Checking CPU Activity and Memory Allocation

Checking CPU Activity and Memory Allocation

After looking at disk I/O and paging, next check CPU activity and memory allocation.

Checking The CPU

A CPU can execute only one process at any given instant. If the CPU becomes overloaded, processes have to wait instead of executing. You can't change the speed of the CPU (although you may be able to upgrade to a faster CPU or add CPU boards to your system if your hardware allows it), but you can monitor CPU load and try to distribute the load. Table 11-6 shows the fields to check for indications that a system is CPU bound.

Indications of a CPU-Bound System
Field Value sar Option
%idle (% of time CPU has no work to do) <5 sar -u
runq-sz (processes in memory waiting for CPU) >2 sar -q
%runocc (% run queue occupied and processes not executing) >90 sar -q

Indications of a CPU-Bound System
Field	Value	sar Option
%idle (% of time CPU has no work to do)	<5	sar -u
runq-sz (processes in memory waiting for CPU)	>2	sar -q
%runocc (% run queue occupied and processes not executing)	>90	sar -q

You can also use the top(1) or gr_top(1) commands to display processes having the highest CPU usage. For each process, the output lists the user, process state flags, process ID and group ID, CPU cycles used, processor currently executing the process, process priority, process size (in pages), resident set size (in pages), amount of time used by the process, and the process name. For more information, see the top(1) or gr_top(1) reference pages.

To increase CPU performance, you'll want to make the following modifications:

off-load jobs to non-peak times or to another machine, set efficient paths, and tune applications.
eliminate polling loops (see select(2)).
increase the slice-size parameter (the length of a process time slice). For example, change slice-size from Hz/30 to Hz/10. However, be aware that this may slow interactive response time.
upgrade to a faster CPU or add another CPU.

Checking Available Memory

"Checking for Excessive Paging and Swapping" described what happens when you don't have enough physical (main) memory for processes.

This section discusses a different problem - what happens when you don't have enough available memory (sometimes called virtual memory), which includes both physical memory and logical swap space.

The IRIX virtual memory subsystem allows programs that are larger than physical memory to execute successfully. It also allows several programs to run even if the combined memory needs of the programs exceed physical memory. It does this by storing the excess data on the swap device(s).

The allocation of swap space is done after program execution has begun. This allows programs with large a virtual address to run as long as the actual amount of virtual memory allocated does not exceed the memory and swap resources of the machine.

Usually it's very evident when you run out of memory, because a message is sent to the console that begins:

Out of logical swap space...

If you see this message:

the process has exceeded ENOMEM or UMEM.
there is not enough physical memory for the kernel to hold the required non-pageable data structures.
there is not enough logical swap space.

You can add virtual swap space to your system at any time. See "Swap Space" if you wish to add more swap space. You need to add physical swap space, though, if you see the message:

Process killed due to insufficient memory

The following system calls will return EAGAIN if there is insufficient available memory: exec, fork, brk, sbrk (called by malloc), mpin, and plock. Applications should check the return status and exit gracefully with a useful message.

To check the size (in pages) of a process that is running, execute ps -el (you can also use top(1)). The SZ:RSS field will show you very large processes.

By checking this field, you can determine the amount of memory the process is using. A good strategy is to run very large processes at less busy times.

Determining the Amount of System Memory

To see the amount of main memory, use the hinv(1) command. It displays data about your system's configuration. For example:

Main memory size: 64 Mb

Maximizing Memory

To increase the amount of virtual memory, increase the amount of real memory and/or swap space. Note that most of the paging/swapping solutions also apply to ways to conserve available memory. These include:

limiting the number of programs
using shared libraries
adding more memory
decreasing the size of system tables

However, the most dramatic way to increase the amount of virtual memory is to add more swap space. The previous section that described using partitions explained how to do this and it is covered completely in "Swap Space".