Subject: Re: PMC strawman
To: Allen Briggs <firstname.lastname@example.org>
From: Matt Thomas <email@example.com>
Date: 02/14/2003 19:42:44
At 06:46 PM 2/14/2003, Allen Briggs wrote:
>On Fri, Feb 14, 2003 at 10:51:52AM -0800, Matt Thomas wrote:
> > Note that to use perfctrs with individual pthreads we need
> > to add a PMC glob to struct mcontext. Does anyone have a
> > problem with that?
>You mean a glob to treat the PMC registers as part of the context?
>If it's only conditionally restored, that seems reasonable to me.
>It's what? Another 44 bytes of mcontext overhead? It might be
>nice to perhaps have a way to avoid that for production system
>code, though. Maybe a compile-time knob?
There would be a bit to note whether that blob has info. I'd
make the blob explicitly opaque to userspace.
> > But what about the kernel? I assume that doing performance
> > of user processes and the kernel concurrently will be desired.
> > If so, then on user traps and execptions, the user PM context
> > will need to be saved in the trapframe and then the kernel's
> > loaded from cpu_info (so it's per-cpu).
>This is something that I kind of ignored with the XScale
>implementation. It would certainly be useful to have,
>though I would certainly want to avoid the overhead unless
>PERFCTRS is enabled.
Of course it would conditional. I'd also prefer to have
separate contents for interrupt / non-interrupt code.
> > Though the current PMC framework doesn't allow for it, it
> > might be useful for multiple processes/threads to be able to
> > share one PMC context.
>What do you think this should look like?
Right now the pmc counters are per proc, not lwp, which needs to be
fixed. I was thinking about creating a pmc context that a process
could establish and share among it's children (or all of its threads).
Make it refcnt'ed and on last reference, free it.
Then l_md.md_pmc would be a pointer to it. To access it will be
fairly cheap, the adding of PMCs is not:
Note that it can't be in the PCB since PCB's are not shared.
Matt Thomas Internet: firstname.lastname@example.org
3am Software Foundry WWW URL: http://www.3am-software.com/bio/matt/
Cupertino, CA Disclaimer: I avow all knowledge of this message