Rewording my question in an attempt to make it On-Topic:
We have a client (only one client out of many) that is consistently getting an Out of Memory exception with our software. I feel like we've eliminated the usual suspects that would cause this and am looking for ideas of what other things (less standard causes) that might cause an OOM. Specifically, since this seems to be specific to a single customer, could it be caused by something wrong in the hardware, OS, or .Net install?
Here are the things I am aware of that cause an OOM and why I believe we've eliminated them as suspects:
1 - OOM caused by system running out of memory.
Why Not? Because the system has several GB available when these exceptions occur.
2 - OOM caused by process running out of memory due to over allocation or memory leaks.
Why Not? Because the process is using only about 100MB of memory at the time of the exceptions. We have monitored the memory usage for days (on the system in question) and have not noticed any significant increase in memory usage.
3 - OOM caused by running out of other system resources such as file handles, etc.
Why Not? The exceptions are happening, exclusively, during run-of-the-mill memory allocations, not while opening a file or connecting to a socket.
4 - OOM caused by attempting to allocate a large array with excessive memory fragmentation.
Why Not? The memory blocks that we are allocating are fairly small (640x480x2, for the most part). With so much memory available, I have trouble believing that it could be so fragmented that something like that would fail.
So, just to be clear, I am not asking "Why doesn't my code run?" My code does run, on all machines but one. I'm not asking anyone to debug my code. My question is: "What other possible causes, besides those we've eliminated, could be resulting in an Out of Memory exception?" Or, "Am I missing something that could have caused me to eliminate one of the known causes prematurely?"
As an FYI for anyone struggling with similar issues. I think we've finally hunted down the cause of this bug. Turns out the OpenGL drivers on certain cheaper on-board Intel graphics cards had a problem with the way we were writing bitmap data to the same texture ID over and over. I changed the code to delete the texture and allocate a new ID each time and the problem seems to have gone away.
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 been reading about out of memory for some time now and I figured out that in most cases out of memory exception (at least in .NET) isn't really caused by system actually running out of memory but rather system could not allocate chunks of requested memory block due to fragmentation.
What I don't really understand is I've been in a situation where I still get out of memory exception even if I try to allocate a large chunk of contiguous memory on application startup (eg: loading 100 images). Since the application has just started up, it is assumed that not much allocations / de-allocations have been done prior to that, so there should be many free contiguous blocks available. In that case why would the application still get hit by memory fragmentation issue?
Note that I'm also fairly certain that the issue was not caused by the system actually running out of memory quota allocated for my application because loading 100 images in my specific case only takes ~200 mb or so.
In my experience, Out of Memory mostly means poor object management. It's symptomatic of creating too many objects too fast and GC is having a hard time keeping up. Setting aside the few projects that take and never give memory back (like a SQL Server) out of memory can be prevented with caching and a well defined object life cycle.
In my C# program I do various memory-consuming operations. Depending on the amount of memory currently available and various non-constant circumstances, the program fails with OutOfMemoryException at different stages.
I would like to stop processing at some point when it is more or less obvious that the program will fail with an OOM in the near future.
However, there is no fixed threshold for this; Other users may have more (or less) memory, another OS with its memory specifics and so on.
I do not want to just check that the software consumes more than eg 500MB as this may be too high or too low a restriction.
Is there any reliable way to predict an upcoming OOM in .NET?
MemoryFailPoint should do what you want.
http://msdn.microsoft.com/en-us/library/system.runtime.memoryfailpoint.aspx
The sort answer is that its probably not the best solution - instead of anticipating when you are going to run out of memory and trying to do something about it, you should probably either try and reduce the memory consumption of your application or just wait until the OutOfMemoryException exception is thrown and then clean up.
An out of memory exception is thrown whenever the .Net runtime fails to allocate the memory that was just requested - this could be because the machine has run out of physical memory and the swap file is disabled (or the machine has run out of disk space), or it could be because the virtual memory space of the process is too fragmented to allocate the desired block of memory (which can happen when working with large objects). Predicting when this is going to be is fairly difficult.
You can use the MemoryFailPoint class to check to see if a certain amount of memory is going to be available before starting an operation that uses up a large amount of memory, however this class does not guarantee that the memory will remain available for the duration of the operation and so your application could still fail with an OOM exception anyway. Although this class can be useful in some scenarios to attempt to reduce OOM exceptions (and in turn to avoid corrupting application state), its probably not going to be a "magic bullet" solution to your problem.
Many of the OOM exceptions I observe so far were caused by improper written application with memory leak or third party components. Consider using a data structure optimized for your operation.
Detecting OOM is quite difficult because there are randomly thrown. Maybe you can use MemoryFailPoint and Performance counters (available memory) to ensure there is enough available memory.
I'm trying to insert data with big column values (1-25Mb) and after a couple seconds, one of my nodes dies, either by throwing an OOM or by being stuck in an endless GC loop.
It usually tries to flush CFs, but then it says Unable to reduce heap usage since there are no dirty column families.
Since the log advised me to reduce memtable/cache sizes, I tried to figure out what was using up all this memory in order to adapt my settings, so I ran nodetool flush / invalidaterowcache / invalidatekeycache and then triggered a GC through jconsole.
Unfortunately, my memory usage stayed high (>60%) even though the server is idling.
So, my problem is Why is the server running out of memory when inserting big values? and also, why isn't the server giving some memory back?
Edit
I did a heapdump and the heap is full of byte[], mainly referenced by of org.apache.cassandra.io.sstable.IndexSummary$KeyPosition.
I don't understand how this is possible since everything is supposed to have been flushed.
It seems to me that you hit the infamous memory fragmentation issue. I'm not sure whether Cassandra takes away some of the fragmentation issues, but generally, in .NET and potentially any Windows program, can run into this.
When you select anything above 85000 bytes (yes, odd number, but it's what it is), objects are stored in the Large Object Heap. The LOH gets GC'ed only as generation 2, but worse, it gets never compacted. The reason is partly caused by the way the OS is implemented.
Result: when you store objects of say 2MB, 5MB, 3MB, 2MB, 3MB and objects of 2MB get GC'ed you have potentially 4MB free. But if you then try to create a new object of 3MB, it cannot be placed there because of the fragmentation (2 holes of 2MB) and moves to the top of the heap. Eventually, this runs out of room. So: there can be enough memory available, but you will get an OOM regardless, due to this fragmentation.
This issue is mostly seen on 32 bit x86 applications on 64 bit (WOW64) and 32 bit Windows. 64 bit applications also have the fragmentation issue, but since virtual memory is much larger, you first hit paging the memory (becoming real slow) before you hit actual fragmentation issues.
If this is indeed the issue (you can check the fragmentation visually with VMMap and with WinDbg) you can solve it by creating a large pool of bytes and reuse your own pool, thus preventing fragmentation.
I investigated the heap dump with MAT and it turns out that the OutOfMemory happened because a lot of memory was used by Thrift.
Since I had to transfer big chunks of data for my column values, I changed those settings to 128, to "be safe":
thrift_framed_transport_size_in_mb
thrift_max_message_length_in_mb
But it turns out that Thrift allocates one byte[2 * thrift_max_message_length_in_mb] per receiving thread, and I had three of those. So I was using 768Mb just for receive buffers...
Changing the settings to 32 fixed my issue.
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