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Why will there be a memoryleak if I don't dispose the stream? #365

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szguoxz opened this issue Dec 21, 2024 · 11 comments
Open

Why will there be a memoryleak if I don't dispose the stream? #365

szguoxz opened this issue Dec 21, 2024 · 11 comments

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@szguoxz
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szguoxz commented Dec 21, 2024 •
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I don't quite understand. I thought the blocks are Garbage Collected if I do not dispose the stream. Why is there a memory leak?

Basically I am having trouble under this paragraph in the readme doc on this project:

If you forget to call a stream's Dispose method, this could cause a memory leak. To help you prevent this, each stream has a finalizer that will be called by the CLR once there are no more references to the stream. This finalizer will raise an event or log a message about the leaked stream.

So what the heck is it talking about? There will be a memory leak? Or could cause a memory leak depend on my Luck?
@udlose
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udlose commented Dec 21, 2024

I posted this answer on SO some years ago. It should help explain:

What happens if I don't call Dispose on the pen object

@szguoxz
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szguoxz commented Dec 21, 2024

I know what will happen if you don't call dispose, that's why I am having this question.
Because in the document of README of this project, it said, if you do not dispose the stream, you will get memoryleak. I wonder if the DOC is wrong here. I thought eventually it will be garbage colelcted, thus there will be no memoryleak.

@sgorozco
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sgorozco commented Dec 23, 2024 •
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You have a case here; the documentation wording might be a bit clearer - basically the documentation is stating that there exists a safety-net finalizer that will prevent a memory leak in case you forget to dispose the stream. The documentation also states that you should treat forgetting to dispose the stream as a problem, that is why the library gives you a lot of support to track undisposed instances. The problem with finalizers is that you do not know when they will eventually run, so you might end up pooling up more resources than strictly required (imagine having 50+ free buffers waiting to be finalized and returned to the pool and still having to allocate extra buffers because the pool has been drained, then eventually the finalizers run and now you end up with a surplus of pooled buffers). Whether this is a serious problem, I guess it will depend on the use-case, the amount of free memory, and your own idiosyncrasies. The library will still greatly reduce strain on the garbage collector, but will not perform optimally.

@szguoxz
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szguoxz commented Dec 24, 2024 via email

@szguoxz
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szguoxz commented Dec 24, 2024 via email

@udlose
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udlose commented Dec 24, 2024

Basically I need to know that I have airbag in my car even I should not depend on it. The doc makes it sound like I do not have an airbag in my car which makes me really nervous that I dare not to drive anymore. Get Outlook for iOShttps://aka.ms/o0ukef
…
________________________________ From: Calvin Guo @.> Sent: Tuesday, December 24, 2024 3:14:12 PM To: microsoft/Microsoft.IO.RecyclableMemoryStream @.>; microsoft/Microsoft.IO.RecyclableMemoryStream @.> Cc: Author @.> Subject: Re: [microsoft/Microsoft.IO.RecyclableMemoryStream] Why will there be a memoryleak if I don't dispose the stream? (Issue #365) So the document needs to be fixed. I just needs to know if for some reason I forgot, it is still covered! Get Outlook for iOShttps://aka.ms/o0ukef
________________________________ From: Sergio Gerardo Orozco @.> Sent: Tuesday, December 24, 2024 12:47:02 AM To: microsoft/Microsoft.IO.RecyclableMemoryStream @.> Cc: szguoxz @.>; Author @.> Subject: Re: [microsoft/Microsoft.IO.RecyclableMemoryStream] Why will there be a memoryleak if I don't dispose the stream? (Issue #365) You have a case here; the documentation wording might be a bit clearer - basically the documentation is stating that there exists a safety-net finalizer that will prevent a memory leak in case you forget to dispose the stream. The documentation also states that you should treat forgetting to dispose the stream as a problem, that is why the library gives you a lot of support to track undisposed instances. The problem with finalizers is that you do not know when they will eventually run, so you might end up pooling up more resources than strictly required (imagine having 50+ free buffers waiting to be finalized and returned to the pool and still having to allocate extra buffers because the pool has been drained, then eventually the finalizers run and now you end up with a surplus of pooled buffers). Whether this is a serious problem, I guess it will depend on the use-case, the amount of free memory, and your own idiosyncrasies. The library will still reduce some strain on the garbage collector, but will not perform optimally. — Reply to this email directly, view it on GitHub<#365 (comment)>, or unsubscribehttps://github.com/notifications/unsubscribe-auth/AEHJXCYJJ37VJCED5N42QW32HA5ANAVCNFSM6AAAAABUABXQBKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDKNRQGAYTINRRGE. You are receiving this because you authored the thread.Message ID: @.***>

I was trying to say the same thing. However, you were able to explain it much more clearly than I was :). I'll have to use your analogy in the future - I think you said it perfectly!

@sgorozco
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Basically I need to know that I have airbag in my car even I should not depend on it. The doc makes it sound like I do not have an airbag in my car which makes me really nervous that I dare not to drive anymore. Get Outlook for iOShttps://aka.ms/o0ukef
…

If you take a look at the logic that handles the buffer pooling in RecyclableMemoryStreamManager.cs I think that even in the absence of the Finalizer, there would actually be no memory leaks, you would simply be allocating a buffer indirectly via the class and it would be collected when no more references to it exist, like an ordinary array allocated via new() - the allocated buffer is not held in an inner collection that would prevent the buffer from being collected.

The Finalizer more than preventing a memory leak, is actually ensuring that a non-disposed buffer is eventually recycled.

@szguoxz
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szguoxz commented Dec 26, 2024 via email

@benmwatson
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benmwatson commented Jan 9, 2025 •
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Please absolutely suggest some new wording if you feel that would be better. I can see why it might be misleading, but there is a logic to it.

What constitutes a memory leak is in some ways a matter of opinion and degrees. Yes, the GC will clean up any dropped buffers eventually, so in the larger context, there is no leak per-se.

However, the intent of this library to minimize the allocations and expense of managing large buffers, especially on the large object heap. By failing to dispose, you're in the worst of both worlds--you've allocated a large pool AND you're not re-pooling items. You could call this a mistake or a leak--either way, you're using way more memory than intended. The heap WILL grow larger until the GC kicks off. On big servers (the primary use case IMO), if you have a large amount of physical memory and your buffers are all on the LOH, this could be a very long period of growth until the GC kicks in. (To give an example, we measure the number of full, compacting GCs in occurrences per DAY). If you are watching the memory of the process, this will look very much like a memory leak, and you'll wonder why the library isn't helping.

(As an aside, one thing that is particular about RMS is that it is very "opinionated"--it solves a niche problem in a particular way and the code wants to be used just like that, with some configuration options around the edges. That's one reason why this library has not been subsumed into the overall .NET framework libraries.)

@udlose
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udlose commented Jan 9, 2025 •
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This is one subject that has caused many .NET developers to ask the same question.

In short, a "memory leak" in the .NET world is just hanging on the memory longer than you'd like to or should. A memory leak in the unmanaged world means the memory is lost not only to your application, but also the Operating System. In .NET, the "leak" only affects the .NET process where the "leak" takes place.

There are a very small handful of exceptions where a memory leak in .NET can actually affect the OS. The OP's question and a number of posts on SO have led me to start working a post on my blog about the difference between the two, what a leak is in .NET, what causes them, and how to avoid them. Not sure if I should make them a series of smaller posts or a larger one covering it all?

Does anyone have any suggestions? @sgorozco @szguoxz @benmwatson

@szguoxz
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szguoxz commented Jan 10, 2025 via email

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@udlose @sgorozco @benmwatson @szguoxz