Date: Fri, 10 Dec 2004 12:16:36 +0100 From: Andre Oppermann <andre@freebsd.org> To: Simon 'corecode' Schubert <corecode@fs.ei.tum.de> Cc: Sam Leffler <sam@errno.com> Subject: Re: cvs commit: src/sys/netinet ip_fw_pfil.c Message-ID: <41B98594.A2FBC9E@freebsd.org> References: <200412091641.iB9GflnD067866@repoman.freebsd.org> <41B8BF81.2000701@errno.com> <41B8EA66.4875E2D9@freebsd.org> <200412101103.27119.corecode@fs.ei.tum.de>
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Simon 'corecode' Schubert wrote: > > On Friday, 10. December 2004 01:14, Andre Oppermann wrote: > > > >>> Check that DUMMYNET_LOADED before seeking dummynet m_tag. > > > >>This change may be a mistake unless you can prove some significant > > > >>performance gain. > > > > Checking for DUMMYNET_LOADED is a simple pointer compare to NULL and > > > > doesn't add a memory write. Not a big difference for sure but not > > > > hurting either. > > > If tag lookup cost becomes an important consideration in writing code > > > then we need to address that basic functionality. > > Actually this is a good argument and reasoning and I buy into it. Not > > that is matter this much in this case but having nice and clean code > > wins big over time. I've spent and am still spending too much time > > cleaning up old BSD PDP-11 "optimizations" and other shortcuts. > > Forgive me if I'm way off here, I not experienced with the network stack as > you are. > > First of all I also think that this check here might be > over(micro)optimization. Nevertheless, if tags appear to be a (small) > performance hit, I conclude that there should be a fast way to check for the > (possible?) existance of a tag. So for example having a bit field signaling > the presence of the most important tags which are checked in the hot code > path might help, so is this really stupid[tm]? Please have a close look at sys/sys/mbuf.h; that should answer all outstanding questions for this case. -- Andre
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