On Tue, Apr 29, 2003 at 02:33:36PM -0500, Robin Holt wrote:

On Tue, Apr 29, 2003 at 09:20:41PM +0200, Andi Kleen wrote:

quoted

A much better way would be to use the POSIX aio interfaces. They support
zero copy transmit, but don't require COW. Instead they just tell
the user process when it is safe to touch the buffer again.

There was already some code to do aio TCP sending, but it didn't
do zero copy and was not merged for some reason.

Also you can already do zero copy transmit using sendfile() 

Users would need to rewrite all their apps to use either the async or
sendfile method.  That assumption seems a little broad.

In my experience only a few programs are performance critical in this 
way; and their developers/users usually do not mind changing their programs 
a bit to get the best performance. In fact they are always happy when
they get such knobs from you ;)

     
I don't disagree that implementing the remainder of the AIO system
calls would also be good, but is there something wrong with getting
write et. al. to work with zero copy?

You have to ask DaveM/Alexey - they had it, but rejected it, apparently
also based on some bad experiences on other operating systems.

I can see their point - e.g. in the worst case each write could
trigger two TLB flush IPIs to all CPUs in the system (one to COW it 
and another to un COW it). You can copy a lot of data in the time
it takes to process all of them, especially on a big machine.

-Andi