Date: Mon, 07 Feb 2005 17:05:39 -0600 From: Jon Noack <noackjr@alumni.rice.edu> To: freebsd4@fadesa.es Cc: freebsd-stable@freebsd.org Subject: Re: 50% of packets lost only on local interfaces Message-ID: <4207F443.8020400@alumni.rice.edu> In-Reply-To: <4207DEEA.E7278E38@fadesa.es> References: <42073FD8.5CCA7EC5@fadesa.es> <20050207102140.GA56842@xor.obsecurity.org> <42074353.9E3EECBE@fadesa.es> <4207C5C7.80707@alumni.rice.edu> <4207DEEA.E7278E38@fadesa.es>
next in thread | previous in thread | raw e-mail | index | archive | help
José M. Fandiño wrote: > Jon Noack wrote: > >>>> Finally, I found the culprit: >>>>> >>>>> CFLAGS="" \ 100% of the transmited traffic is received >>>>> COPTFLAGS="" / >>>>> >>>>> CFLAGS= -pipe \ 50% of the transmited traffic is received >>>>> COPTFLAGS= -pipe / >>>> > >>> That would be exceedingly strange, because the above two options >>>> are supposed to produce *no differences at all* with the code >>>> generation. >>>> > >>> I'd believe that -O and no -O could behave differently, although >>>> I don't know why you'd want to compile without -O. >>> > >> because by the time I was compiling the system I was no interested >>> in compiler optimizations. Now I prefer a lightly optimized >>> kernel than a system with 50% of packet lost in local interfaces >>> ;-) >> > > -O is the default for -STABLE; anything else might very well cause >> problems. In fact, check out the CFLAGS section of >> /usr/share/examples/etc/make.conf: > > "Note that optimization settings other than -O and -O2 are not >> recommended or supported for compiling the world or the kernel - >> please revert any nonstandard optimization settings to "-O" before >> submitting bug reports without patches to the developers." > > I think this comment was referring to higher optimizations levels > than -O2, anyway removing "-O" shouldn't be so harmful. The explanation I've heard (I have no actual knowledge here, I'm just good at repeating others) is that gcc doesn't compile any ASM with -O0 (which is what you get with CFLAGS="-pipe"). This Breaks Things(tm): http://docs.freebsd.org/cgi/mid.cgi?20020623214947.J84322 kern/52764 is another example: http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/52764 More generically it makes sense that gcc treat code differently with -O0 than with -O. With the vast majority of users using -O and different code paths being taken with -O0, it doesn't surprise me at all that there are issues. It should be assumed that nonstandard means exactly what it says: something other than -O (or more recently -O2). If it breaks, try -O. If it's still broken with -O, report away. Regards, Jon P.S. Historically, the reason to use -O0 was for easier debugging. It appears that steps have been taken to ease debugging with -O to the point that it is no longer necessary to use -O0 (with the FreeBSD kernel and world, at least); I don't recall a FreeBSD developer ever asking someone to recompile with -O0 to help solve a problem...
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?4207F443.8020400>