Messages in this thread |  | | From | Systemkennung Linux <> | Subject | Re: CONUNDRUM. | Date | Thu, 13 Feb 1997 21:19:41 +0100 (MET) |
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Hi,
> > Because its incredibly non obvious. Some of the folk doing big number > > crunches noticed that the size of your environment changed the > > performance. [...] > > hm, maybe time to add 'misalignment fault' counting support to the kernel? > Has anyone tried this? Does it work for the FPU too? Is it reliable? ;) > > i remember Dave has made this for the Sparc, and there is something like > this for x86 too.
For most (all?) RISCs this alignment problem is a non issue because the hardware does not handle it in a completly transparant way. On Linux where quite a lot of lowlevel system code (fdisk etc.) is written very dirty this is a problem. Therefore the RISC ports of Linux have kernel emulation software that executes the misaligned accesses in software.
This software emulation is many dozen times slower than the real thing and so no hardware counters need to exist. During my normal daily use of Linux/MIPS it's usually a real bug in a piece of user software when something triggers the emulation. In the kernel only the stuff dealing with PC-style partition tables, the fat filesystem and the IDE stuff trigger alignment exceptions and some of these haven been fixed for 2.0. Probably this is only a topic at all for 386 and to a lesser degree for 68k.
Ralf
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