Skip to main content

Calculating System Parameters for UNIX®, Linux, and macOS

Calculating System Parameters for UNIX®, Linux, and macOS

This section explains how you can calculate the best parameters for your system in these sections:

For optimal Caché performance, you need to calculate proper values for certain Caché system parameters. These values allow you to determine whether you need to adjust certain system level parameters. The values you choose should minimize swapping and paging that require disk accesses, and thus improve system performance.

Review this section carefully and calculate the proper values for both your operating system and Caché before proceeding. Use the tables provided here to record the current and calculated values for your system level parameters. You can then refer to these tables when you install Caché. After your system is running, you may need to adjust these values to gain optimal performance.

If you are not already familiar with the memory organization at your operating system level, consult the appropriate system documentation.

Determining Initial System Configuration

This section covers some basic configuration topics for UNIX, Linux, and macOS systems. Because requirements vary by platform, consult your platform documentation for additional information.

See the section Managing Caché Memory for information on the two primary ways that you can manage memory in Caché.

Configuring Large and Huge Pages

As noted in Large and Huge Pages, the use of large and huge memory pages where supported can be of significant performance benefit and is highly recommended.

Support for Huge Memory Pages for Linux

The default memory page size on Linux systems is 4 KB. Most current Linux distributions include an option for Huge Pages, that is, a memory page size of 2 MB or 1 GB depending on system configuration. Use of Huge Pages saves memory by saving space in page tables. When Huge Pages are configured, the system automatically uses them in memory allocation. InterSystems recommends the use of Huge Pages on systems hosting Caché under most circumstances.

On Linux platforms, if shared memory is allocated in Huge Pages, they are automatically locked in memory and no further action is required. You can configure Caché to lock the shared memory segment in memory to prevent paging as described in the memlock entry of the Configuration Parameter File Reference.


With the 2.6.38 kernel, some Linux distributions have introduced Transparent Huge Pages (THP) to automate the creation, management, and use of HugePages. However, THP does not handle the shared memory segments that make up the majority of Caché’s memory allocated, and can cause memory allocation delays at runtime that may affect performance, especially for applications that have a high rate of job or process creation. For these reasons, InterSystems recommends that THP be disabled on all systems hosting Caché. For more detailed information on this topic, see Linux Transparent Huge Pages and the impact to CachéOpens in a new tab on InterSystems Developer Community.

To configure Huge Pages on Linux, do the following:

  1. Check the status.

    /proc/meminfo contains Huge Pages information. By default, no Huge Pages are allocated. Default Huge Page size is 2 MB. For example:

    HugePages_Total:     0
    HugePages_Free:      0
    HugePages_Rsvd:      0
    Hugepagesize:     2048 KB
  2. Change the number of Huge Pages.

    You can change the system parameter directly: For example, to allocate 2056 Huge Pages, execute:

    # echo 2056 > /proc/sys/vm/nr_hugepages

    Alternatively, you can use sysctl(8) to change it:

    # sysctl -w vm.nr_hugepages=2056  

    Huge pages must be allocated contiguously, which may require a reboot. Therefore, to guarantee the allocation, as well as to make the change permanent, do the following:

    1. Enter a line in /etc/sysctl.conf file:

      echo "vm.nr_hugepages=2056" >> /etc/sysctl.conf  
    2. Reboot the system.

    3. Verify meminfo after reboot; for example:

      [root woodcrest grub]# tail -4 /proc/meminfo
      HugePages_Total:  2056
      HugePages_Free:   2056
      HugePages_Rsvd:      0
      Hugepagesize:     2048 KB
  3. Verify the use of Huge Pages by Caché.

    When Caché is started, it reports how much shared memory was allocated; for example, a message similar to the following is displayed (and included in the cconsole.log file):

    Allocated 3580MB shared memory: 3000MB global buffers, 226MB routine buffers

    The amount of memory available in Huge Pages should be greater than the total amount of shared memory to be allocated; if it is not greater, Huge Pages are not used.


    Huge Pages are allocated from physical memory. Only applications and processes using Huge Pages can access this memory.

    Physical memory not allocated for Huge Pages is the only memory available to all other applications and processes.

    It is not advisable to specify HugePages_Total much higher than the shared memory amount because the unused memory will not be available to other components.

    If Caché fails to allocate Huge Pages on start-up and switches to standard pages, Caché will be allocating shared memory from the same memory pool as all other jobs.

    When Caché is configured to lock the shared memory segment in memory to prevent paging, Huge Pages can provide the required increase in the maximum size that may be locked into memory, as described in the Locked-in Memory section of the Red Hat Linux Platform Notes in this chapter.

Support for Large (16 MB) Pages on IBM AIX®

AIX® supports multiple page sizes: 4 KB, 64 KB, 16 MB, and 16 GB. Use of 4 KB and 64 KB pages is transparent to Caché. In order for Caché to use 16 MB large pages, you must configure them within AIX®. AIX® does not automatically change the number of configured large or huge pages based on demand. Currently, Caché does not use 16 GB huge pages.

Large pages should be configured only in high-performance environments because memory allocated to large pages can be used only for large pages.

To allocate large pages, users must have the CAP_BYPASS_RAC_VMM and CAP_PROPAGATE capabilities or have root authority unless memlock=64.

By default, when large pages are configured, the system automatically uses them in memory allocation. If shared memory cannot be allocated in large pages then it is allocated in standard (small) pages. For finer grain control over large pages, see memlock in the Caché Parameter File Reference.

Configuring Large Pages for AIX®

Configure large pages using the vmo command as follows:

vmo -r -o lgpg_regions=<LargePages> -o lgpg_size=<LargePageSize>

where <LargePages> specifies the number of large pages to reserve, and <LargePageSize> specifies the size, in bytes, of the hardware-supported large pages.


On systems that support dynamic Logical PARtitioning (LPAR), you can omit the -r option to dynamically configure large pages without a system reboot.

For example, the following command configures 1 GB of large pages:

# vmo -r -o lgpg_regions=64 -o lgpg_size=16777216

Once you have configured large pages, run the bosboot command to save the configuration in the boot image. After the system comes up, enable it for pinned memory using the following vmo command:

vmo -o v_pinshm=1

However, if memlock=64, vmo -o v_pinshm=1 is not required. For more information on memlock, see memlock in the Caché Parameter File Reference.

Calculating Disk Requirements

Your Caché instance needs disk space for the following items:

  • 67 MB for Caché.

  • 3 MB for the Caché Server Pages (CSP).

  • 3.5 MB for Caché ODBC support.

  • 2.5 MB for the Caché manager sources.

  • 6.6 MB for the Caché engine link libraries.

  • Space for your Caché application database.

  • Approximately 12.5% of the buffer pool size for the initial size of the write image journal file. If your disk does not have enough space for the write image journal file, when you start Caché it displays a message indicating that the system did not start.

  • Desired space for journal files.

Although you do not need to remove any installation files after completing the installation procedure, you can do so if you are short on disk space. The installation program tells you how much space can be saved, and asks if you want to delete the installation files.

Determining the Number of Global Buffers

Caché supports the following maximum values for the number of global buffers:

  • For 32-bit platforms, any 8-KB buffers that are:

    • Less than 2GB for 32-bit platforms

    The 2-GB value is the total address space the operation system allocates for the process data, which includes not only shared memory, but other Caché and operating system data as well. Therefore, it represents an upper limit that is not achievable in practice.

  • For 64-bit platforms:

    The number of global buffers is limited only by the operating system and the available memory.

For guidelines for your initial allocation of memory to the database cache (the global buffer pool), see Calculating Initial Memory Requirements; for the procedure for this allocation, see Allocating Memory Within Caché. For further information about the database cache, see globals in the “config” section of the Caché Parameter File Reference and Memory and Startup Settings in the “Configuring Caché” chapter of the Caché System Administration Guide.

Determining Number of Routine Buffers

Caché supports the following maximum value for the number of routine buffers:


Set your values to less than this maximum number of buffers.

For guidelines for your initial allocation of memory to the database cache (the global buffer pool), see Calculating Initial Memory Requirements; for the procedure for this allocation, see Allocating Memory Within Caché. For further information about the database cache, see routines in the “config” section of the Caché Parameter File Reference and Memory and Startup Settings in the “Configuring Caché” chapter of the Caché System Administration Guide.

Determining Maximum Number of Users

The maximum users allowed by Caché is the lowest of the following values:

  • License limit

  • # of semaphores - 4

For more information, see Determining License Capacity and Usage in the “Managing Caché Licensing” chapter of the Caché System Administration Guide.

Determining Maximum Database Size

The ulimit parameter in UNIX® determines the maximum file size available to a process. For the Caché Manager group, the value of ulimit should either be unlimited or as large as the largest database you may have.

For more information, see Configuring Databases in the “Configuring Caché” chapter of the Caché System Administration Guide.

Configuring UNIX® Kernel Parameters

The following sections describe issues related to tuning and performance on various UNIX® platforms:

Setting Values for Tunable UNIX® Parameters

Caché uses a configurable number of semaphores, in sets whose size you define. The parameters SEMMNI, SEMMNS, and SEMMSL reflect the number of semaphores per set and the total number of semaphores Caché uses. The UNIX®/Linux parameters that govern shared memory allocation are SHMMAX, SHMMNI, SHMSEG, and SHMALL. Caché uses shared memory and allocates one segment of shared memory; the size of this segment depends on the area set aside for global buffers and routine buffers. It uses the following formula to determine the segment's minimum size:

                       space required for routine buffers
                     +  space required for global buffers
                     +                               4 MB
                     =         Shared memory segment size

If you are distributing your data across multiple computers, Caché allocates a second segment; by default, there is no memory allocated for the second segment. (If you plan to use distributed data, contact your vendor or InterSystems support for configuration guidelines.) You can alter NBUF and NHBUF according to other system requirements. Because Caché does all its own disk buffering, you should keep NBUF and NHBUF small. The following table lists the most common names of the UNIX® parameters that you may need to change, the minimum value InterSystems recommends for each parameter, and a brief description of each. Verify that your parameter values are set to at least the minimum value. Certain parameters may not be implemented on all platforms or may be referred to differently. Refer to platform-specific tuning notes for more information.

Tunable UNIX® Parameters
Kernel Parameter Recommended Minimum Value Definition
CDLIMIT Number of bytes in largest virtual volume Maximum size of a file.
MSGMAX 2 KB Maximum message size, in bytes.
MSGMNI Number of Caché instances x 3; each Caché instance uses three message queues Maximum number of uniquely identifiable message queues that may exist simultaneously.
NOFILES 35 Number of open files per process.
SEMMNI Product of SEMMNI and SEMMSL must be greater than the # of user processes + 4 Number of semaphore identifiers in the kernel; this is the number of unique semaphore sets that can be active at any one time.
SEMMNS 128 or ... Total number of semaphores in the system. User processes include jobbed processes and all other semaphores required by other software.
Number of processes expected to run. If the process table might expand, use a larger number to provide for expansion.
SEMMSL See SEMMNI Maximum number of semaphores per identifier list.
SHMALL 60 KB or ... Maximum total shared memory system-wide. Units should be in KB. 1000 represents the MCOMMON shared region.
1000 + total global buffer space+ total routine buffer space *
SHMMNI 3 Maximum number of shared memory identifiers system-wide.
SHMSEG 3 Number of attached shared memory segments per process.
SHMMAX 60 KB or ... Maximum shared memory segment size in KB.
1000 + total global buffer space+ total routine buffer space

* This is the minimum value for SHMALL required for Caché UNIX®. You must also take into account any other applications that use shared memory. If you are unsure of other shared memory use, calculate SHMALL as SHMSEG multiplied by SHMMAX, in pages; this larger value suffices in all cases.


Enough swap space must be created to support the memory allocated, unless the operating system documentation explicitly states otherwise. On certain operating systems, Caché creates locked shared memory segments, which are not pageable but still may need swap space.

Adjusting Maximum File Size

The hard limit for the maximum file size (RLIMIT_FSIZE) on any system running Caché must be unlimited. Set the value to unlimited on the operating system before installing. Make sure that the limit is set to unlimited for both the root user and the user who will run Caché. Caché also sets the process soft limit to RLIMIT_FSIZE in its daemons to prevent I/O errors.


Caché will not install or start up if RLIMIT_FSIZE is not set to unlimited.

See the operating system documentation for your platform for instructions on how to set the system hard limit for the maximum file size, RLIMIT_FSIZE.

FeedbackOpens in a new tab