On Wed, 14 Feb 2007, Benjamin LaHaise wrote:
> On Wed, Feb 14, 2007 at 11:45:23AM -0800, Davide Libenzi wrote:
> > Sort of, except that the whole thing can complete syncronously w/out
> > context switches. The real point of the whole fibrils/syslets solution is
> > that kind of optimization. The solution is as good as it is now, for
>
> Except that You Can't Do That (tm). Try to predict beforehand if the code
> path being followed will touch the FPU or SSE state, and you can't. There is
> no way to avoid the context switch overhead, as you have to preserve things
> so that whatever state is being returned to the user is as it was. Unless
> you plan on resetting the state beforehand, but then you have to call into
> arch specific code that ends up with a comparable overhead to the context
> switch.
I think you may have mis-interpreted my words. *When* a schedule would
block a synco execution try, then you do have a context switch. Noone
argue that, and the code is clear. The sys_async_exec thread will block,
and a newly woke up thread will re-emerge to sys_async_exec with a NULL
returned to userspace. But in a "cachehit" case (no schedule happens
during the syscall/*let execution), there is no context switch at all.
That is the whole point of the optimization.
- Davide
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