Java Virtual Machine - Tuning the GC

In the last chapter, we learnt about various Generational Gcs. In this chapter, we shall discuss about how to tune the GC.

Heap Size

The heap size is an important factor in the performance of our Java apppcations. If it is too small, then it will get filled frequently and as a result, will have to be collected frequently by the GC. On the other hand, if we just increase the size of the heap, although it need to be collected less frequently, the length of the pauses would increase.

Further, increasing the heap size has a severe penalty on the underlying OS. Using paging, the OS makes our apppcation programs see much more memory than is actually available. The OS manages this by using some swap space on the disk, copying inactive portions of the programs into it. When those portions are needed, the OS copies them back from the disk to the memory.

Let us suppose that a machine has 8G of memory, and the JVM sees 16G of virtual memory, the JVM would not know that there is in fact only 8G available on the system. It will just request 16G from the OS, and once it gets that memory, it will continue using it. The OS will have to swap a lot of data in and out, and this is a huge performance penalty on the system.

And then comes the pauses which would occur during the full GC of such virtual memory. Since the GC will act on the entire heap for collection and compaction, it will have to wait a lot for the virtual memory to be swapped out of the disk. In case of a concurrent collector, the background threads will have to wait a lot for data to be copied from the swap space to the memory.

So here the question of how we should decide on the optimal heap size comes. The first rule is to never request the OS more memory than is actually present. This would totally prevent the problem for frequent swapping. If the machine has multiple JVMs installed and running, then the total memory request by all of them combined is less than the actual RAM present in the system.

You can control the size of memory request by the JVM using two flags −

-XmsN − Controls the initial memory requested.

-XmxN − Controls the maximum memory that can be requested.

The default values of both these flags depend upon the underlying OS. For example, for 64b JVMs running on the MacOS, -XmsN = 64M and -XmxN = minimum of 1G or 1/4th of the total physical memory.

Note that the JVM can adjust between the two values automatically. For example, if it notices that too much GC is happening, it will keep increasing the memory size as long as it is under -XmxN and the desired performance goals are met.

If you know exactly how much memory your apppcation needs, then you can set -XmsN = -XmxN. In this case, the JVM does not need to figure out an “optimal” value of the heap, and hence, the GC process becomes a pttle more efficient.

Generation Sizes

You can decide on how much of the heap do you want to allocate to the YG, and how much of it you want to allocate to the OG. Both of these values affect the performance of our apppcations in the following way.

If the size of the YG is very large, then it would be collected less frequently. This would result in lesser number of objects being promoted to the OG. On the other hand, if you increase OG’s size too much, then collecting and compacting it would take too much time and this would lead to long STW pauses. Thus, the user has to find a balance between these two values.

Below are the flags that you can use to set these values −

-XX:NewRatio=N: Ratio of the YG to the OG (default value = 2)

-XX:NewSize=N: YG’s initial size

-XX:MaxNewSize=N: YG’s max size

-XmnN: Set NewSize and MaxNewSize to the same value using this flag

The initial size of the YG is determined by the value of NewRatio by the given formula −

(total heap size) / (newRatio + 1)

Since the initial value of newRatio is 2, the above formula gives the initial value of YG to be 1/3 of the total heap size. You can always override this value by exppcitly specifying the size of the YG using the NewSize flag. This flag does not have any default value, and if it is not set exppcitly, the size of the YG will keep getting calculated using the above formula.

Permagen and Metaspace

The permagen and the metaspace are heap areas where the JVM keeps classes’ metadata. The space is called the ‘permagen’ in Java 7, and in Java 8, it is called the ‘metaspace’. This information is used by the compiler and the runtime.

You can control the permagen’s size using the following flags: -XX: PermSize=N and -XX:MaxPermSize=N. Metaspace’s size can be controlled using: -XX:Metaspace- Size=N and -XX:MaxMetaspaceSize=N.

There are some differences how the permagen and the metaspace are managed when the flag values are not set. By default, both have a default initial size. But while the metaspace can occupy as much of the heap as is needed, the permagen can occupy no more than the default initial values. For example, the 64b JVM has 82M of heap space as maximum permagen size.

Note that since the metaspace can occupy unpmited amounts of memory unless specified not to, there can be an out of memory error. A full GC takes place whenever these regions are getting resized. Hence, during startup, if there are a lot of classes that are getting loaded, the metaspace can keep resizing resulting in a full GC every time. Thus, it takes a lot of time for large apppcations to startup in case the initial metaspace size is too low. It is a good idea to increase the initial size as it reduces the startup time.

Though the permagen and metaspace hold the class metadata, it is not permanent, and the space is reclaimed by the GC, as in case of objects. This is typically in case of server apppcations. Whenever you make a new deployment to the server, the old metadata has to be cleaned up as new class loaders will now need space. This space is freed by the GC.