Pseudo terminals, ptys, are used for two purposes in Oracle Solaris software:
Supporting remote logins by using the telnet, rlogin, or rsh commands
Providing the interface through which the X Window system creates command interpreter windows
The default number of pseudo-terminals is sufficient for a desktop workstation. So, tuning focuses on the number of ptys available for remote logins.
The default number of ptys is now based on the amount of memory on the system. This default should be changed only to restrict or increase the number of users who can log in to the system.
Three related variables are used in the configuration process:
pt_cnt – Default maximum number of ptys.
pt_pctofmem – Percentage of kernel memory that can be dedicated to pty support structures. A value of zero means that no remote users can log in to the system.
pt_max_pty – Hard maximum for number of ptys.
pt_cnt has a default value of zero, which tells the system to limit logins based on the amount of memory specified in pct_pctofmem, unless pt_max_pty is set. If pt_cnt is non-zero, ptys are allocated until this limit is reached. When that threshold is crossed, the system looks at pt_max_pty. If pt_max_pty has a non-zero value, it is compared to pt_cnt. The pty allocation is allowed if pt_cnt is less than pt_max_pty. If pt_max_pty is zero, pt_cnt is compared to the number of ptys supported based on pt_pctofmem. If pt_cnt is less than this value, the pty allocation is allowed. Note that the limit based on pt_pctofmem only comes into play if both pt_cnt and ptms_ptymax have default values of zero.
To put a hard limit on ptys that is different than the maximum derived from pt_pctofmem, set pt_cnt and ptms_ptymax in /etc/system to the preferred number of ptys. The setting of ptms_pctofmem is not relevant in this case.
To dedicate a different percentage of system memory to pty support and let the operating system manage the explicit limits, do the following:
Do not set pt_cnt or ptms_ptymax in /etc/system.
Set pt_pctofmem in /etc/system to the preferred percentage. For example, set pt_pctofmem=10 for a 10 percent setting.
Note that the memory is not actually allocated until it is used in support of a pty. Once memory is allocated, it remains allocated.