Re: Inter-process send()/recv() using zero-copy ?

[Arjan van de Ven]

On Wed, 23 Sep 2009 08:01:27 +0200
Xavier Roche <roche+kml2@xxxxxxxxxxx> wrote:

> Hi folks,
> 
> I was wondering if there was a way to have zero-copy send()/recv(),
> when the socket is connected to the local machine (to another process
> on the same machine, for example) ?
> 
> Such feature would be only feasible with page-aligned blocks, from an
> a mmap'ed block to another one, I guess.
> 
> Typical case:
> 
> Process #1 (uid A)
> buff = mmap(0, size, ..) /* anonymous or not */
> ...
> send(s, buff, size, 0)
> munmap(buff, size)
> 
> Process #2 (uid B)
> buff = mmap(0, size, .. | MAP_ANONYMOUS, ..)
> recv(s, buff, size, 0)
> 
> In an ideal fantasy world, the first process would use send() to 
> transmit the complete page-aligned memory block to the other side,
> and the second process would use recv() to get the memory block on a
> similar anonymously mmap'ed block, and the only operation the kernel
> would do would be to share the memory block between the two processes
> with copy-on-write.
> 
> On the real world, the same operation requires a first read of the
> whole memory block (possibly partially on disk) and a complete write
> (possibly partially on disk, too) with two copies of the same memory
> region at the end.
> 
> Two solutions can be used to 

the problem you have is that
1) memory copies are cheap
   (say, 3000 cycles/page or less)
2) page table operations (mmap etc) are very expensive.

these two combined tend to not make it a win to substitute simple
copies with complex pagetable tricks.

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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