Badkey Corner | ConstrainedSHM and PercentAvailSysResources in notes.ini

Tuesday 13th, March 2007

ConstrainedSHM and PercentAvailSysResources in notes.ini03/13/2007 12:19 PM

ConstrainedSHM in notes.ini

We where working on Domino servers on a VMWare environment and we noticed that 'Domino was eating' a lot of memory.

PANIC: Cannot attach to shared memory region
This error may happen on servers with very high process local memory usage. An example would be the http server serving up large databases or if port compression is enabled. In Domino the private and shared memory must reside in a limed virtual address space, which is usually 4 gigabytes. This error happens when Domino runs out of virtual memory. In order to prevent this from happening there are 2 new notes.ini entries:

ConstrainedSHM=1
ConstrainedSHMSizeMB= size in mega-bytes

The ConstrainedSHM=1 will restrict shared memory to a set of default sizes as follows:
Windows and Mac platforms 2 giga-bytes
AIX platforms 2.4 giga-bytes
Solaris and Linux 3 giga-bytes

The ConstrainedSHMSizeMB="size in mega-bytes" will restrict memory to the size in mega-bytes specified.

The PercentAvailSysResources may also be used if partition servers are used.

In Domino Release 5.0.4 or later, you can control the amount of memory allocated to your server by using the PercentAvailSysResources variable.

Before the introduction of this variable in 5.0.4, each Domino server assumed it had 100 percent of system memory available to it. With the PercentAvailSysResources variable, you assign a portion of memory to the server by specifying a value from 2 to 100, which represents a percentage of a system's total physical memory. For example, if you want to dedicate 75 percent of system memory to Domino, add the following line to the NOTES.INI file:

PercentAvailSysResources=75

This effectively leaves 25 percent of memory for other applications. To follow through with the example, a Domino server on a system with 256MB of RAM and the NOTES.INI setting PercentAvailSysResources=75 would reserve 192MB for itself .

You can also allocate memory on partitioned servers using the PercentAvailSysResources variable. For example, if you have a Domino server with two partitions and want to control the memory resources allocated to each partition, you could set the NOTES.INI of Partition 1 with the line PercentAvailSysResources=60 and the NOTES.INI of Partition 2 with the line PercentAvailSysResources=40. In this case, Partition 1 would use 60 percent of the system memory resources and Partition 2 would use 40 percent. Make sure that the sum of the values you specify in each partition does not exceed 100 percent.

To reserve system memory for other applications, you can choose values that total less than 100 percent.

All of these Notes.INI values interact as follows:

On AIX (a segmented memory platform):
In essence Domino takes the lower of "Mappable shared memory size Dynamic from maxdata" ,"ram on the system * (PercentAvailSysResources/100)", "Notes.ini ConstrainedSHMSizeMB" and "ConstrainedSHM". It should be noted that ConstrainedSHMSizeMB will round down on AIX only to a 256 mega-byte boundary. example if set to 650 the it will be set to 512.

On all other platforms:
In essence Domino takes the lower of "ram on the system * (PercentAvailSysResources/100)", "Notes.ini ConstrainedSHMSizeMB" and "ConstrainedSHM". Domino will size it's shared memory usage to live within the limits derived above.

ConstrainedSHM /ConstrainedSHMSizeMB- introduced in ND6, this parameter actually enforces an overall limit to the amount of shared memory a Domino Server will use.

Unlike PercentAvailSysResources, this parameter not only affects the size of the UBM, but also enforces a hard limit on the total amount of shared memory that a single Domino Server will allocate. Better still, the default values for this parameter take into account the limits of a 32-bit process address space, scaling memory usage more effectively than the use of PercentAvailSysResources.

Note: ND7.0.2FP1 on AIX has introduced a new parameter than is the preferred method of constraining shared memory, one that is more intelligent regarding the AIX segmented architecture. This is the new parameter AIX_LIMIT_SHM_SEGMENTS; additional documentation is forthcoming for this parameter.

Source:
Notes/Domino Best Practices: Performance Reference #: 7008849
PercentAvailSysResources Provides Inconsistent Results on Very Large Memory Systems Reference #: 1099088
How does the notes.ini parameter PercentAvailSysResources work. Reference #: 1088944

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