Re: [9fans] gar nix!

[erik quanstrom <quanstro@quanstro.net>]

hey, ron.

On Fri Sep 16 01:57:04 EDT 2011, rminnich@gmail.com wrote:
> for the 2M pages -- I'm willing to see some measurement but let's get
> the #s -- I've done some simple measurements and it's not the hit one
> would expect. These new machines have about 10 GB/s bandwidth (well,
> the ones we are targeting do) and that translates to sub-millisecond
> times to zero a 2M page.   Further, the text page is in the image cache.
> So after first exec of a program, the only text issue is locating the
> page. It's not simply a case of having to write 6M each time you exec.

however, neither the stack nor the heap are.  that's 4MB that need to be
cleared.  that sounds like an operation that could take on the order of
ms, and well-worth measuring.

maybe it might make sense to use 4k pages for stack, and sometime for
the heap.

> I note that starting a proc, allocating and zeroing 2 GiB, takes
> *less* time with 2M pages than 4K pages -- this was measured in May
> when we still were supporting 4K pages -- the page faults are far more

i'll try to get some numbers soon, but i think a higher priority is to get
a smp setup.  is anyone testing with smp right now?

are you sure this isn't the difference between throughput and latency?
did you try a small-executable test like
	for(i in `{seq 1 1000000})dd -if /dev/zero -of /dev/null -quiet 1
?  now that the code has been removed, it's a little harder to replicate your
numbers.

> expensive than the time to write the memory. Again, YMMV, esp. on an
> Atom, but the cost of taking (say) 6 page faults for a 24k text
> segment that's already in memory may not be what you want.
>
> There are plenty of games to be played to reduce the cost of zero
> filled pages but at least from what I measured the 2M pages are not a
> real hit.

i'm okay with the atom suffering a little bit (odd how far down the food
chain one can get 64 bits!), i'm actually more concerned about
being punshed severely for forking on a beefy but busy machine.
the atom is just my test mule.

(can't one just preemptively map the whole text on first-fault?)

- erik

p.s.  i guess the claim i thought i saw that you need 1gb pages
isn't correct.  that's good, but i need to track down why i don't see 'em on the atom.