"BGL", Giant and kernel lockups

Greg 'groggy' Lehey grog at FreeBSD.org
Sat Oct 1 18:17:23 PDT 2005

On Saturday,  1 October 2005 at  4:11:25 -0400, Kris Kennaway wrote:
> On Fri, Sep 30, 2005 at 11:58:14PM -0400, Ansar Mohammed wrote:
>> I love FreeBSD. I have been using it since 2.x. I have never had any
>> problems with it. Rock Solid. Never a single kernel panic.
>> Then I come across this article in a comparison between Linux and
>> FreeBSD saying that FreeBSD has kernel locking
>> issues. Specifically, a problem nicknamed BGL or Big Giant Lock. It
>> seems that it affects SMP systems under high load.
> Sounds like a pretty simplistic summary, but as I mentioned it's out
> of date anyway.

I don't think that answers his question.

I think that most people (with the exception of Erik Trulsson) have
missed the fact that Ansar has a misconception about the meaning of
"BGL".  This is not a "problem", and it has nothing to do with kernel
stability (lockups); it has to do with the way the kernel protects its
data integrity in normal operation.

All kernels perform locking for this purpose.  Older UNIX (including
BSD) and Linux systems used a thing called the "Big Kernel Lock" or
Giant to ensure that only one process, system wide, had access to the
entire kernel at a time.  This was a performance issue, not a
stability issue.  All modern systems have relaxed this requirement.

You may like to look at my (now somewhat out of date) papers on
FreeBSD SMPNg, available at http://www.lemis.com/grog/Papers/.  There
should be more up-to-date information on the current state of the
FreeBSD implementation.

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