Red Hat Linux Platform Notes

Shared Memory Limit

The default shared memory limit (shmmax) on Linux platforms is 32 MB. This value is too small for InterSystems IRIS, but it can be changed in the proc file system without a restart. The new memory limit remains in effect until you restart the Red Hat Linux system.

For example, to allow 128 MB, type the following command:

$ echo 134217728 >/proc/sys/kernel/shmmax

You can put this command into a startup script.

Alternatively, you can use sysctl(8), if available, to set this parameter permanently. Add a line to the/etc/sysctl.conf similar to the following:

kernel.shmmax = 134217728

This file is usually processed at startup, but sysctl can also be called explicitly later.

Other parameters are sufficiently sized for an InterSystems IRIS application. To view the values of other parameters, look in the files /usr/src/linux/include/asm-xxx/shmparam.h and /usr/src/linux/include/linux/sem.h.

For more information, reference “The proc File SystemOpens in a new tab” chapter of the Red Hat Enterprise Linux 4: Reference Guide.

Locked-in Memory

On Linux platforms, if shared memory is allocated in huge pages, the pages are automatically locked in memory and no further action is required. See the Configuring Huge Pages on Linux section in this chapter for information about allocating huge pages.

If not using huge pages, you can configure InterSystems IRIS to lock the shared memory segment in memory to prevent paging. This is described in the LockSharedMemory section of the “memlock” entry in the Configuration Parameter File Reference.

Otherwise, you must increase the maximum size that may be locked into memory. The default value is 32 KB. View the current value using the ulimit command.

For example, to display all current limits:

bash$ ulimit -a 
core file size (blocks, -c) unlimited 
data seg size ( KBytes, -d) unlimited 
file size (blocks, -f) unlimited 
pending signals (-i) 1024 
max locked memory (KBytes, -l) 32 <---------- THIS ONE 
max memory size (KBytes, -m) unlimited 
open files (-n) 1024 
pipe size (512 bytes, -p) 8 
POSIX message queues (bytes, -q) 819200 
stack size ( KBytes, -s) 10240 
cpu time (seconds, -t) unlimited 
max user processes (-u) 49000 
virtual memory ( KBytes, -v) unlimited 
file locks (-x) unlimited 

To display only max-locked memory, use the -l option:

bash$ ulimit -l 
32 

If you have privileges, you can alter the value directly using the ulimit command; however, it is better to update the memlock parameter in the /etc/security/limits.conf file. If the memlock limit is too low, Linux reports a ENOMEM - "Not enough memory" error, which does not make the cause obvious. The actual memory is allocated; it is the lock that fails.

For more information, see memlock in the Configuration Parameter File Reference.

Adjusting for Large Number of Concurrent Processes

Make the following adjustments if you are running a system that requires a large number of processes or telnet logins.

1. In the /etc/xinetd.d/telnet file, add the following line:

   instances = unlimited
   
   

   

2. In the /etc/xinetd.conf file, add or change the instances setting to:

   instances = unlimited
   
   

   

3. After you make these modifications, restart the xinetd services with:

   # service xinetd restart
   

   

4. The default pty (pseudo terminal connection) limit is 4096. If this is not sufficient, add or change the maximum pty line in the /etc/sysctl.conf file. For example:

   kernel.pty.max=10000
   

   

Dirty Page Cleanup

On large memory systems (for example, 8GB or larger), when doing numerous flat-file writes (for example, InterSystems IRIS backups or file copies), you can improve performance by adjusting the following parameters, which are located in proc/sys/vm/:

• dirty_background_ratio — Maximum percentage of active that can be filled with dirty pages before pdflush begins to write them. InterSystems recommends setting this parameter to 5.

• dirty_ratio — Maximum percentage of total memory that can be filled with dirty pages before processes are forced to write dirty buffers themselves during their time slice instead of being allowed to do more writes. InterSystems recommends setting this parameter to 10

You can set these variables by adding the following to your /etc/sysctl.conf file:

vm.dirty_background_ratio=5 
vm.dirty_ratio=10

These changes force the Linux pdflush daemon to write out dirty pages more often rather than queue large amounts of updates that can potentially flood the storage with a large burst of updates.”

Using Kerberos

To use Kerberos on the Red Hat Linux platform, you must install the krb5-devel package in addition to the krb5-libs package. Installing krb5-devel establishes the required symbolic links for using Kerberos. The package is required for production environments, not only development environments. See the Red Hat NetworkOpens in a new tab web site for more information about these components.