On Sun, Oct 22, 2006 at 02:56:29PM -0700, Jason Thorpe wrote:
> 
> On Oct 22, 2006, at 12:41 PM, Thor Lancelot Simon wrote:
> 
> >What do you think is going on?
> 
> I'm not sure yet.  Are you absolutely sure it's a problem with  
> servicing the interrupt on time?

Well, if I run systat vmstat (with interval 1) while this is going on,
when LFS starts to whack the disk, I see the usual 2000-3000 network
interrupts per second fall off to near zero until the writes (and
disk controller) drop back to zero. 

So, while LFS is writing, I will see 10, sometimes 30, very occasionally
as many as 300 interrupts per second on the network controller's interrupt
line, right as TCP backs off and throughput goes to hell.  When LFS isn't
writing, I see, as I said, 2000-300 network interrupts per second -- or
as many as 7,000, if I turn interrupt moderation down.

So the reasonable inference, to me, really seems to be that LFS, when it
writes flat-out for 5 or 10 seconds at a time, is causing the network
interrupts to not be serviced, which is what's causing TCP to back off.

-- 
  Thor Lancelot Simon	                                     tls@rek.tjls.com

  "We cannot usually in social life pursue a single value or a single moral
   aim, untroubled by the need to compromise with others."      - H.L.A. Hart