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Capacity Planning

Capacity planning is calculating the number of machines your application requires. This is one of the most important tasks you must complete before moving into production and making your application available to your customers/users. Typically, it should be done during the early stages of your project, in order to budget for the hardware and relevant software products your system will use. At a minimum, capacity planning involves estimating the number of CPUs and cores plus the memory each machine must have.

Another important deployment decision is to calculate the maximum number of In-Memory-Data-Grid (IMDG) partitions the application requires. This deployment parameter determines the scalability of your application IMDG. When the application is deployed, the number of IMDG partitions remains constant, but their physical location, their activation mode (primary or backup), and hosting containers are dynamic.

The IMDG instances can fail and relocate themselves to another machine automatically. They can relocate as a result of an SLA (Service Level Agreement) event (for example - CPU and memory utilization breach), or they can relocate based on manual intervention by an administrator. This IMDG “mobility” is a critical capability that, beyond providing the application with the ability to scale dynamically and self-heal itself, can prevent over-provisioning and unnecessary overbudgeting of your hardware. You can start small, and grow as needed with your hardware utilization. Your capacity planning should take this fact into consideration.

To avoid over provisioning, you should start small, and expand your IMDG capacity when needed. The maximum number of IMDG partitions can be calculated, based on a simple estimation of the number of machines you have available, or based on the size and quantity of objects your application generates. This allows your application to scale while remaining resilient and robust.

This topic addresses the following capacity planning issues:

  1. How do I calculate the footprint of objects when stored within the IMDG?
  2. What is the balance between the amount of active clients, machine cores, and the IMDG process heap size?
  3. How should the number of IMDG partitions be calculated?

It is not necessary to specify the maximum number of partitions for services that are not co-located with IMDG instances. These can be scaled dynamically without having to specify maximum instances.

Object Footprint Within the IMDG

The object footprint within the IMDG is determined based on the following:

  • The original object size - the number of object fields and their content size.
  • The JVM type (32- or 64-bit) - a 64-bit JVM may consume more memory due to the pointer address size.
  • The number of indexed fields - every indexed value means another copy of the value within the index list.
  • The number of different indexed values - more different values means a different index list.
  • The object UID size - the UID is a string-based field, and consumes about 35 characters. You may have the object UID based on your own unique ID.

Calculating the Object Footprint

The best way to determine the exact object footprint is via a simple test that allows you to perform some extrapolation when running the application in production.

To calculate the object footprint:

  1. Start a single IMDG instance.
  2. Take a measurement of the free memory.
  3. Write a sample number of objects to the IMDG (~100,000 is a good number).
  4. Measure the free memory again.

This test provides a very good understanding of the object footprint within the IMDG. Bear in mind that if you have a backup running, you need double the amount of memory to accommodate your objects.

The following sample calculation of an object footprint uses a 32- and 64-Bit JVM with different amounts of indexes. The numbers below are for example only, and real results may vary based on the actual Object data and index values.

The test was set up as follows:

  • Version XAP 7.1.2.
  • All objects values are different.
  • The Object has one String field, one Integer field, one Long field and one Double field.
  • Footprint measured in Bytes.
  • Basic Index type is used. An Extended Index type will have an additional footprint (20%) compared to a Basic Index type.
Indexes Footprint 32-Bit JVM - Windows Footprint 64-Bit JVM - Linux Footprint 64-Bit JVM - UseCompressedOops - Linux
0 331 306 308
1 - String 456 705 493
2 - String+Integer 493 989 671
3 - String+Integer+Long 533 1002 680
4 - String+Integer+Long+Double 571 1026 691



Active Clients vs. Cores vs. Heap Size

The IMDG kernel is a highly multi-threaded process, and therefore has a relatively large number of active threads handling incoming requests. These requests can come from remote clients or co-located clients, such as:

  • Any remote call involves a thread on the IMDG side that handles the request.
  • A notification delivery may involve multiple threads sending events to registered clients.
  • Any destructive operation (write, update, take) also triggers a replication event that is handled via a dedicated replication channel, which uses a dedicated thread to handle the replication request to the backup IMDG instance.
  • There is periodic background activity, used to monitor the relevant components that are using its own dedicated threads within the IMDG kernel.

A co-located client does not go through the network layer, and interacts with the IMDG kernel very fast. This means that the machine CPU core that has been assigned to deal with this thread activity is very busy, and does not wait for the operating system to handle IO operations. Taking this fact into consideration means we can have less concurrent clients served by the same core when compared to remote client activity.

The number of active threads and machine cores is an important consideration when calculating the maximum heap size to be allocated for the JVM running the GSC. You should keep memory in reserve for the JVM garbage collection activity, to deal with cleaning allocated resources and reclaiming unused memory. A large heap size means a potentially large number of temporary objects that can generate work for the garbage collection activity. You should have a reasonable balance between the number of cores the machine is running, the number of GSCs/IMDG that are running, and the number of active clients/threads accessing the IMDG.

A machine running 4 quad-core cores with fast CPUs (3GHz clock) can handle 20-30 concurrent collocated clients and 100-150 concurrent remote clients without any special delay. This JVM configuration should have at least a 2-3GB heap size to handle the IMDG data and additional resources that utilize the memory. With the above, we assume the application business logic is very simple and does not have any IO operations, and the IMDG persistency mode is asynchronous.

Calculating the Number of IMDG Partitions

Calculating the number of IMDG partitions required by an application is essentially on the maximum number of machines available. Theoretically, in an ideal world where you have unlimited budget and resources, you might have a dedicated machine per IMDG instance hosted within a dedicated Grid Container (GSC). In the real world, in order to avoid a large hardware budget, you can initially have multiple IMDG instances running on the same machine within a single GSC. This deployment topology can be modified later to move closer to the ideal of a dedicated GSC hosting a single IMDG instance (one IMDG instance per GSC).

The initial required number of GSCs per machine is calculated based on the machine’s physical RAM, and the amount of heap memory you want to allocate for the JVM running the GSC. In many cases, the heap size is determined based on the operating system: for a 32-bit OS, you can allocate a 2GB maximum heap size, and for a 64-bit OS, you need 6-10GB maximum heap size (the JVM -Xmx argument). For performance optimization, you should have the initial heap size the same as the maximum size. The sections below demonstrate capacity planning using a simple, real-life example.

Here are a few basic formulas you can use:

Amount of GSCs per Machine = Amount of Total Machine Cores/2
Total Amount of GSC = Amount of GSCs per Machine X Initial amount of Machines
GSC max heap Size = min (6, (Machine RAM Size * 0.8) / Amount of GSCs per Machine))
Amount of Data-Grid Partitions = Total Amount of GSC X Scaling Growth Rate / 2


  • Number of Total Machine Cores - total number of cores the machine is running. For a quad-core with 2 CPUs (Duo) machine this value is 8.
  • Number of Data-Grid Partitions - number of IMDG Partitions you need to set when deploying.
  • GSC max heap Size - JVM Xmx value
  • Number of GSCs per machine - number of GSCs you run per machine. This is a GSA parameter.
  • Total amount of data in-memory - number that should be estimated based on the object footprint you are storing within the space.
  • Scaling Growth Rate - expansion ratio, usually between 2-10. This value determines how much to expand the data grid capacity without downtime.
  • Initial number of machines - initial available machines to have when deploying the data grid.
  • Machine RAM Size - amount of physical RAM a machine has.
  • Total number of GSCs - total number of GSCs that are initially running when deploying the data grid.

Example 1 - Basic Capacity Planning

In this example, there are initially 2 machines used to run the IMDG application. 10 machines may be allocated for the project within the next 12 months. Each machine has 32GB of RAM with 4 quad-core CPUs. This provides a total of 64GB of RAM. Later, when all 10 machines are available, there will be a potential 320GB of total memory. The memory is used both by the primary IMDG and the backup IMDG instances (exact replica of the primary machines).

The machines are running a Linux 64-bit operating system. Allocating 6GB per JVM as the maximum heap size for the GSC results in 5 GSCs per machine; 10 GSCs initially across 2 machines. When all of the 10 machines are in use, there will be 50 GSCs.


Figure 1: 2-Machine Topology, 5 IMDG Instances per GSC, total 64GB RAM

When a maximum of 40 GSCs are hosting the IMDG, it is a good idea to have half of them (20 GSCs) running primary IMDG instances and the other half running backup instances. Use this number to define the number of partitions the IMDG is deployed with; with the 2 machines there are initially 10 GSCs hosting 40 IMDG instances. Later, as needed, these IMDG instances will be relocated to new GSCs that are started on new machines. Each machine will start 4 GSCs, which will join the GigaSpaces grid and allow the administrator to manually or automatically expand the capacity of the IMDG during runtime.


Figure 2: 10-Machine Topology. - 1 IMDG Instance per GSC, total 320GB RAM

This rebalancing of the IMDG instances can be done via the UI, or via a simple program using the Admin API.

Being able to determine the number of IMDG partitions prior to the application deployment allows you to have exact knowledge of how your routing field values should be distributed, and how your data will be partitioned across the different JVMs that will be hosting your data in memory.

Example 2 - Capacity Planning when Running on the Cloud

In a cloud environment, you have access to essentially unlimited resources. You can spin up new virtual machines and have practically unlimited amounts of memory and CPU power. In this type of environment, calculating the maximum number of IMDG partitions is not based on the maximum number of machines you might have allocated for your project because theoretically, you can have an unlimited number of machines started on the cloud to host your IMDG. Still, you must have some value for the maximum number of IMDG partitions when deploying your application. In this case, calculate the number of IMDG partitions based on the amount of memory your application might generate and store within the IMDG.

For example, if you have an application using 3 types of classes to store its data within the IMDG:

  • Class A - object average size is 1KB
  • Class B - object average size is 10KB
  • Class C - object average size is 100KB

The application needs to generate 1 million objects for each type of class during its life cycle:

  • Class A - total memory needed = 1KB X 1M = 1GB
  • Class B - total memory needed = 10KB X 1M = 10GB
  • Class C - total memory needed = 100KB X 1M = 100GB

The total memory required to store the application data in memory = 111GB

When using machines with 32GB of RAM, 4 machines are needed to run enough primary IMDG instances to store 111GB of data in memory, and another 4 machines are needed for the backup IMDG instances. For a 64-bit operating system, the numbers are 5 GSCs, each having a 6GB maximum heap size for a total of 40 GSCs (5 X 4 X 2). With 20 GSCs used to run primary instances, it is reasonable to target 80 as the potential final number of partitions needed (each GSC will initially host 4 IMDG instances). This means that we have 160 IMDG instances (half primary and half backups) hosted within 40 GSCs. Theoretically, this allows us to expand the IMDG to run across 160 machines (one GSC per machine). This means 160 X 10GB as the heap size = 1.6TB of IMDG memory capacity to host the IMDG objects. This is a huge amount of capacity for the IMDG, and actually 10 times larger than the estimated size. It provides a lot of room for error in case our initial memory utilization was wrong.

Used Memory Utility

Checking the used memory on all primary instances can be done using the following (we assume we have one GSC per primary instance):

GigaSpace gigaSpace = new GigaSpaceConfigurer(new UrlSpaceConfigurer(spaceURL)).gigaSpace();
Future<Long> taskresult = gigaSpace.execute(new FreeMemoryTask());
long usedMem = taskresult.get();
System.out.println("Used Mem[MB] " + (double)usedMem/(1024*1024));

The FreeMemoryTask implementation:

import java.util.Iterator;
import java.util.List;

import org.openspaces.core.executor.DistributedTask;

import com.gigaspaces.annotation.pojo.SpaceRouting;
import com.gigaspaces.async.AsyncResult;

public class FreeMemoryTask implements DistributedTask<Long, Long>{

    Integer routing;
    public Long execute() throws Exception {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Calling GC...");
        System.out.println("Done GC..." + 
                " Used memory " + (rt.totalMemory() - rt.freeMemory() )+
                " Free Memory " + rt.freeMemory() + 
                " MaxMemory " + rt.maxMemory() + 
                " Committed memory "+ rt.totalMemory());
        return (rt.totalMemory() - rt.freeMemory() );

    public Long reduce(List<AsyncResult<Long>> _usedMemList) throws Exception {
        long totalUsed =0;
        Iterator<AsyncResult<Long>> usedMemList = _usedMemList.iterator();
        while (usedMemList.hasNext())
            totalUsed  = totalUsed +;
        return totalUsed ;

    public Integer getRouting() {
        return routing;

    public void setRouting(Integer routing) {
        this.routing = routing;