If broken it is, fix it you should

Using the powers of the debugger to solve the problems of the world - and a bag of chips    by Tess Ferrandez, ASP.NET Escalation Engineer (Microsoft)

.NET Memory usage - A restaurant analogy

.NET Memory usage - A restaurant analogy

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My favourite author Simon Singh is a wiz at analogies. In his book The big bang he explains concepts like the doppler effect and the theory of relativity using analogies with frogs and trains that makes it not only easy to understand but you will remember them forever because of the picture they paint in your head.

The other day at work I heard one of my colleagues explaining memory usage and why you get out of memory exceptions to one of his customers using a restaurant analogy.  I've talked about OOMs and memory management in an earlier post but I found the analogy so amusing that I thought I'd share it (and yes, before you say it, i do admit i might have stretched the analogy a little too far:), and that it doesn't hold a candle to Simon Singhs analogies, but then again he sells books and i just rant in a blog...).

Disclaimer:  In order to not get too longwinded I will simplify a lot of things, and say that for example the GC allocates 64 MB segments even though this differs between different framework versions and the size of the objects you allocate (read large object heap). Some other details are also dependent on configuration settings (i.e. using the /3GB switch etc.) but I will exclude such details from the analogy.

Analogy Part 1 - General memory usage

If you have read my earlier posts you will know that a process on a 32-bit system can typically address 2 GB of virtual address space. This is the memory that you have to work with, independently of how much RAM you have.  More RAM is good for performance since you page less with more RAM, but it doesn't do anything to expand the 2 GB address space.

Picture this 2 GB address space as being the floor space of a restaurant.

When you allocate an object (whether it is .net or non-.net) you typically follow a two step process.  You reserve the memory and then you commit space inside your reservation.

The reservation is equal to reserving a table at the restaurant.  And just like in a restaurant, depending on the memory manager you use (we will get to that later) you will reserve the memory in chunks.  Let's say for example you are a party of 3. It is not likely that there will be a table for 3 in the restaurant, but rather you would get a table for 4 out of which you use 3 seats and waste one seat.  

In memory terms the space for the table you have reserved is called reserved memory (virtual bytes), and the actual space you use (for the 3 seats) is comitted memory (private bytes).   The floor space that is not yet reserved is free memory.

On a pretty good restaurant night your restaurant/memory might look something like this where the blue areas are reserved space, red means committed space and white is free space.

      

Now, if someone calls in to make a reservation for 3 they will get the answer that the restaurant is full, since the only way to seat 3 people together is to seat them on a 4 seat table.  Even though you could fit in two 2-seat tables that wouldn't be good since they all want to sit together.

Similarily when you make memory allocations you won't split a memory reservation out into different locations, it has to be allocated in one chunk or not at all. So the memory result in this case would be "out of memory", even though there is plenty of space left.

An observant person might also note that if we put the tables closer together so that they are completely side-by-side you could easily fit in a new table of 4, but reserved memory areas much like tables at some restaurants can't be moved.

When we talk about memory fragmentation we either talk about the free but unusable (because it is not large enough to fit a new table) we have, or how much of our reserved memory we are not using (difference between virtual bytes and private bytes).

Analogy Part 2 - The .NET GC

Most of the time when you create objects in an application, whether it is .NET or not you use some kind of memory manager (NTHeap, C++ Heap, GC etc.), and in the restaurant case you can think of the memory manager as an hostess that reserves seats for you and ushers you to the location where you are to be seated.  For example if you call malloc you don't have to provide an address where you want your allocation to lie, instead you say that you want memory of a certain size and malloc returns, ok, you will be seated at table 1 in the "C++ heap" area.

The .NET GC takes this one step further and pre-reserves a large table for anyone who might want to use .NET objects in the process (let's say a 64 seat table).  And when anyone creates a .NET object, the GC ushers them to the next available seat on that table.  Once in a while the usher will walk around the table to check if someone is done eating and ask them to leave, and then scoots the rest of the people down the table.   Some people might be waiting on other people to finish up before they can leave (references), so they get to stay too. And some people may be really annoying and say, dude, i got a window seat, i am sooo not moving (pinned objects) which means that the rest of the people can't be scooted down towards the end of the table either.

Any empty seats between people are referred to as .NET memory fragmentation.

Once the 64 seat table is filled up the GC needs to reserve a new 64 seat table if it needs to accomodate newcommers, and if it can't you will get an out of memory exception.

But, how does it really look

Ok, enough with the analogy, here is what memory looks like in a real ASP.NET application

       

Again, the red parts are committed memory, the blue parts are reserved memory that is not committed and the white space is free space.

The dots you see towards the end of the memory space are probably dlls, and although just like in the restaurant scenario there is a lot of white space, it is likely that none of the gaps between the small red dots are large enough to house a 64 MB segment and thus the next time we fill up a GC segment and need a new one to accommodate a new object, we will get an out of memory exception.

The reason these small red dots (dlls) are spaced out like this is because they are loading at the prefered base addresses for those particular dlls.  You can't really do much about that type of fragmentation since it is hard to know in advance what a "good" prefered base address would be, but what you can do something about is finding out where the memory you are actually using is going.

A comment on performance counters and how not to use taskmanager

Throughout the analogy I talked about private bytes and virtual bytes and these are the two most important performance counters to look at when defining memory usage or memory leaks. 

There is another counter called working set which simplified consists of how much memory is in the memory pages that are currently or was recently touched by threads in the process or approximately, how much of the memory that is used by the process is currently in RAM.  The working set counter might be interesting if you have issues with too much paging and many processes on the same box competing about the RAM, but in order to determine how much memory you are using (reserved or committed) it offers little or no help.

If you want to see this in action, you can create a winforms application and allocate a bunch of objects and see the workingset go up, and then if you minimize the app, the working set drops.  This doesn't by any means mean that you have just released all this memory. It just means that you are looking at a counter that is totally irrelevant for determining how much stuff you store in memory :) Yet... this is the counter that people most often look at to determine memory usage...   

I know that by now you are probably thinking "yeah right", you haven't even heard of this counter before, why would I say that this is the counter most people look at???  The answer is, because most people use task manager to look at memory usage of a process, and specifically look at the Memory Usage column. Surprise surprise:) what this actually shows you is the working set of the process...

If you want to see private bytes which is a far more interesting counter, you sould look at the column in task manager that is labeled Virtual Memory Size (yeah, that's really intuitive:)), or better yet, look in performance monitor at process\private bytes and process\virtual bytes, there is no reason not to if your intent is to investigate high memory usage or a memory leak.

 

So tonight, go out, grab a bite to eat and see memory management in action:)  I bet you will probably find a lot more similarities than the ones me and my pal came up with...

Laters,





  • Hi,

    I have a problem...

    Please help me out.. i m using asp.net 2005 and my application is to uplaod multiple images on the server, but it gets crash down when multiple user tries to uplaod images. the error is like this

    aspnet_wp.exe  (PID: 3040) was recycled because memory consumption exceeded the 609 MB (60 percent of available RAM).

    For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

    Means something related to memory, server is having 1GB RAM..

    Please help me out, i m stuck.............

    Thanks in advance

  • you might want to read through

    http://blogs.msdn.com/johan/archive/tags/Upload/default.aspx

    Regarding the 60%, this is the default setting for 1.1 in machine.config,  you can change it if you dont want it to recycle at 60% of RAM size, but you will probably get out of memory exceptions if your images are very large

  • I was just reading through this post and marveling at the simplicity and elegance of the explanation, after which I happened to scroll back up to the top.  It took me a couple of moments to realize that's probably a picture of the writer up at the top of the page.

    So you have someone walking around with that kind of brain power that also looks like that?!  Tess is either a mythical avatar that can't be found in nature or she won the gene pool lottery.

  • Good article, here i want to relate it with my scenario.

    I initialize all the object, which will be needed, in advance and put them in a map and i return the clone of object on demand.

    I want you to shed light on this approach and what to know is there ever a chance of memory leaks in this scenario. Thanks

  • Hi

    Really great article.

    I have a query may be you can help.

    In our application we create big arrays of the size

    float[] abc = new float[65000] many such arrays are created. As many as 512 or 1024. We get outofmemory exception frequently.

    The problem is even if we set these arrays to null memory is not released.

    We doubt that GC is not reclaiming the memory.

    We did memory profiling using some tools then the tools shows that the arrays are removed.

    Can you suggest something for this problem.

  • JC,

    Because of the size these will end up on the large object heap which is not garbage collected as frequently as small objects.

    Depending on framework version etc. and allocation pattern you may also be causing some significant fragmentation on the large object heap here.

    The GC should collect them if they are ready to be collected the next time a full GC is initiated.

    I am thinking the reason for your OOMs is probably that you allocate a lot of these at the same time (i.e. their lifetimes intersect) and my best advice to you would be to make them slightly smaller so that they dont end up on the large object heap.    Monitor the perf counter for LOH to see how much you allocate on the LOH.

  • Hi Tess

       Great article! But the memory map pictures are not loading on this page...can this be fixed?

    Thanks

  • Hi Tess

     Great article....but the memory map pics are not loading on this page....can this be fixed?

  • sandeep, are you talking about pictures?  I can see the pics fine, do you get an error of some kind?

  • For me, either in IE or Firefox, it just shows a [x] in those places where images should be loaded...I would appreciate if u can mail them to me at this Id : sandeeparora_1704@yahoo.com

    Thanks in advance

    Sandeep

  • yes the picture does not show up the first time page loads completely. however i was able to see them in IE after couple of refreshes.

    article made it's point without the images as well. Thanks Tess. Great article.

  • Yes Tess,

    The images are not visible, I have IE9 and FF3.5. In IE it simply show 'X' mark and FF, just nothing there !!

    Even after few refresh.

    Thanks

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