instrumenting cache followup and request for info

[Chuq]

I want to thank the people who took time to respond to my inquiry about
instrumenting caches. As it turns out, there doesn't seem to be any way to
directly monitor cache activity without using a number of logic analyzer
tied directly to the bus. One way of indirectly monitoring it is to turn
the cache on and off during timesharing, which can be done (on the vax, at
least) in console mode by poking at appropriate places (see the hardware
manual for the right places. Note-- if you can't figure it out from the
hardware manual you probably don't want to try it).

I've been delving into the innards of the kernel with vmstat and
gprof/kgmon trying to track down where all of our CPU cycles have been
going and have found something that looks interesting. Our 4.2 780 is still
running DZ's using the dma simulation code. We are getting an enormous
number of interrupts generated (with 8 DZ boards, who wouldn't???). Each
interrupt seems to be dropping through the Swtch process scheduler routine,
which sets up a new process for the CPU to play with. Needless to say, we
are spending a LOT of time scheduling processes. I've timed some I/O
intensive routines on our 780 and our 750 (which does have DH's) and found
that the amount of time in Switch is about 30% for the 780 and about 2.5%
for the 750. Non-trivial, to say the least. We were under the impression
that the dma simulation code kept the system from dropping through the
scheduler, but this doesn't seem to be the case. Has someone else found a
way to keep this enormous DZ overhead from crippling the system? From all
indications the best we can do is about a 765. I've got people looking into
buying DH's (of course...) but I was wondering if someone had a software
fix out there. Has anyone else out there looked into this?

chuq

s
-- 
>From the depths of the Crystal Cavern:		Chuq Von Rospach
{amd,decwrl,fortune,hplabs,ihnp4}!nsc!chuqui	nsc!chuqui at decwrl.ARPA

Dreams, dreams, enchanter! Gone with the harp's echo when the strings fall
mute; with the flame's shadow when the fire dies. Be still, and listen.