A    Tuning a Low Memory System Workstation

The appropriate tuning for any system depends on how the system is used and how it is configured. The following sections describe tuning considerations that can be very important for low-memory (24-MB) Alpha systems. Note that some of these tuning considerations can also apply to Digital UNIX workstations in general, regardless of size.

A.1    Attribute Settings for Low Memory System Workstations

The following attribute settings are applied automatically when installing Digital UNIX on a 24-MB Alpha system.

generic:
 lite-system=1

 

proc:
 ncallout_alloc_size=4096

 

vfs:
 bufcache=2
 max-vnodes=1000
 min-free-vnodes=150
 vnode-age=2
 namei-cache-valid-time=30
 name-cache-size=150

 

io:
 bdevsw-size = 70
 cdevsw-size = 125
 max-iosize-read = 65536
 max-iosize-write = 65536
 basic-dma-window-size = 0
 cam_ccb_pool_size = 100
 cam_ccb_low_water = 50
 cam_ccb_increment = 25

 

network:
 arptab_nb=19

 

vm:
 vm-aggressive-swap = 1

The default attribute settings for 24-MB Alpha systems increase the amount of physical memory available to user applications by reducing the amount of memory used for system caches. These settings may also work well on 32-MB or larger Alpha systems that are being used as personal workstations (that is, not being used as timesharing systems or file servers). You can apply the settings to your workstation by issuing the following command:

#  sysconfigdb -f /etc/sysconfigtab.lite -m

The settings can be removed by issuing the following command:

#  sysconfigdb -f /etc/sysconfigtab.lite -r

After issuing either of these sysconfigdb commands, the system must be rebooted before the attribute changes will take effect. (See Appendix B for details on the sysconfigtab attributes.)

A.2    Swap Space and Memory Tuning on Low-Memory Systems

Swapping can cause problems with low-memory systems. When operating in deferred mode (also referred to as overcommittment or lazy mode), low-memory systems will use more swap space. Low-memory systems have to overcommit much more physical memory than high-memory systems. As a result, low-memory systems will do more page outs and will use more swap space.

When swap space is exhausted, you will receive warning messages, and processes will be killed. The only solution to this problem is to increase either memory or swap space. See Section 3.4.2.1 for information on swapping modes. (Low-memory systems operating in immediate mode do not have special problems with swapping.)

Low-memory systems can also have special problems with the UBC (unified buffer cache). If vmstat output shows excessive page ins but few or no page outs, the value of ubc-borrowpercent is probably set too low. It is particularly important to watch for this on low-memory systems because they tend to reclaim UBC pages much more aggressively than systems with more memory, and this condition can have an adverse effect on system performance. See Section 3.4.1.1 for information about attribute settings affecting the UBC.

A.3    X Window System Considerations

On low-memory systems, you may want to consider the following adjustments that affect memory use by the X Window system:

• If a shortage of memory (and the paging and swapping associated with limited memory) is slowing the performance of your system, you may need to reset the X server to free up memory resources.

  Space that has been allocated to the X server is never freed to the system until the X server is terminated; it can be reused by the X server, but it is not freed. As a result, the amount of memory resources allocated to the X server is the largest amount of memory resources ever used by the X server at any single point in time in either the current session or any prior session. This amount can be rather large if at some point you opened a large number of windows, displayed PostScript text, and performed other windowing operations that consume a lot of memory.

  To reset the X server and eliminate any reserve of unused memory that may have built up over time, you need to specify the -terminate option in the Xserver.conf file (/var/X11/Xserver.conf) and then kill the X server (kill pid). Xdm will automatically restart your X server. (Use the ps command to locate the pid for the X server, that is, the pid for the /usr/bin/X11/X command.) From that point forward, with the -terminate option in effect, the X server will reset upon each logout.

  The Xserver.conf file contains information on how to add options. For example, the entry for the -terminate option, with no other options present in the entry, would have the following form:

  args < -terminate >
  

  If you are running xdm, you can establish the -terminate option in either the Xserver.conf file or the Xservers file (/var/X11/xdm/Xservers).

• On low-memory systems, you should always compile Motif applications with the -shared option (the default) instead of the -non_shared option. Applications compiled with the -shared option have much smaller executable files and consume much fewer system resources than applications compiled with the -non_shared option. This is especially true for Motif applications; for example, a Motif application that is 400K bytes in the -shared case may be as large as 4M bytes in the -non_shared case - a factor of 10 difference in size.