A Tuning Special Configurations

This appendix provides information about tuning special configurations. See Section 2.11 for information about modifying system attributes.

A.1 Tuning Internet Servers

Internet servers require that you modify the default values of some system attributes. Internet servers include World Wide Web servers, proxy servers, mail servers, and ftp servers. See Chapter 6 for detailed information about these attributes.

You can modify the following socket subsystem attributes and specify the values as indicated:

somaxconn = 65535

sominconn = 65535

You can modify the following inet subsystem attributes and specify the values as indicated:

tcphashsize = 16384

ipport_userreserved = 65535

pmtu_enabled

You can modify the following vm subsystem attributes and specify the values as indicated:

ubc-maxpercent = 100

vm-mapentries = 20000

vm-maxvas = 10737418240

vm-vpagemax = 131072

You can modify the following proc subsystem attributes and specify the values as indicated:

maxusers = 512

max-proc-per-user = 512

max-threads-per-user = 4096

A.2 Tuning a Low-Memory Workstation

The following sections describe tuning considerations for low-memory (24-MB) Alpha systems. Some of these tuning considerations may also apply to DIGITAL UNIX workstations in general, regardless of size.

A.2.1 Attribute Settings for Low-Memory Workstations

The following attribute settings are automatically used 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 entering the following command:

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

The settings can be removed by entering the following command:

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

After entering either of these sysconfigdb commands, you must reboot the system to apply the new attribute values.

A.2.2 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 more physical memory than high-memory systems. As a result, low-memory systems will do more pageouts 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 Chapter 4 for information on swap 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 Unified Buffer Cache (UBC). If vmstat output shows excessive pageins but few or no pageouts, the value of the ubc-borrowpercent attribute may be too small. It is particularly important to watch for this on low-memory systems, because they tend to reclaim UBC pages more aggressively than systems with more memory, and this condition can have an adverse effect on system performance. See Chapter 4 for information about attribute settings affecting the UBC.

A.2.3 X Window System Considerations for Low-Memory Workstations

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). The X Display Manager, xdm, will automatically restart your X server. (Use the ps command to locate the process identification (PID) number for the X server.) With the -terminate option in effect, the X server will now 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, is as follows:
```
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, compile Motif applications with the -shared option (the default) instead of the -non_shared option. Applications compiled with the -shared option have smaller executable files and consume 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 400 KB with the -shared option may be as large as 4 MB with the -non_shared option.