We have a 64bit C#/.Net3.0 application that runs on a 64bit Windows server. From time to time the app can use large amount of memory which is available. In some instances the application stops allocating additional memory and slows down significantly (500+ times slower).When I check the memory from the task manager the amount of the memory used barely changes. The application keeps on running very slowly and never gives an out of memory exception.
Any ideas? Let me know if more data is needed.
You might try enabling server mode for the Garbage Collector. By default, all .NET apps run in Workstation Mode, where the GC tries to do its sweeps while keeping the application running. If you turn on server mode, it temporarily stops the application so that it can free up memory (much) faster, and it also uses different heaps for each processor/core.
Most server apps will see a performance improvement using the GC server mode, especially if they allocate a lot of memory. The downside is that your app will basically stall when it starts to run out of memory (until the GC is finished).
* To enable this mode, insert the following into your app.config or web.config:
<configuration>
<runtime>
<gcServer enabled="true"/>
</runtime>
</configuration>
The moment you are hitting the physical memory limit, the OS will start paging (that is, write memory to disk). This will indeed cause the kind of slowdown you are seeing.
Solutions?
Add more memory - this will only help until you hit the new memory limit
Rewrite your app to use less memory
Figure out if you have a memory leak and fix it
If memory is not the issue, perhaps your application is hitting CPU very hard? Do you see the CPU hitting close to 100%? If so, check for large collections that are being iterated over and over.
As with 32-bit Windows operating systems, there is a 2GB limit on the size of an object you can create while running a 64-bit managed application on a 64-bit Windows operating system.
Investigating Memory Issues (MSDN article)
There is an awful lot of good stuff mentioned in the other answers. However, I'm going to chip in my two pence (or cents - depending on where you're from!) anyway.
Assuming that this is indeed a 64-bit process as you have stated, here's a few avenues of investigation...
Which memory usage are you checking? Mem Usage or VMem Size? VMem size is the one that actually matters, since that applies to both paged and non-paged memory. If the two numbers are far out of whack, then the memory usage is indeed the cause of the slow-down.
What's the actual memory usage across the whole server when things start to slow down? Does the slow down also apply to other apps? If so, then you may have a kernel memory issue - which can be due to huge amounts of disk accessing and low-level resource usage (for example, create 20000 mutexes, or load a few thousand bitmaps via code that uses Win32 HBitmaps). You can get some indication of this on the Task Manager (although Windows 2003's version is more informative directly on this than 2008's).
When you say that the app gets significantly slower, how do you know? Are you using vast dictionaries or lists? Could it not just be that the internal data structures are getting so big so as to complicate the work any internal algorithms are performing? When you get to huge numbers some algorithms can start to become slower by orders of magnitude.
What's the CPU load of the application when it's running at full-pelt? Is actually the same as when the slow-down occurs? If the CPU usage decreases as the memory usage goes up, then that means that whatever it's doing is taking the OS longer to fulfill, meaning that it's probably putting too much load on the OS. If there's no difference in CPU load, then my guess is it's internal data structures getting so big as to slow down your algos.
I would certainly be looking at running a Perfmon on the application - starting off with some .Net and native memory counters, Cache hits and misses, and Disk Queue length. Run it over the course of the application from startup to when it starts to run like an asthmatic tortoise, and you might just get a clue from that as well.
Having skimmed through the other answers, I'd say there's a lot of good ideas. Here's one I didn't see:
Get a memory profiler, such as SciTech's MemProfiler. It will tell you what's being allocated, by what, and it will show you the whole slice n dice.
It also has video tutorials in case you don't know how to use it. In my case, I discovered I had IDisposable instances that I wasn't Using(...)
Related
I heard many times that once C# managed program request more memory from OS, it doesn't free it back, unless system is out of memory. Eg. when object is collected, it gets deleted, and memory that was occupied by the object is free to reuse by another managed object, but memory itself is not returned to operating system (for example, mono on unix wouldn't call brk / sbrk to decrease the amount of virtual memory available to the process back to what it was before its allocation).
I don't know if this really happens or not, but I can see that my c# applications, running on linux, use small amount of memory on beginning, then when I do something memory expensive, it allocates more of it, but later on when all objects get deleted (I can verify that by putting debug message to destructors), the memory is not free'd. On other hand no more memory is allocated when I run that memory expensive operation again. The program just keep on eating the same amount of memory until it is terminated.
Maybe it is just my misunderstanding of how GC in .net works, but if it really does work like this, why is that? What is a benefit of keeping the allocated memory for later, instead of returning it back to the system? How can it even know if system need it back or not? What about other application that would crash or couldn't start because of OOM caused by this effect?
I know that people will probably answer something like "GC manages memory better than you ever could, just don't care about it" or "GC knows what it does best" or "it doesn't matter at all, it's just virtual memory" but it does matter, on my 2gb laptop I am running OOM (and kernel OOM killer gets started because of that) very often when I am running any C# applications after some time precisely because of this irresponsible memory management.
Note: I was testing this all on mono in linux because I really have hard times understanding how windows manage memory, so debugging on linux is much easier for me, also linux memory management is open source code, memory management of windows kernel / .Net is rather mystery for me
The memory manager works this way because there is no benefit of having a lot of unused system memory when you don't need it.
If the memory manager would always try to have as little memory allocated as possible, that would mean that it would do a lot of work for no reason. It would only slow the application down, and the only benefit would be more free memory that no application is using.
Whenever the system needs more memory, it will tell the running applications to return as much as possible. The same signal is also sent to an application when you minimise it.
If this doesn't work the same with Mono in Linux, then that is a problem with that specific implementation.
Generally, if an app needs memory once, it will need it again. Releasing memory back to the OS only to request it back again is overhead, and if nothing else wants the memory: why bother?. It is trying to optimize for the very likely scenario of wanting it again. Additionally, releasing it back requires entire / contiguous blocks that can be handed back, which has very specific impact on things like compaction: it isn't quite as simple as "hey, I'm not using most of this : have it back" - it needs to figure out what blocks can be released, presumably after a full collect and compact (relocate objects etc) cycle.
I have an application which has to process hundred of thousands of records. Right now, I can only process 500 of them at a time. Each batch can take up to 5 minutes to process/analyze (total of ~10 hours of processing). The reason for that limit of 500 records is memory consumption. I think that one of the main reason why our program takes so much memory is that fact that we don't set the size of lists or dictionaries (e.g. new List() instead of new List(100000)). I did the changes to set the size of collection so that .NET stop creating and copying new lists with extra capacity.
Here's my question: how to prove that a version of a program is more memory efficient? Are there performance counters I should look at? Tools? Monitoring?
There are tools like .net memory profiler from where you can have in-depth analysis of memory management and memory leaks of a .net application.
.NET Memory Profiler is a powerful tool for finding memory leaks and optimizing the memory usage in programs written in C#, VB.NET or any other .NET Language. With the help of the profiling guides, the automatic memory analyzer, and specialized trackers, you can make sure that your program has no memory or resource leaks, and that the memory usage is as optimal as possible.
I don't think a profiler would give you a real overview of how much more efficient one version is than the other.
I recommend using memory performance counters to do this. You can setup a few data collection sessions using perfmon, for both of your app's versions.
You should monitor at first only the process memory (of your process, of course). You can also add some of the rest of system memory performance counters, just to see how the overall virtual memory status changes during your process' lifetime.
If there is a difference and you still can't get a good overview and conclusion, you could start digging in the .NET CLR memory performance counters. Not as easy to analyze than the previous ones I mentioned, but much more detailed.
You should probably run each session for one hour or two in order to get some good data, given the lengthy processing done by your app.
You can use the in-built Performance Profiler tool if you have Visual Studio Ultimate: Debug -> Start Performance Analysis. If you don't have this available, you can use dotTrace by JetBrains, which accomplishes the same thing.
You can also measure execution time by using the Stopwatch class. Stopwatch is specifically designed to measure elapsed time and may (if available on your hardware) provide good granularity/accuracy using an underlying high-frequency hardware timer, compared to DateTime.Now. By using the Stopwatch class and comparing the two execution times you can see which runs faster. This does not give any information with CPU or Memory Usage.
Using lots of memory does normally not really slow you down until you go out of real memory and start using swapfiles. You can check that in Task manager while your app is running. Where you can also see the peak and working amount of memory of your app.
I would profile a bit (there are several memory profilers available) to see what objects are taking up your memory.
As for the lists; I do not think setting the capacity of Lists will fix your problem. The List class grows by doubling the capacity (tested as I could not find documentation). Worst case you are using twice the memory. If you create new lists inabundance you could create an object cache to re-use the lists. But in my experience .NET this does not help much in .NET as the runtime is quite efficient.
You can use CLR GC ETL events. Download perfview from Microsoft, run your application under it, and then just check the GC page.
Just check "total memory allocation" and "time spent in GC" would give you an idea of how managed memory is used.
If you want more details, check the CLR allocation tick events.
Perfview can also analyse your managed heap live objects.
Have simple C# console app which imports text data into SQL.
It takes around 300K in memory and 80% in CPU. There are 2Gb RAM available at any time and yet the Page Fault shows 500K.
The app is 32 bit and OS is either W2000 or XP 32 bit and .NET 3.5
Anyone can explain what could be the problem and how can I investigate this further?
EDIT: I am now certain that the page faults are related to the disk I/O (read). I commented out SQL part and the pure disk read generates that high number alone.
EDIT2: There are 200 hard faults/sec and 4000 soft faults/sec on average.
I wonder if the same would appear on W2008
First, how do you measure the memory the app is using? If you're looking at "working set" that's only the part that resides in physical memory. You should also take a look at the "VM Size" (or "Commit Size") where the actual virtual memory your process takes up.
If Windows kernel Balance Set Manager thinks that your app is inactive, or should be left behind to give other processes more power, it can decide to reduce the working set size. If working set size is smaller than what your application actually needs to work on, you could easily see a lot of page faults because it simply becomes a race between The Balance Set Manager and the application. Usually balance set manager monitors memory usage and can also decide to increase working set size accordingly. However, this might be prevented in certain circumstances like low physical free memory, high I/O (cache stress on physical memory), low process priorty, background/foreground status of the application etc.
It can simply be the behavior of .NET garbage collector due to vast amount of small memory blocks getting allocated and disposed in a very short time, causing a stress on both memory allocation and releasing. The "VM Size" could stay around the same size but behind the scenes it could be continously allocating/freeing memory, causing continous page faults.
Also know that the DLLs the process is using are also accounted for the process statistics. Not your app but one of the COM or .NET DLL you are using might be causing this behavior as well. You can deduce actual culprit by changing your application's behavior (e.g. removing DB access code and only leave object allocation code behind) to see which component is actually causing thrashing.
EDIT: About your question on GC impact on memory thrashing: The CLR actually grows the heap dynamically and gives the memory back to the OS as needed. That does not occur synchronously. GC runs behind the scenes and frees memory in large chunks to prevent hindering application performance. Say you are allocating many small objects and freeing them almost immediately. That causes many references to stay for a moment in memory before freeing. It is easy to imagine that it becomes like a head-to-head race between the garbage collector and the memory allocating code. While GC eventually catches up, the required new memory must be satisified from a "new memory", not the old one because old one is not freed up yet. Since actual memory we are working on stays around the same, balance set manager may not think of giving our process more memory because we're on the edge, always around the same physical memory size but constantly need "newly allocated memory" not "more memory", therefore page faults.
Page faults are normal. Memory gets swapped out and when you next access it that's a page fault and the system brings it back. This is by design.
I've got an app running on my machine right now with 500 million page faults. There's nothing to worry about!
Page faults means Memory issues
Consider increasing memory, if you have excessive page faults.
Have a large working set size.
The working set is the set of memory pages currently loaded in RAM. This is measured by Process\Working Set. A high value might indicate that you have loaded a number of assemblies.
Process\Working Set has no specific threshold value to watch, although a high or fluctuating value can indicate a memory shortage. A high or fluctuating value accompanied by a high rate of page faults clearly indicates that your server does not have enough memory.
Further reading:
Check memory under System Resources in following MSDN article:
http://msdn.microsoft.com/en-us/library/ff647791.aspx#scalenetchapt15_topic9
Please provide some code to investigate.
A possible answer to this I am currently testing it on my application. Break up your working set into smaller chunks and work with the chunks.
For instance I have a large list of objects (9000-30000). If I break up that list into a chunk of 500 or so at a time it should maintain the 500 objects in memory while I work on them.
You will want to increase or decrease the size of your chunk until you can work with it fast enough that the OS will maintain it in memory. This is theory I haven't fully tested it yet. But it should work.
After reading a few enlightening articles about memory in the .NET technology, Out of Memory does not refer to physical memory, 597499.
I thought I understood why a C# app would throw an out of memory exception -- until I started experimenting with two servers-- both are having 2.5 gigs of ram, windows server 2003 and identical programs running.
The only significant difference between the two being one has 7% hard drive storage left and the other more than 50%.
The server with 7% storage space left is consistently throwing an out of memory while the other is performing consistently well.
My app is a C# web application that process' hundreds of MBs of String object.
Why would this difference happen seeing that the most likely reason for the out of memory issue is out of contiguous virtual address space.
All I can think of is that you're exhausting the virtual memory. Sounds like you need to run a memory profiler on the app.
I've used the Red Gate profiler in similar situations in the past. You may be surprised how much memory your strings are actually using.
Is the paging file fragmentation different on each machine? High fragmentation could slow down paging operations and thus exacerbate memory issues. If the paging file is massively fragmented, sort it out e.g. bring the server off-line, set the paging file size to zero, defrag the drive, re-create the paging file.
It's hard to give any specific advice on how to deal with perf problems with your string handling without more detail of what you are doing.
Why would this difference happen
seeing that the most likely reason for
the out of memory issue is out of
contiguous virtual address space?
With 7% free hard disk your server is probably running out of space to page out memory from either your process or other processes, hence it has to keep everything in RAM and therefore you are unable to allocate additional memory more often than on the server with 50% free space.
What solutions do you guys propose?
Since you've already run a profiler and seen at least 600MB+ of usage with all the string data you need to start tackling this problem.
The obvious answer would be to not hold all that data in memory. If you are processing a large data set then load a bit, process it and then throw that bit away and load the next bit instead of loading it all up front.
If it's data you need to serve, look at a caching strategy like LRU (least recently used) and keep only the hottest data in memory but leave the rest on disk.
You could even offload the strings into a database (in-memory or disk-based) and let that handle the cache management for you.
A slighty left-of-field solution I've had to use in the past was simply compressing the string data in memory as it arrived and decompressing it again when needed using the SharpZipLib. It wasn't that slow surprisingly.
I would agree that your best bet is to use a memory profiler. I've used .Net Memory Profiler 3.5 and was able to diagnose the issue, which in my case were undisposed Regex statements. They have demo tutorials which will walk you through the process if you're not familiar.
As you your question, any single reference to the strings, the jagged array for instance, would still prevent the string from disposing. Without knowing more about your architecture, it would be tough to make a specific recommendation. I would suggest trying to optimize your app before extending memory though. It will come back to bite you later.
An OutOfMemoryException is more likely to indicate fragmentation in your page file - not that you are out of RAM or disk space.
It is generally (wrongly) assumed that the page file is used as a swap disk - that RAM overflow is written to the page file. All allocated memory is stored in the page file and only data that is under heavy usage is copied to RAM.
There's no simple code fix to this problem other than trying to reduce the memory footprint of your application. But if you really get desperate you can always try PageDefrag, which is a free application originally developed by SysInternals.
There is a few tricks to increase memory (I dont know if it works with a web-app, but it looks like it does):
"Out of memory? Easy ways to increase the memory available to your program"
http://blogs.msdn.com/b/calvin_hsia/archive/2010/09/27/10068359.aspx
What is the largest heap you have personally used in a managed environment such as Java or .NET? What were some of the performance issues you ran into, and did you end up getting a diminishing returns the larger the heap was?
I work on a 64-bit .Net system that typically uses 9-12 GB, and sometimes as much as 20GB. I have not seen any performance problems even while garbage collecting, and I have been looking hard as I was not expecting it to work so well.
An earlier version hung on to some objects for too long resulting in occasional GCs that freed up 3GB+. Even then, there was no noticeable impact on performance. The system is running on a 16-core server with 32GB RAM, which probably helps...
In .Net, on Windows 32-bit, You can only really get to about 1.4 GB of memory usage before things start getting really screwy (out of memory exceptions). This is due to a limitation in 32 bit windows that limits a single process to using more than 2 GB of RAM. There is /3GB switch you can put in your boot.ini, but that will only bring you a little bit further. If you want to use lots of memory, you should seriously consider running on a 64 bit version of windows.
I currently have a production application with 6 GB of memory. You'll need a 64-bit box as well for the JVM to be able to address that much.
The garbage collector is really the only thing (that I've found so far) where performance degrades with size, and then only if you manually kick off a System.GC, which forces the JVM to bring everything to a screeching halt as it traverses 6 GB worth of objects. Takes a good 20 seconds, too. The default GC behavior does not do this, BTW, you have to be dumb enough to make it do that. Also worth researching JVM tuning at this size.
You can also find things like distributed and clustered JVMs, sorry, don't have any good references as I didn't look into this option too closely, although I did find references to larger installations.
I am unsure what you mean by heap, but if you mean memory used, I have used quite a bit, 2GB+. I have a web app that does image processing and it requires loading 2 large scan files into memory to do analysis.
There were performance issues. Windows would swap out lots of ram, and then that would create a lot of page faults. There was never any need for anymore than 2 images at a time as all requests were gainst those images (I only allowed 1 session per image set at a time)
For instance, to setup the files for initial viewing would take about 5 seconds. Doing simple analysis and zooming would be fairly fast once in memory, in the order of .1 to .5 seconds.
I still had to optimize, so I ended up preparsung the files and chopping into smaller peices and worked only with the peices that were required by the user at the time.
I have used from 2GB to 5GB of memory in java, but usually when I get to more than 2GB I really start thinking about memory optimization. Diminishing returns can vary from not optimizing when it's necessary because you have a lot of memory, to not having memory available for the OS/Disk caches (which can help your application overall).
For Java, I recommend watching your memory usage per generation over time. Do you create a lot of temporary objects or have long-lasting objects that consume a lot of memory? A lot of optimization of memory can be done when knowing those things.