Re: Sleazy FPU optimization

[David Laight <david%l8s.co.uk@localhost>]

On Mon, Feb 11, 2008 at 07:43:09PM +0100, Joerg Sonnenberger wrote:
> On Mon, Feb 11, 2008 at 05:27:16PM +0000, Matthias Scheler wrote:
> > Linux 2.6.20 and above has an interesting optimization for lazy FPU
> > context saving:
> 
> We've been discussing whether or not it is worth to do lazy FPU context
> saving at all. Most programs will use the FPU on i386 anyway, e.g. if
> printf is used. I'm not sure it will actually help.

1) Why would using printf for integers need the fpu

2) a fair number of context switches don't end up in userspace (or
   only return there for a veru short time) doing a full FP save/restore
   on these is likely to be expensive.

3) I suspect that some programs (like make) hardly ever use the FPU at all,
   but will do a lot of context switches.

        David

-- 
David Laight: david%l8s.co.uk@localhost