Giant lock, bce and uhc using the same irq

[Kris Kennaway]

On Mon, Nov 17, 2008 at 05:14:21PM +0200, Murat Balaban wrote:
> Hello hackers,
> 
> In one of my production servers (64-bit Intel Xeon machine) running
> 
> 6.3-RELEASE-p4 (amd64) FreeBSD 6.3-RELEASE-p4 #0: Fri Sep 12 17:07:19
> EEST 2008
> 
> I see this "top -S" output excerpt:
> 
> 32 root          1 -68 -187     0K    16K *Giant 0  48.3H  3.08% irq17:
> bce1 uhci1
> 
> What I get from above is that bce and uhc drivers both use irq17. I
> assume
> bce is SMPng'ed, and uhc is not.
> 
> Does uhc being Giant locked, affect bce performance, because they use
> the same
> interrupt handler thread?

bce will not need to acquire Giant at all so they will not fight for
the lock.  However both of them will have to wake up to check each
interrupt so there is some time spent there.  Also if you have some
other workload on the system that is still heavily dependent onGiant
(e.g. MSDOSFS, etc) then that will interfere with uhci while
interrupts are coming in.

Kris

P.S. In 8.0 there is a new USB stack that is Giant-free.

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