News Group Posts

lock process to specific cpu; lockpid

From: Bela Lubkin <belal@sco.com>
Subject: Re: how to force running on a selected cpu on smp machine
Date: Tue, 19 Nov 2002 15:18:32 GMT
References: <3DD517A1.E7641423@oce.de>
<20021115103759.H3983@mammoth.ca.sco.com>
<3DD69630.6000601@t-online.de>
<20021116111309.M3983@mammoth.ca.sco.com>
<20021116113134.N3983@mammoth.ca.sco.com>
<3DDA0188.56DAE28@oce.de> 

Carsten Hammer wrote:

> For a shell script solution:
> 
> Has anybody ever used lockpid to *start* a prozess on a certain cpu? It should be
> possible using a tricky wrapper script around it by delaying execution of  a
> forked process and using lockpid in the meantime, isnt it?

It looks like you don't even have to be very tricky:

  # sh -c 'lockpid -l 2 $$; while :; do :; done' &
  # sh -c 'lockpid -l 2 $$; while :; do :; done' &
  # mpstat

You'll see that CPU 2 is 100% busy, CPU 1 isn't.  In each case, '$$' is
the PID of the respective new process.  The "while" is being run in the
same process that got locked.  Next:

  # sh -c 'lockpid -l 2 $$; awk "BEGIN { while(1); }"'&
  # sh -c 'lockpid -l 2 $$; awk "BEGIN { while(1); }"'&
  # mpstat

Two CPUs will be lit up.  `awk` is a subprocess of the locked process,
and locking doesn't carry across to children.

Finally,

  # sh -c 'lockpid -l 2 $$; exec awk "BEGIN { while(1); }"'&
  # sh -c 'lockpid -l 2 $$; exec awk "BEGIN { while(1); }"'&
  # mpstat

Now `awk` is being exec'd within the same process that was locked.
exec(S) does not clear CPU locks.

>Bela<

PS: I was testing this on a 4-CPU box, so I always had spare CPUs to use
for killing these runaways...  Except once when I mistakenly locked my
shell onto the same CPU I was abusing.  Then things got pretty difficult
for a while.  My mistake:

  # (lockpid -l 2 $$; while :; do :; done) &

I was using ksh -- not sure if sh would do the same -- '$$' here was the
PID of the parent shell, so I got locked onto CPU 2.  Then I ran a
correct statement, locking a runaway onto CPU 2 ... and shell
interaction got really slow.  It might be good to use `nice` somewhere
in the equation to make the runaways back off.  But I always get
confused about the syntax of `nice`...
 

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