SUMMARY: 2 CPUS better than 4?

From: Doug Rinckes (
Date: Wed Dec 16 1992 - 14:04:04 CST

I requested the circumstances in which having 4 CPUs in an MP system actually
degraded performance.

Thanks for all the answers, everyone, and here is the generally accepted one:

SunOS 4.1.2 and 4.1.3 limit the system to only one processor running the kernel
at any one time. On I/O bound systems, other processors need to grab the kernel.
Under extreme circumstances, the time spent by the processors checking for the
availability of the kernel can impact on performance. In this case, the more
processors you have, the more time they spend checking for the lock on the kernel
and transferring processes.

Suggested conditions where this may occur are:
        Heavily loaded databases
        Heavily loaded NFS server

There are patches to improve the situation, and it is fixed properly in Solaris 2.x.

Credit is due to: (Rand S. Huntzinger)
        aloft!bill (B. Shorter) (Olivier MARCE) (Lance Eric Greenwade)
        msf@fsg.COM (Mike Fischbein)
        Kerien Fitzpatrick <>
        Steve Lodin <>
        khb@Eng.Sun.COM (Keith Bierman fpgoup)

Doug Rinckes, Network Administrator |
  Telecommunications Systems Support Centre New Zealand Ltd
        Sitting in New Zealand, mailing thru Japan

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