It will come as no surprise to most Linux users that the kernel has grown
over time. In general, the expansion in the kernel has been more than
offset by the increasing power of the systems that it runs on, but there is
still a price to be paid for kernel bloat. Extra memory has to be paid
for, and other overhead - such as cache misses - can hurt the overall
performance of the system.
Andi Kleen has been putting some effort into making the kernel smaller
through the use of some relatively new and obscure gcc options. He starts
with -Os, as do most kernel shrinkers; this one simply tells the
compiler to optimize for size rather than strictly for performance.
Anecdotal evidence suggests that -Os not only produces a smaller
kernel, but the resulting code also often runs faster as well.
The next step was to use
-funit-at-a-time. This option is new; it will be part of the
upcoming gcc 3.4 release. It causes the compiler to load the entire
source file into memory before it begins generating code; the result is
better inlining and dropping of unused functions. The result was a little
over 3% reduction in kernel text size. The reasons for this shrinkage require
further investigation; it may be that there is a significant amount of dead
code in the kernel.
Finally, Andi has also enabled
-mregparm=3, which instructs the compiler to pass up to three
function arguments in registers, rather than on the stack. This option
helps even more than -funit-at-a-time. Using all three options,
Andi is able to reduce the text size by over 700KB.
There is one potential problem with -mregparm=3, however: it
changes the calling conventions within the kernel, and thus breaks binary
modules. As one might imagine, some kernel developers are more worried
about this than others. Red Hat kernel packager Arjan van de Ven has stated that he is using this option, and
intends to build production kernels that way as well. As always, sympathy
for the difficulties encountered by distributors of binary-only modules is
low. If the kernel hackers decide that this option is worth using, they'll
not let some broken binary modules stop them.
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