On Sun, 30 Dec 2001 23:01:50 +0100, Richard Zidlicky wrote: > On Sat, Dec 29, 2001 at 07:39:33PM +0100, Thierry Godefroy wrote: > > On Sat, 29 Dec 2001 19:13:48 +0100 (CET), [EMAIL PROTECTED] wrote: > > > > > Hi! > > > > > > I have measured the time it takes to boot Linux on Q40 (starting at the > > > penguin): > > > > > > Kernel 2.2.17 : 62 sec > > > Kernel 2.4.17 : 36 sec > > > > > > I used the same installation, of course. > > > Thierry, your kernel seems to be mega fast. > > > > Well, Linux v2.4 is known (and designed) to be faster than v2.2, but > > beyong that, I used the 68060 optimisation flag (-m68060) to compile > > the Q60 kernel and the -m68040 flag for the Q40 kernel: the proper > > flag can be selected when configuring the kernel (make menuconfig > > or make xconfig), by selecting ONLY ONE of the 68020 / 68030 / 68040 / > > 68060 support options. > > It alwasy worth doing that. > > > If you select two of them (e.g. 68040 & 68060 > > for the Qx0), then no optimization flag is used at all (!) for the > > compilation (thus defaulting to a 68000-compatible code and also > > probably avoiding any 68000 instruction that would not work on higher > > processor, e.g MOVE.W SR,Dn which is priviledged on 68020+). > > not correct, we use gcc configured for linux-m68k which has DEFAULT_TARGET > set to 68020.
I was not aware of this fact... > So the effect of actually using -m68040 will be next to zero for non-FPU code. But even 68020, 68040 and 68060 have got significant differences between them (cache size and organization, MMU functionnalities, a few instructions differ also, some others running more or less faster depending on the processor model, etc...) for a GOOD optimizing compiler (or a good assembler programmer ;-) to take advantage of a targeted optimization... > The reason why selecting a single CPU variant makes the kernel faster is > that the differences between the m68k CPU's wrt cache, MMU, FPU and exception > handling are rather big, so each CPU needs its own cache and MMU handling > code. Sure ! BTW don't even think (or try) to run a 68040 optimized kernel on a Q60: it won't even initialize (because of the MMU and registers initialization at the very start of the kernel code) ! > This code is mostly inlined and if support for more CPU variants is > compiled in the distinction has to happen at runtime. This is a very valid point, although a well optimized code should test for the processor just once and then be able to just test a flag (or even self-modify itself !) IOT choose the good inlined part of the cache/MMU/exception processing routines to run. > It has also significant impact on code size. I did not check this, but it seems logical that it has some impact... Thierry.
