more people (which may or may not be true, but I’ll reserve the right to make that call:)) I’ll answer with a blog post like in this case.

I will never give out any names of people who email me or include any sensitive data on the blog such as machine names, custom classes or anything else that can identify the sender, but if you email me and do not want me to paraphrase your question on the blog, please let me know.