Date: Wed, 16 Nov 2005 13:07:26 -0800
From: Paul Jackson <pj@sgi.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: mrmacman_g4@mac.com, serue@us.ibm.com, linux-kernel@vger.kernel.org,
       frankeh@watson.ibm.com, haveblue@us.ibm.com
Subject: Re: [RFC] [PATCH 00/13] Introduce task_pid api
Message-Id: <20051116130726.2412fbc9.pj@sgi.com>
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> Could we switch
> to 128-bits so that pids are never reused or something like that?

Not easily.  We've got a very cool pid-dispenser at this point that
has excellent performance and scalability, but requires a bit map,
one bit per potential pid.  That bitmap can't exceed a small percentage
of main memory on most any configuration, constraining us to perhaps
20 to 30 bits.  The code currently has a 22 bit arbitrary limit.
Something like 30 bits would usually only make sense on the terabyte
NUMA monster boxes.

128-bit UUID technology scales fine, but adds quite a few compute
cycles per allocation, and would blow out a whole lot of user code
expecting to put a pid in a machine word.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401
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