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From: John Baldwin <jhb@freebsd.org>
To: freebsd-arch@freebsd.org
Subject: Re: mbuf and uio
Date: Fri, 7 Feb 2014 11:23:38 -0500
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 (bigwig.baldwin.cx); Fri, 07 Feb 2014 11:32:31 -0500 (EST)
Cc: Alfred Perlstein <bright@mu.org>
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On Friday, February 07, 2014 11:08:33 am Alfred Perlstein wrote:
> On 2/7/14 4:51 AM, Luca Ferrari wrote:
> > Hi all,
> > I'm just wondering why mbufs seems to be much more important from an
> > administrator point of view than uio. I mean, both structures are used
> > to move data thru a stack (network or i/o), but the mbufs get
> > accounted by serveral tools (like netstat and so on) while uio does
> > not.
> > Am I totally wrong on this?
> uios are transient structures that can be allocated on the stack at any 
> time.  In general you'll have at most 1 uio per process/thread active.
> 
> tracking them would not really gain us much.

The other thing is that uios do not allocate their own storage for
I/O buffers.  They merely serve as metadata so that data can be copied between 
userland buffers and other buffers in the kernel (such as the VM pages that 
back a file).   The actual data used for I/O does not belong to the uio.  
mbufs, on the other hand, store packet data.

-- 
John Baldwin