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Chapter 9

Physical Memory Control Using the Resource Capping Daemon

The resource capping daemon rcapd regulates physical memory consumption by processes running in projects that have resource caps defined.

Resource Capping Daemon Overview

A resource cap is an upper bound placed on the consumption of a resource, such as physical memory. Per-project physical memory caps are supported.

The resource capping daemon and its associated utilities provide mechanisms for physical memory resource cap enforcement and administration.

Like the resource control, the resource cap can be defined by using attributes of project entries in the project database. However, while resource controls are synchronously enforced by the kernel, resource caps are asynchronously enforced at the user level by the resource capping daemon. With asynchronous enforcement, a small delay occurs as a result of the sampling interval used by the daemon.

For information about rcapd, see the rcapd(1M) man page. For information about projects and the project database, see Chapter 5, Projects and Tasks and the project(4) man page. For information about resource controls, see Chapter 7, Resource Controls.

How Resource Capping Works

The daemon repeatedly samples the resource utilization of projects that have physical memory caps. The sampling interval used by the daemon is specified by the administrator. See Determining Sample Intervals for additional information. When the system's physical memory utilization exceeds the threshold for cap enforcement, and other conditions are met, the daemon takes action to reduce the resource consumption of projects with memory caps to levels at or below the caps.

The virtual memory system divides physical memory into segments known as pages. Pages are the fundamental unit of physical memory in the Solaris memory management subsystem. To read data from a file into memory, the virtual memory system reads in one page at a time, or pages in a file. To reduce resource consumption, the daemon can page out, or relocate, infrequently used pages to a swap device, which is an area outside of physical memory.

The daemon manages physical memory by regulating the size of a project workload's resident set relative to the size of its working set. The resident set is the set of pages that are resident in physical memory. The working set is the set of pages that the workload actively uses during its processing cycle. The working set changes over time, depending on the process's mode of operation and the type of data being processed. Ideally, every workload has access to enough physical memory to enable its working set to remain resident. However, the working set can also include the use of secondary disk storage to hold the memory that does not fit in physical memory.

Only one instance of rcapd can run at any given time.

Attribute to Limit Physical Memory Usage

To define a physical memory resource cap for a project, establish a resident set size (RSS) cap by adding this attribute to the project database entry:

rcap.max-rss

The total amount of physical memory, in bytes, that is available to processes in the project.

For example, the following line in the /etc/project database sets an RSS cap of 10 gigabytes for a project named db.

db:100::db,root::rcap.max-rss=10737418240

Note - The system might round the specified cap value to a page size.

`rcapd` Configuration

You use the rcapadm command to configure the resource capping daemon. You can perform the following actions:

Set the threshold value for cap enforcement
Set intervals for the operations performed by rcapd
Enable or disable resource capping
Display the current status of the configured resource capping daemon

To configure the daemon, you must have superuser privileges or have the Process Management profile in your list of profiles. The Process Management role and the System Administrator role both include the Process Management profile. See "RBAC Elements: Reference Information" in System Administration Guide: Security Services.

Configuration changes can be incorporated into rcapd according to the configuration interval (see rcapd Operation Intervals) or on demand by sending a SIGHUP (see the kill(1) man page).

If used without arguments, rcapadm displays the current status of the resource capping daemon if it has been configured.

The following subsections discuss cap enforcement, cap values, and rcapd operation intervals.

Memory Cap Enforcement Threshold

The memory cap enforcement threshold is the percentage of physical memory utilization on the system that triggers cap enforcement. When the system exceeds this utilization, caps are enforced. The physical memory used by applications and the kernel is included in this percentage. The percentage of utilization determines the way in which memory caps are enforced.

To enforce caps, memory can be paged out from project workloads.

Memory can be paged out to reduce the size of the portion of memory that is over its cap for a given workload.
Memory can be paged out to reduce the proportion of physical memory used that is over the memory cap enforcement threshold on the system.

A workload is permitted to use physical memory up to its cap. A workload can use additional memory as long as the system's memory utilization stays below the memory cap enforcement threshold.

To set the value for cap enforcement, see How to Set the Memory Cap Enforcement Threshold.

Determining Cap Values

If a project cap is set too low, there might not be enough memory for the workload to proceed effectively under normal conditions. The paging that occurs because the workload requires more memory has a negative effect on system performance.

Projects that have caps set too high can consume available physical memory before their caps are exceeded. In this case, physical memory is effectively managed by the kernel and not by rcapd.

In determining caps on projects, consider these factors.

Impact on I/O system

The daemon can attempt to reduce a project workload's physical memory usage whenever the sampled usage exceeds the project's cap. During cap enforcement, the swap devices and other devices that contain files that the workload has mapped are used. The performance of the swap devices is a critical factor in determining the performance of a workload that routinely exceeds its cap. The execution of the workload is similar to running it on a machine with the same amount of physical memory as the workload's cap.

Impact on CPU usage

The daemon's CPU usage varies with the number of processes in the project workloads it is capping and the sizes of the workloads' address spaces.

A small portion of the daemon's CPU time is spent sampling the usage of each workload. Adding processes to workloads increases the time spent sampling usage.

Another portion of the daemon's CPU time is spent enforcing caps when they are exceeded. The time spent is proportional to the amount of virtual memory involved. CPU time spent increases or decreases in response to corresponding changes in the total size of a workload's address space. This information is reported in the vm column of rcapstat output. See Monitoring Resource Utilization With rcapstat and the rcapstat(1) man page for more information.

Reporting on shared memory

The daemon cannot determine which pages of memory are shared with other processes or which are mapped multiple times within the same process. Since rcapd assumes that each page is unique, this results in a discrepancy between the reported (estimated) RSS and the actual RSS.

Certain workloads, such as databases, use shared memory extensively. For these workloads, you can sample a project's regular usage to determine a suitable initial cap value. Use output from the prstat command with the -J option. See the prstat(1M) man page.

`rcapd` Operation Intervals

You can tune the intervals for the periodic operations performed by rcapd.

All intervals are specified in seconds. The rcapd operations and their default interval values are described in the following table.

Operation	Default Interval Value in Seconds	Description
`scan`	15	Number of seconds between scans for processes that have joined or left a project workload. Minimum value is 1 second.
`sample`	5	Number of seconds between samplings of resident set size and subsequent cap enforcements. Minimum value is 1 second.
`report`	5	Number of seconds between updates to paging statistics. If set to `0`, statistics are not updated, and output from `rcapstat` is not current.
`config`	60	Number of seconds between reconfigurations. In a reconfiguration event, `rcapadm` reads the configuration file for updates, and scans the `project` database for new or revised project caps. Sending a `SIGHUP` to `rcapd` causes an immediate reconfiguration.

To tune intervals, see How to Set Operation Intervals.