Date: Sun, 16 Nov 2003 23:14:23 -0500 From: Michael Edenfield <kutulu@kutulu.org> To: Harald Schmalzbauer <h@schmalzbauer.de> Cc: current@freebsd.org Subject: Re: General debug/kernel question Message-ID: <20031117041423.GA15294@wombat.localnet> In-Reply-To: <200311170243.18436@harrymail> References: <200311170243.18436@harrymail>
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[-- Attachment #1 --] * Harald Schmalzbauer <h@schmalzbauer.de> [031116 20:43]: Content-Description: signed data > Salve, > > I always thought that building a kernel with debug symbols would increase the > kernel size dramatically. But if I understand things right the additioal > "symbols" (code snippets?) are not in the kernel but in a different file > which makes the kernel the same size like without debug=-g. Is there any > reason to not build it with debug=-g? The debugging information doesn't appear in the installed kernel (/boot/kernel/kernel for example). The build process makes a kernel, with debug symbols, in /usr/obj/usr/src/sys/KERNCONF. It strips the debugging information out before installing the file, though, because it's pretty big. The downside to adding -g to the build is just that you take up more space in /usr which you may never use. > Also I thought debug kernels suffer from reduced performance. I also have DDB > in my kernel and don't _feel_ any difference. So again, is there any reason > not to put DDB into the kernel? DDB itself probably will never affect your performance in a noticeable way unless your system crashed. The problem with DDB is that, if you're not present at the console when you get a kernel panic, the system hangs waiting for you. You can get around this by adding DDB_UNATTENDED, but IMO this is pretty much worthless for anyone who's not a kernel developer: you don't get to see the DDB screen and write down the backtrace, and you don't get a crash dump that you can extract with savecore. Unless you plan to actually do debugging live in your kernel I'd leave DDB off and set up a dump device instead. This lets you go back after a crash and extract information, by way of your debug kernel, at your leisure. The kernel debugging options that really kill performance are WITNESS and especially INVARIANTS. They cause a lot more debugging code to be compiled into the kernel that's normally cut out, and run lots of extra checks on locking and types and such at run-time. WITNESS also tends to procude excessive messages for potentially harmless problems. --Mike [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.3 (FreeBSD) iD4DBQE/uEsfCczNhKRsh48RAms7AJdFhlCGlpwUlt27amSwTf1iTqrWAJ4wj1Rc CZ41Wv9qNqSauvU/pDPqHA== =RZmv -----END PGP SIGNATURE-----help
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