The truth about sadcnt/nautopush/pt_cnt and npty.
2007-12-25 11:30:00
Please help me stamp out the myth that is being perpetuated about
increasing the number of ptys.
Here's is how it's really done for system V ptys, in 99.9% of the cases,
this is all you need:
set pt_cnt = <num>
followed by a "reconfigure reboot".
Now, if you really, really need more BSD ptys, here's how:
set npty = <num> (doesn't need to be the same as SV ptys)
but you *must* also edit /etc/iu.ap (see in the appended FAQ entry) and do
a reconfigure boot.
This is what the myth says:
set pt_cnt=<number desired> RIGHT
set npty=<same number as pt_cnt> WRONG
set nautopush=<same number as pt_cnt> WRONG
set sadcnt=<2 * pt_cnt number> WRONG
And here's why the WRONG are wrong:
- there is no relation between npty and pt_cnt.
pt_cnt sets the number of System V ptys; these are the standard
pseudo terminals used by rlogind/telnetd/xterm/dtterm and
public domain programs.
There is no system imposed maximum for pt_cnt; > 3000 can
certainly be used
npty is used for BSD style ptys; code that uses them cannot use
more than 256 ptys; the system imposes a limit of 176.
You can set npty > 176, but you won't be able to use the
extra ptys, you'll just pin down and waste some physical memory.
- there is no need to set nautopush
nautopush is a variable that sizes a table of "autopushable"
major device numbers. Basically, it's the maximum size of
/etc/iu.ap.
It need not be increased when minor numbers of a device are
increased.
System V ptys are not autopush devices
- there's no need to set sadcnt
(Sun documentation to the contrary not withstanding)
Sadcnt limits the number of simultaneous opens to
/dev/sad/{user,admin}
No part of the system ever keeps a /dev/sad device open for
an extended period of time: typically, an open is done, a ioctl
is done and the device is closed.
Ergo, there is *never* a need to increase sadcnt
- final argument:
The default system parameters disagree with the proposed
settings:
nautopush = 32; pt_cnt = 48; sadcnt = 16
I'm at a loss to explain how these myths came into existance; at no
point in time that I can trace was setting nautopush or sadcnt necessary.
I am trying to correct the official Sun documentation; if you still find
some, please let me know.
(Yes, I know about the 2.7 and earlier answerbooks; they even mention
"sad_cnt"; there is no such parameter)
Please leave sadcnt and nautopush alone.
- text/plain attachment: PseudoTTY-FAQ
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