Date: Thu, 19 Jul 2001 00:52:05 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: John Baldwin <jhb@FreeBSD.org> Cc: hackers@FreeBSD.org, Matthew Jacob <mjacob@feral.com> Subject: Re: x86 unaligned access followup. Message-ID: <3B5691A5.66486B63@mindspring.com> References: <XFMail.010718112506.jhb@FreeBSD.org>
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John Baldwin wrote: > > Also note that this will play hell with some of the recent > > copy avoidance changes made by Bill Paul to the ethernet > > drivers, to avoid the expense of copying the packet, with > > the knowledge that there would be an increased overhead in > > the resulting packet field unaligned accesses when decoding > > IP packets... [ ... ] > I didn't say it was a good thing, I was just saying how it was done. :) > From Bill's description of the network stuff, it sounds like the unaligned > access on x86 is cheaper than the copy that would otherwise be done, so we > don't want AC checks in that case anyways. I don't know if I entriely believe that... it depends on how big the average packets are. For a web server, for example, there is the initial request, and then there's ACK's, so the copy overhead is negligible, and might outweigh the unaligned access overhead, particularly if you could delay the copy until it was abosolutely necessary (e.g. avoid copying the 14 byte ethernet header, etc., and do the copy in ip_input() instead). -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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