Using GC Efficiently – Part 1

re: Using GC Efficiently – Part 1

Wayne — Wed, 16 Jun 2004 00:44:00 GMT

Good stuff. Keep it coming, please.

re: Using GC Efficiently – Part 1

SubZero — Wed, 16 Jun 2004 03:36:00 GMT

Wow - I used to get paid (approx. $500 per article) for writing things like this.

Using GC Efficiently

Cook Computing — Wed, 16 Jun 2004 10:24:00 GMT

Microsoft developer Maoni has some more interesting GC-related information, this time on Using GC Efficiently . While you don't want to get bogged down in micro-optimization too early, it important to bear stuff like this in mind when designing high...

re: Using GC Efficiently – Part 1

Vishal Joshi — Wed, 16 Jun 2004 22:24:00 GMT

A very good article... I recommend you to send it to MSDN Knowlege Base...

re: Using GC Efficiently – Part 1

avnrao — Mon, 21 Jun 2004 09:08:00 GMT

cool stuff.. please continue this blog.
pls answer these few questions..

1. how do i find out how many objects exist in Gen 0,1 and 2 separately?
2. In collection section of your blog..you mentioned that GC fires when any of the three conditions meet. what happens to the objects if none of the conditions meet.
say.. I call a method, create some object object X and finish the method. say my Gen 0 doesnt reach the threshold (is there any default??), when will GC be collected? until GC starts, will the object be alive even if my application is stopped?

pls do answer.

thank you,
Av.

re: Using GC Efficiently – Part 1

Maoni — Mon, 21 Jun 2004 22:17:00 GMT

Hi Avnrao, to answer your questions:

1) You can get the generation sizes from perfmon counters. Please see http://blogs.msdn.com/maoni/archive/2004/06/03/148029.aspx. We don't expose the number of objects. It's more useful to know how much space they occupy rather than how many of them there are. Use CLRProfiler if you need to know what the objecs are on the heap.

2) If no GC happens, your object will just stay alive in the heap. I am not sure what you mean by "my application is stopped" - does that mean your applicatoin's process is idle or terminated or something else?

re: Using GC Efficiently – Part 1

Luke S. — Thu, 24 Jun 2004 05:45:00 GMT

That's an interesting read thanks for posting.
Though now I can understand more why .NET GC is slow and inferior to native allocations:
1. you say you are allocating (reserving) 16MB per segment. What does that do to my PTEs? My simple calc says that you would need 4000 PTEs per your GC heap, thus 12000 for all three of them, 48kb of non paged memory? How does that compare to native HelloWorld where all my code might fit in few kbs?
2. strategy for allocation: if you do not collect in G0 until 16MB isn't filled, what does that do to my L2 cache? I don't know about you guys at MS, but my best CPU has 2MB. Does that mean that instead of being local and fitting nice in L2 I am bouncing other data out? I guess that doesn't help perf there?
3. what objects go to GC? If I have for (…){String s1;…} does s1 it goes to GC or you keep it on stack? From Rotor I understand it will be put on GC and though compiler has perfect control and info over lifetime of S1? Why not to put in on stack, or at least to move your GC pointer back and forth?

I wonder is there any a theoretical case where manged code with GC would be anywhere close to well crafted native allocation? Doubt it...

re: Using GC Efficiently – Part 1

Maoni — Thu, 24 Jun 2004 18:40:00 GMT

Luke, you got this completely wrong. Let me explain:

1) PTEs are not allocated for reserved memory. And we don't allocate a segment for each generation.
2) See 1), we don't allocate a whole segment for Gen0. Gen0 is a lot smaller than the whole segment.
3) In your code example, it is clear that the compiler could figure out that the scope and lifetime are the same so the object could be safely be allocated on the stack without any checks. Otherwise, in a normal case, where the object is passed to a method, you couldn’t allocate it on the stack even with checks. So it could be done with a smart compiler and some changes in the EE but it isn't clear how much it buys you in practice because when the lifetime isn't clear, the compiler has to heap allocate.

Finally, do you have evidence that GC is slower than native allocations? Because that's the opposite of our experience.

re: Using GC Efficiently – Part 1

Akshay Kumar — Fri, 09 Jul 2004 06:32:00 GMT

Hi Maoni,
This is excellent stuff . But I have to say this .
I do not fine relation between this theory with ASP.NET technology and Win forms technology.
e.g What impact does session management has when done is a certain fashion on GC. b'cos its MS developer who wrote session management or cache management engine.
So how are objects in session disposed and GC'ed.
There has to be a relationship established for overall performance increase.

this comment is not for you but its for people writing scalability guides and MS product documentation( as I have no access to them) may be you can send my gripe to them.
Once again good post.
Regards,
Akshay

re: Using GC Efficiently – Part 1

Maoni — Fri, 09 Jul 2004 13:39:00 GMT

Hi Akshay, thanks for your feedback. I've asked our ASP.NET and WinForms perf people whether they have any documents of such. I'll post the responses once I have them.

re: Using GC Efficiently – Part 1

Maarten — Thu, 15 Jul 2004 05:00:00 GMT

Naomi, Could you explain what is in the 8 byte sync block you mention? What is is used for?

Thank you

re: Using GC Efficiently – Part 1

Maoni — Thu, 15 Jul 2004 05:40:00 GMT

Maarten, take a look at Jeff Richter's MSDN article http://msdn.microsoft.com/msdnmag/issues/03/01/NET/
It explains a sync block use. It's also used for various other things in the runtime but what Jeff mentioned in his article is one of the most important usages of sync blocks. Basically think of it as some data associated with the object and allocated on demand.

Backlog

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In Using GC Efficiently – Part 2 I talked about different flavors of GC that exist in the CLR and how

So, what’s new in the CLR 2.0 GC?

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Certainly that’s one of the most frequently asked questions I get (at the PDC too!). So since PDC already

Doubt in JIT and Method Call

Ramachandran — Mon, 11 Dec 2006 10:19:57 GMT

Hi Maoni,

I read about method calls and JIT in Jeff's CLR via C# book. There i read the following concepts.

When a method is about to call, iT finds all the types references/used in the method and creates type objects for those types and loads the MSIL of the methods/types and associate them with the type object.

When a method is called, it loads the MSIL JITs it and Updates the type object to update the JIT compiled code instead of MSIL code.

When a non virtual method is called it **finds the type object and finds the address** of the JIT compiled code or MSIL code and do the above things.

How does it finds the exact address of the type object?

How this happens if it is a value type?.

regards

RAM

re: Using GC Efficiently – Part 1

Raj — Wed, 20 Dec 2006 09:47:25 GMT

Hi Maoni,

nice Article on GC.

I have some doubts in it.

you said when EE stars it allocate one segment for small objects and one segement for LOH. in which generations will it creates?

2)some time back i read one article in which he stated that when an application started it will create 2 threads one for main and one for GC in this situation how the memory reclaims for the following scenario.

I created a reference type object in a method so that the memory is created in Gen0 and the Gen0 is not full suddenly my application terminated. what about the memory allocated?

re: Using GC Efficiently – Part 1

maoni — Wed, 20 Dec 2006 10:51:48 GMT

Raj,

Re 1) LOH is always treated as gen2 objects - meaning they are only collected with gen2 colletions. For SOH (Small Object Heap) the initial objects are gen0 then the survivors get promoted to gen1; then you start allocating in gen0 again. The segment often looks like this:

from lowest address to highest address

gen2 | gen1 | gen0 | end space of this segment

Take a look at my slide deck from the last PDC at

http://216.55.183.13/pdc2005/slides/FUN421_Stephens.ppt

(note that last time I tried I couldn't open this in IE but I could save it)

re 2) regardless of what GC does, all private memory associated with the process will be freed by the OS.

re: Using GC Efficiently – Part 1

Bill — Sun, 14 Jan 2007 07:52:24 GMT

Maoni,

Can you recommend a good resource on how to troubleshoot an ASP .Net application that has too many gen2 collections. I am having a hard time determining why the gen 2 collections are happening. Our research indicates we have too many segements being allocated or the sytem is in a low memory situation (possibly both).

Thanks,

Bill

re: Using GC Efficiently – Part 1

maoni — Mon, 15 Jan 2007 10:36:04 GMT

Take a look: http://msdn.microsoft.com/msdnmag/issues/06/11/CLRInsideOut/default.aspx?loc=en

re: Using GC Efficiently – Part 1

Bill — Mon, 15 Jan 2007 19:22:49 GMT

Thanks for the troubleshooting article :)

Can you explain in more detail the situation where garbage collection is triggered by:

System is in low memory

Do you mean the entire machine is low on memory or just the .net process? What are the thresholds that must be crossed for this situation to happen??

Bill

re: Using GC Efficiently – Part 1

maoni — Tue, 16 Jan 2007 00:45:35 GMT

Bill, we measure the machine memory load, not per process. On different OSs the values are different but you can consider a GC is likely to be triggered if the machine mem load is in the high 80's. However we don't do full GCs all the time when the memory load is high - we tend to do a full GC only every few GCs.

re: Using GC Efficiently – Part 1

Bill — Thu, 18 Jan 2007 18:20:42 GMT

Maoni,

You mentioned "if you allocate a large object that you expect to not move, you should make sure to pin it".

In several of my company's applications developers have loaded a DataSet with static data during the ASP .Net application start. In your opinion we can help the GC by pinning the DataSet so the GC doesn't have to "look" at the static data during collections?

Would this code look like this??

DataSet ds = new DataSet();

... code to load ds ...

GCHandle pinHandle = GCHandle.Alloc(ds,GCHandleType.Pinned);

Thanks,

Bill

re: Using GC Efficiently – Part 1

maoni — Sat, 20 Jan 2007 01:07:36 GMT

When I said "if you allocate a large object that you expect to not move, you should make sure to pin it", my intention was to tell you that not compacting LOH is an implementation detail that you should not rely on. So to make sure your large object don't move you should pin it.

You shouldn't pin those objects. Since they are allocated when the process starts up, they will likely be moved into a gen2 segment and stay there anyway. Pinning them would only introduce possible fragmentation and won't gain you anything (and makes GC's job harder).

re: Using GC Efficiently – Part 1

pmackay — Fri, 16 Feb 2007 00:40:41 GMT

Maoni,

I checked your FUN421_Stephens.ppt, and in the slide 9, after 500 GC, what could happen with a segment if after the 501th collect, it frees many objects that lived in gen 2?

¿Is that segment going to be like this?

222222222222222222222222222222222222222FFFFFFFFFF

22FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

222222221111111110000000000FFFFFFFFFFFFFFFFFFFFFF

or like this, due to a compact

222222222222222222222222222222222222222FFFFFFFFFF

22222222221111111110000000000FFFFFFFFFFFFFFFFFFFF

Also, Is possible to have something like this?

222222222222222222222222222222222222222222222FFFF

2222222222222222222222222222222222111111111111111

111111111111100000000000FFFFFFFFFFFFFFFFFFFFFFFFF

Thanks,

patrick.

re: Using GC Efficiently – Part 1

maoni — Sat, 24 Feb 2007 02:04:04 GMT

The 1st and the 2nd case you described are possible. The 3rd one is not 'cause there can't be more than one ephemeral segment for each heap.

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re: Using GC Efficiently – Part 1

Andrew Z — Tue, 01 Apr 2008 19:28:46 GMT

Objects moving around in memory? Remind me to stick with native C++!

re: Using GC Efficiently – Part 1

APatel — Wed, 07 May 2008 22:08:56 GMT

Maoni,

We are executing performance tests on a .NET web application and we are running into few issues. If you can shade some light around some of the GC related issues that will be of great help:

About the Application:

It is a web application with a specialized engine to handle inbound requests (a C# class) with a very simple ASPX page which simply passes the ASP.NET Request object to it.

For every request, the Application generates about ~1,000,000 objects (we used Windbg)

W3wp.exe memory usage is around 300 MB

During each request process, we create many XML and execute at least 4 XSLT transformations (we use managed code - compiled XSLT and not MSXML).

Environment: .NET 2.0 with SP1, IIS 6 on Windows Server 2003 R2. The server has single dual-core Xeon and 4 GB of RAM

Observations:

With multi-user load, we see frequent GC Gen 2 collections (around 6 or 7 spikes in % Time spent in GC for Gen 2 every 100 seconds).

The LOH size keeps seesawing (it goes down when Gen 2 GC runs)

CPU usage is also very high (90% +) throughout the load test.

Questions:

I would like to find out GC collection trigger point logic (i.e. number of objects, memory allocation etc.). From your blog entry, I understand that when allocation exceeds Gen 0 threshold, GC gets invoked. What is this “Gen 0 Allocation Threshold”? Are there other such trigger points? We are not invoking GC from the app code.

We have nothing else running on the server and w3wp.exe is taking around 300-400MB of RAM (observed trough the task manager and Perfmon). Why is it not using the remaining available RAM on the system? Are there memory settings for CLR that we can tweak?

Are there any tools that you would suggest for memory profiling? We are using couple of tools including VSTS 2008 but they take long time to collect information (few hours at least). We want to pinpoint the “culprits” which cause allocations in LOH through call stack.

Please let me know if you need more information.

Thanks

APatel

re: Using GC Efficiently – Part 1

Martin Kulov — Thu, 26 Jun 2008 16:33:47 GMT

Rico,

do you have any idea why the two msdn magazine articles that you are refering in the first paragraph and no longer available.

I also found them pretty usefull resource, but now they can not be reached online.

Why it is deleted?

re: Using GC Efficiently – Part 1

David V. Corbin — Sun, 13 Jul 2008 19:43:57 GMT

I am looking for any available information on how the "Budgets" are calculated. I am seeing some anomolies in client code where I believe the budget is getting lower and lower, but can not resolve the potential cause....

Any guidance greadly appreciated...

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[翻译]更有效的使用垃圾收集

tmfc — Tue, 29 Jul 2008 08:53:48 GMT

翻译自Maoni's WebLog 文章Using GC Efficiently – Part 1，Maoni是微软CLR Performance组的成员本文的目标是解释一些东西的代价好让你可以更好使用托管内存-而不是解释GC本身-只是解释如何使用它而已。我假设绝大多数人对于使用垃圾收集感兴趣，而不想自己实现一个。本文假设读者对GC有基础的了解，如果你需要一些关于GC的背景知识，Jeff Richter写了两篇非常好的MSDN文章，1和2。首先我会关注工作站级类型的GC（Wks GC），然后我会解释服务器类型的GC（Svr

re: Using GC Efficiently – Part 1

csjasnoch — Wed, 01 Oct 2008 21:09:08 GMT

Hi Maoni,

I have been digging around and I am trying to find some info on G2 collection. Is it hindered even if the G2 objects are not dead?

We have an app that has around 35mil objects. This is a very OO app that has been built to do image processing and is modifiable on the fly. So reducing these amounts is not likely.

However, the app was set up to use a buffer and recycle the objects. The app crashes when GC begins though. I am wondering is GC working so hard because of other G2 objects that are dieing or is it more likely the 35mil references (not dieing) that are doing it?

re: Using GC Efficiently – Part 1

maoni — Wed, 15 Oct 2008 01:58:29 GMT

csjasnoch, observing a crash while a GC is in process most likely points to a managed heap corruption. I would suggest you to contact our PSS folks if you need help debugging this.

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