Re: ZFS committed to the FreeBSD base.

On Thu, Apr 12, 2007 at 01:51:59PM -0500, Craig Boston wrote:
> 
> For something this low level my opinion is it's better to stay with
> compile time options.  After all, in the above example, cmpxchg8 is a
> single machine instruction.  How much overhead does it add to retrieve a
> variable from memory and check it, then jump to the correct place?
> Enough that it outweighs the benefit of using that instruction in the
> first place?

That's why I suggested the second method (to change fn pointers in the
device struct).

> I agree this makes sense for some things, but atomic operations are
> supposed to be as fast as possible -- preferably single machine
> instructions I can't think of anything short of JIT compiling the kernel
> that wouldn't be a high price to pay.

The problem is that ZFS would be compiled (by default) to work for many
platforms, and thus a majority of systems wouldn't get the nice
optimization.  That's why I think we should do something along the lines of
doing a check for CX8 and changing the pointers in the vfsops and
vop_vector static structures, depending upon the availability of this
optimization.

I guess it really depends upon how much ZFS uses it;  I got the sense that
it is "often".

-- Rick C. Petty