I am wondering what's the JVM behaviour for the following situation:
JVM minimum heap size = 500MB
JVM maximum heap size = 2GB
OS has 1GB memory
After the JVM started and the program runs for a period of time, it uses more than 1GB memory. I wonder if OOM will happen immediately or it will try to GC first!
It depends on how much swap space you have.
If you don't have enough free swap, the JVM won't start as it can't allocate enough virtual memory.
If you have enough free swap your program could start and run. However once a JVM starts swapping its heap the GC times rise dramatically. The GC assumes it can access the heap somewhat randomly.
If your heap can't fit in main memory, the program, and possibly the machine becomes unusable. In my experience, on Windows, a reboot is needed at this point. On Linux, I usually find I can kill the process.
In short, you might be able to start the JVM, but it's a bad idea.
Related
We have a tomcat with following arguments
Xms 1g
Xmx 4g
Parallel GC
It is installed in Ubuntu machine with JVM 1.8.181
Lately GC is being started with full throttle and doesn't let any other process go on. What I don't understand is this takes place when even total JVM is just 2.8 GB while maximum heap can go is 4GB. Why does full GC run when mamory has not reached to max?
When I dug deep, ii found that there is a sudden change in the used and committed memory; from 1+GB to ~4GB. Does that mean that because I had set the min heap to 1 GB, it goes till 1GB only and as soon as it reaches there it increases to a next step? Because of this the Garbage collection takes place?
If yes that does that mean that in order to avoid this situation, I need to increase the min heap?
More info- this is happening when there almost 0 traffic. No background process is going on. I understand it can build up but without using anything, how can it go up! - I need to figure this out myself.
When you set the min heap to 1 GB, it starts with a 1 GB heap, thought the process itself could be a few 100 MB to a few GB more than this depending on which libraries you use. i.e. the resident size can be larger.
As the pressure on the heap grows from activity, it may decide it needs to increase the heap size. There has to be a significant load to trigger this to happen otherwise it won't change the heap.
We migrated web application from jsf1.0 to 1.2 and deployed in Websphere 8.5. EArlier application was deployed in Websphere6.0. We are facing performance issue during SOAK testing. Got some thread hung message in sysout logs also i observe lot of blocking thread in thread dump file and its released on time.
Application performance degrades on time. i can see the performance issue remains same even the application is idle for 1 day .
Main issue is with the High CPU usage and high JVM memory even the application is idle for 1 day. Application is fast after the restart of server. Does the GC will not clear the JVM memory for 1 day or why this CPU is high ?
High cpu with low/declining app throughput is typical of java heap exhaust, when the JVM spends most of its time running GC trying to clear space in the heap to do real work. You should enable verbose GC logging, the GC log will show the heap state and GC activity. If the heap is below 10% tenure/OldGen free (assuming using default gencon collector) after a global/full GC, you are in heap exhaust state.
You could try increasing the heap size, maybe it just needs more space than currently provided. If the heap use (used tenure after global) continues to climb over time, when the workload offered is steady/constant, then the app probably has a memory leak. The objects accumulating in the heap can be seen by taking a core/system dump when the server is near heap exhaust state, and examining the dump with e.g. Eclipse Memory Analyzer.
Netbeans out of memory exception
I increased the -Xmx value in netbeans file.
but the IDE is busy acquiring more memory to scan projects ?
the memory usage increases and the process is slow, and non responsive
Sounds like your system is thrashing. The heap size is now so large that there is not enough physical memory on your system to hold it ... and all of the other things you are running.
The end result is that your system has to copy memory pages between physical memory and the disc page file. Too much of that and the system performance will drop dramatically. You will see that the disc activity light is "on" continually. (The behaviour is worst during Java garbage collection, because that entails accessing lots of VM pages in essentially random order.)
If this is your problem then there is no easy solution:
You could reduce the -Xmx a bit ...
You could stop other applications running; e.g. quit your web browser, email client, etc.
You could buy more memory. (This only works up to a point ... if you are using a 32bit system / 32bit OS / 32bit JVM.)
You could switch to a less memory-hungry operating system or distro.
Sorry for the vagueness, but I'm just trying to understand websphere memory management at a high level.
This is really a question about JVM behavior. As far as I know, there are no JVMs that will block a thread waiting for another thread to finish if it is holding a large amount of memory. I expect both threads to continuously consume memory, and if both are able to allocate memory at the same rate, I would expect them both to get OutOfMemoryError as soon as their combined allocations exceed the max heap size.
I had run valgrind on a sample daemon program. The parent exits after allocating a chunk of 1000B, but the child that runs on the background keeps on allocating 200B of memory on the heap through malloc, after every two seconds.
My question is: does valgrind execute the program on the actual processor, or on a synthetic CPU?
Does it allocate the memory on the actual heap or on a synthetic RAm which doesn't exist?
Since I let the program run for a quite a long duration so much so that the child allocated some 2GB of memory on the heap. On implementing the program on massif, I got one output file for the parent, and on killing the daemon process, I got another massif.out. for the child which showed the allocation of the memory on the heap.
Valgrind run program in its own synthetic CPU, nothing from the program machine code reaches the host CPU.
Memory allocation is hooked with Memcheck, if you use it, otherwise Valgrind calls the libc memory allocation routines.
This facts may complicate Valgrind debugging of system services, indeed.
If you turn on the memcheck(which is the default), then Valgrind will manage the heap, i.e. all the memory related methods (malloc/free/memmove etc.) will be replaced by Valgrind's version of the corresponding methods.
As already told, your program is running on virtual CPU created and managed by valgrind.
There is no notion of synthetic RAM as far I know. In any case, all this is very transparent to the running process(your daemon) and shoudl not change the behavior of your program in any way.
So the answer is YES for synthetic CPU and no for synthetic RAM.