Re: [PATCH] turn off irqdebug by default on Altix

[Jack Steiner <steiner@sgi.com>]

On Wed, Feb 04, 2004 at 02:08:05PM -0500, Martin Hicks wrote:
> 
> 
> On Wed, Feb 04, 2004 at 11:04:43AM -0800, Chen, Kenneth W wrote:
> > This is really confusing with two parameters that do the same thing.
> > What will happen if someone pass noirqdebug and irqdebug at the same
> > time?
> > 
> > What's wrong with boot time parameter of noirqdebug?
> 
> It just means that if you forget to pass this option then your machine
> never boots.  Well, it will boot.  Eventually.  Jack is a patient man,
> and he said he never waited long enough for it to boot.

Actually, I determined that a big system would never boot. Each cpu would have to
do about 1500usec of work every 1000usec.  (highly dependent on the
system interconnect).

Each clock tick, the desc->irq_count is incremented by each cpu. On a big
system, this creates an incredibly hot cache line. On a big system, it takes
more than a millisecond to do the update. At that point, another clock
interrupt is already pending. Forward progress stops.

Another possible approach might be to special case note_interrupt() to
skip the update for the timer tick. (I havent actually looked at the code, but
this should be possible). 


> 
> mh
> 
> -- 
> Martin Hicks || mort@bork.org || PGP/GnuPG: 0x4C7F2BEE

-- 
Thanks

Jack Steiner (steiner@sgi.com)          651-683-5302
Principal Engineer                      SGI - Silicon Graphics, Inc.