em interrupt storm

[Kris Kennaway]

On Tue, Nov 22, 2005 at 08:54:49PM -0800, John Polstra wrote:
> 
> On 23-Nov-2005 Kris Kennaway wrote:
> > I am seeing the em driver undergoing an interrupt storm whenever the
> > amr driver receives interrupts.  In this case I was running newfs on
> > the amr array and em0 was not in use:
> > 
> >    28 root        1 -68 -187     0K     8K CPU1   1   0:32 53.98% irq16: em0
> >    36 root        1 -64 -183     0K     8K RUN    1   0:37 27.75% irq24: amr0
> > 
> ># vmstat -i
> > interrupt                          total       rate
> > irq1: atkbd0                           2          0
> > irq4: sio0                           199          1
> > irq6: fdc0                            32          0
> > irq13: npx0                            1          0
> > irq14: ata0                           47          0
> > irq15: ata1                          931          5
> > irq16: em0                       6321801      37187
> > irq24: amr0                        28023        164
> > cpu0: timer                       337533       1985
> > cpu1: timer                       337285       1984
> > Total                            7025854      41328
> > 
> > When newfs finished (i.e. amr was idle), em0 stopped storming.
> > 
> > MPTable: <INTEL    SE7520BD22  >
> 
> This is the dreaded interrupt aliasing problem that several of us have
> experienced with this chipset.  High-numbered interrupts alias down to
> interrupts in the range 16..19 (or maybe 16..23), a multiple of 8 less
> than the original interupt.
> 
> Nobody knows what causes it, and nobody knows how to fix it.

This would be good to document somewhere so that people don't either
accidentally buy this hardware, or know what to expect when they run
it.

Kris

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