ps acting weird?

[Danny Braniss]

> 
> On Tue, 20 Jan 2009, Danny Braniss wrote:
> 
> > I just stumbled on this, ps(1) gives different info if the user is root or a 
> > simple mortal:
> >
> > simple-mortal> ps p8130
> >  PID  TT  STAT      TIME COMMAND
> > 8130  ??  Is     0:05.72 [java]
> >
> > root# ps p8130
> >
> >  PID  TT  STAT      TIME COMMAND
> > 8130  ??  Is     0:05.73 /usr/local/diablo-jdk1.6.0/bin/java ...
> >
> > was this always like this?
> > btw, this is on 7.1-stable
> 
> This happens when command lines are really long -- the kernel caches certain 
> command line data when it's short (i.e., under a couple of hundred 
> characters), but when it's long the only way to get it is to attach to the 
> process's address space using debugging interfaces and read it from there. 
> This requires ps(1) to have debugging rights for the target process, which 
> will not always be true for simple mortal users, but will often be true for 
> root.
> 
> You can set the kern.ps_arg_cache_limit sysctl to increase the limit, which 
> will take effect when the command line is next changed (typically, when the 
> program is run again, but there are some programs that update their command 
> lines to show status information, in which case it will be when they next 
> update).  This shows up particularly for Java command lines, which are often 
> long.
> 
> I would be careful not to tune it up too high as it will use up kernel 
> memory/address space, but setting it to, say, 4k or 8k on modern systems 
> shouldn't really be a problem, especially since most commands don't have long 
> command lines.  The problem this limit addresses is what happens when maxproc 
> processes all set maximally-sized command lines.  I.e., if your maxproc is 
> 6,000, then fully filling the command line cache gives you 1.5MB of command 
> line in kernel address space and memory - a lot, but very little compared to 
> making the limit 4000 bytes, in which case it's more around 24MB.
> 
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
> 

thanks Robert, it is always educational to read your answers!

i have set kern.ps_arg_cache_limit, as you suggested to 4k, but i think
512 would have been enough (excluding those pathological cases of 'command *')
btw, this tomcat/java command line was 312 (why not use a config file!).

which brings on another issue:

ps -o command= -p 777
/usr/local/diablo-jdk1.6.0/bin/java -Djava.endorsed.dirs= -classpath [...]
while
ps -o comm= -p 777
java

thanks,
	danny