On Tue, Sep 17, 2002 at 02:42:20PM -0400, justinca@cs.cmu.edu wrote: > > > > This requires each CPU to remember the last owner of FPU. > > In order to support possible process migration cases in a SMP > > system, each process also needs to remember the processor > > on which it used FPU last. A process has a valid live FPU > > context on a CPU if those two variables match to each other. > > Therefore we can avoid unnecessary restoring FPU context. > > > > Fairly complex in implementation. > > > > I'd argue for something between 2 & 3. Always save FPU state, and if > you know the state has been preserved for the next run, skip the > restore. > Determining whether the current FPU context is valid for the new process is not easy. It requires last_task_used_math like variable for each CPU. > I'm a bit leery of the whole "don't restore FPU state on context switch > until you use the FPU again" idea as it's added complexity Quite easy to implement. Just turn off ST0_CU1 bit in the status register saved in the kernel stack when a process is switched off. Therefore next use of FPU will cause a trap and do_cpu() does the normal thing. > and I'm not > at all sure you're going to see any measurable performance gain out of > it. I think this gives a big performance improvement because most processes don't use FPU during their runs but they all have used_math flag set! Jun
