Re: NetBSD 5.1 TCP performance issue (lots of ACK)

[David Young <dyoung%pobox.com@localhost>]

On Wed, Nov 02, 2011 at 08:04:40PM +0100, Manuel Bouyer wrote:
> On Sat, Oct 29, 2011 at 10:14:18PM +0200, Martin Husemann wrote:
> > On Sat, Oct 29, 2011 at 10:02:10PM +0200, Manuel Bouyer wrote:
> > > OK, I see. But, is there a platoform where BUS_DMASYNC_PREREAD is not
> > > a NOP ? I can't what kind of work BUS_DMASYNC_PREREAD could have to do ...
> > 
> > On platforms with memory reordering it might be a barrier (i.e. sparc64 does
> > a membar_sync() for it).
> 
> So does x86. But I think it's redundant with the POSTREAD (which is also
> a barrier) we'll do later.

It looks to me, too, like the x86 implementation will make a redundant
x86_lfence() call.

I think it is preferable for the bus_dma implementation to try to avoid
redundant operations than for MI drivers to try to do so.  What do you
think?

Dave

-- 
David Young             OJC Technologies is now Pixo
dyoung%pixotech.com@localhost     Urbana, IL   (217) 344-0444 x24