PAE Slowdown

[Scott Long]

Jeff Kramer wrote:
> At 6:56 PM +0200 10/8/07, Ivan Voras wrote:
>> Jeff Kramer wrote:
>>>  Hey all,
>>>
>>>  I know that AMD64's the preferred way to run >4 gig systems, but I'm
>>>  having a weird situation with 6.2-RELEASE-p8 and 6-STABLE as of last
>>>  night.  When I compile the PAE kernel, my system performance drops like
>>>  a rock.  It still boots and everything still runs, but for instance,
>>>  running the Flops port my megaflops drop from the 950 MFLOPS range to 4
>>>  MFLOPS.  It feels about as fast as a 486.
>>
>> Does vmstat -i show unusually high interrupt rates?
> 
> When it's running ok at idle (4 gig of ram):
> 
> interrupt                          total       rate
> irq1: atkbd0                          77          0
> irq16: twa0                         1084          3
> irq17: atapci0                         1          0
> irq19: fwohci0++                       3          0
> irq20: em0                           161          0
> cpu0: timer                       549165       1920
> Total                             550491       1924
> 
> When it's slow at idle (8 gig of ram):
> 
> interrupt                          total       rate
> irq1: atkbd0                          48          0
> irq16: twa0                         1093          8
> irq17: atapci0                         1          0
> irq19: fwohci0++                       3          0
> irq20: em0                           179          1
> cpu0: timer                       241862       1950
> Total                             243186       1961
> 
> 
> 

The culprit could be the twa driver.  Are you really generating that
much I/O?

Scott