svn commit: r187701 - user/alc/zerocopy/sys/vm

[Alan Cox]

Jeff Roberson wrote:
> On Mon, 26 Jan 2009, Alan Cox wrote:
>
>> Author: alc
>> Date: Mon Jan 26 00:52:09 2009
>> New Revision: 187701
>> URL: http://svn.freebsd.org/changeset/base/187701
>>
>> Log:
>>  Retire the page-based copy-on-write mechanism.
> Hey if we're doing this there are some mbuf fields I could axe too. :-)
>
> This isn't productive for anyone anyway eh?  I've done page-flipping 
> to kernel for nfs and custom protocols but to userland it just doesn't 
> make sense without an api to support it.

It's great for netperf.  :-)  More seriously, it works well for repeated 
transmission of the same immutable buffer.  Otherwise, the overheads 
overwhelm the savings.  For example, after transmission is complete, it 
requires a page fault on each page of the buffer to re-enable write 
access to the buffer.  Need I say more ...

That said, the real motivation for this work is that the page-based 
copy-on-write mechanism is still buggy.  Specifically, it doesn't play 
nice with the buffer cache.  I just don't see the point in investing any 
further effort in fixing it.

Alan