* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> [...]
>
> And I realize that compiler people tend to think that loop
> hoisting etc is absolutely critical for performance, and some
> big hammer like "barrier()" makes a compiler person wince. You
> think it results in horrible code generation problems.
>
> It really doesn't. Loops are fairly unusual in the kernel to
> begin with, and the compiler barriers are a total non-issue.
> We have much more problems with the actual CPU barriers that
> can be *very* expensive on some architectures, and we work a
> lot at avoiding those and avoiding cacheline ping-pong issues
> etc.
Just to underline this point, if barriers caused optimization
problems when GCC builds the kernel then we'd expect to see
various code generation problems: for example the compiler would
not be able to cache things well enough and reorder it to make
the code faster and (often) more compact.
So to test that effect of Linus's claim I picked up a fairly
bleeding edge version of GCC:
gcc version 4.7.0 20120112 (Red Hat 4.7.0-0.6) (GCC)
and performed a test build of the kernel with the majority of
optimization barriers removed (using the v3.2 kernel, x86
defconfig, 64-bit, -O2 optimization level): 1600 barriers were
removed (!) and GCC's hands were thus freed to create more
optimal code [and a very broken kernel], if it could.
I compared the resulting kernel image to an unmodified kernel
image:
text data bss dec hex filename
9781555 982328 1118208 11882091 b54e6b vmlinux.vanilla
9780972 982328 1118208 11881508 b54c24 vmlinux.no-barriers
So the barriers are costing us only a 0.06% size increase - 583
bytes on an almost 10 MB kernel image.
To put that into perspectve: a *single* inline function inlining
decision by the compiler has a larger effect than that. Just a
couple of days ago we uninlined a function, which had an order
of magnitude larger effect than this.
The other possible dimension would be the ordering of
instructions.
To test for that effect I disassembled the two kernel images and
performed a function by function, instruction by instruction
comparison of instruction ordering. The summary is that GCC was
able to remove only 86 instructions (0.005%) and reordered
around 2400 instructions (0.15%) - out of about 1,570,000
instructions.
Or, put differently, for the 1600 barriers in this particular
kernel build, there's about 1.5 instructions reordered and 0.05
instructions removed.
I also inspected the type of reordering: the overwhelming
majority of reordering happened within a jump-free basic block
of instructions and did not affect any loops.
Thus much of the effect of barriers kernel is only the crutial
effect that we want them to have: to reorder code to have a
specific program order sequence - but in the process the
barriers() cause very, very small optimization quality side
effects.
So the numbers support Linus's claim, the kernel incurs very
little optimization cost side effects from barriers.
Thanks,
Ingo
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