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Date:      Sun, 17 Feb 2002 17:20:57 -0800
From:      Terry Lambert <tlambert2@mindspring.com>
To:        David Greenman <dg@root.com>
Cc:        Roy Sigurd Karlsbakk <roy@karlsbakk.net>, Dag-Erling Smorgrav <des@ofug.org>, Thomas Hurst <tom.hurst@clara.net>, hiten@uk.FreeBSD.org, hackers@FreeBSD.ORG, freebsd-questions@FreeBSD.ORG
Subject:   Re: in-kernel HTTP Server for FreeBSD?
Message-ID:  <3C7056F9.A9F37535@mindspring.com>
References:  <Pine.LNX.4.30.0202171935060.6486-100000@mustard.heime.net> <3C703A92.2EBD3E67@mindspring.com> <20020217170929.D80718@nexus.root.com>

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David Greenman wrote:
> >"Zero copy" usually means "zero unnecessary copies"; but
> >what someone thinks of as "necessary" is really based on
> >their bias towards an existing implementation.
> 
>    "zero copy" these days has come to mean no copies that involve
> the CPU. In my experiance, raw memory bandwidth to DMA packets
> to/from main memory is not the bottleneck on modern hardware.

I'll agree with your experience.  At this point, the limiting
factor is PCI bandwith, at least for general purpose hardware.

The "AGP approach", where the main memory *is* the NIC
memory, gets around this limitation, but doesn't kick the
CPU out of the picture, even then.

In practice, I've found that it is sometimes better to let
the CPU do the copying, rather than the DMA engine on the
device, since it's often faster at it.  This assumes that
the CPU is not otherwise engaged (e.g. doing cryptography),
and there's enough spare cycles around the CPU can do the
job faster.

Moore's law and Intel really screw with the business model
required to build special purpose network processors, don't
they?  8-) 8-).

-- Terry

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