Introduction part:
----------
I'm doing a performance tuning effort all over the place in
frequently-used Linux programs:
I'm trying to mark as many data areas as possible const (and static)
in various programs (at those places where it's actually feasible,
that is).
This should have the following benefits:
a) more .rodata (read-only) pages in programs, which means that
pages (since they're read-only and can thus be restored from the binary any
time) can simply be discarded instead of having to be paged out when
free memory is getting low
b) more page sharing between program instances due to less
modified / read-write (and thus unique instance *per-program*!) memory pages
c) various const compiler optimizations kicking in
d) less linker work to be done (static)
(use "objdump -x" to analyze what can be improved)
In smaller programs marking variables as const won't be enough to get
several whole pages (4K size) back to read-only, but it goes into the
right direction after all, since we don't intend to actually increase
the number of read-write pages...
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I marked several busybox variables const, resulting in the following improvement:
busybox before:
13 .rodata 0000edc5 0808c080 0808c080 00044080 2**5
CONTENTS, ALLOC, LOAD, READONLY, DATA
15 .data 0000045c 0809b000 0809b000 00053000 2**5
CONTENTS, ALLOC, LOAD, DATA
-rwxr-xr-x 1 root staff 5499979 May 8 00:16 busybox
busybox after:
13 .rodata 0000ee05 0808c080 0808c080 00044080 2**5
CONTENTS, ALLOC, LOAD, READONLY, DATA
15 .data 00000428 0809b000 0809b000 00053000 2**5
CONTENTS, ALLOC, LOAD, DATA
-rwxr-xr-x 1 root staff 5499943 May 8 00:15 busybox
Thank you,
Andreas Mohr
(patch attached, against current SVN)
busybox.diff [^] (2,764 bytes) 05-07-05 15:31 
