Re: [PATCH] sched: CPU_THRESHOLD

Timothy Miller
2003-08-25T16:54:19Z

Ricardo Nabinger Sanchez wrote:
> hello Frank,
>
>
>>- *imbalance = (max_load - nr_running) / 2;
>>+ *imbalance = (max_load - nr_running) >> 1;
>
>
> I think it is a good coding practice to keep things human-readable.
> In this code snippet, the division by 2 is quickly understood by most
> readers (specially those who didn't write it). The right shift may
> obfuscate the real meaning of this operation, which is a single
> division by 2, not a bit-oriented expression.
>
> Assuming that sched.c will be compiled with optimizations enabled, the
> compiler will change the human-readable division by a fast machine
> right shift operation, whenever possible (gcc surely will).
>
> Thus, we keep the kernel code more readable, and sometimes let the
> compiler apply newer (and hopefully faster) optimizations than some
> tricks we have known as fastest available.
>
> Regards, and please let me know what do you think about it.
>

Note that right-shift rounds towards negative infinity, while divide
rounds towards zero.

If the operands are unsigned, then rsh and divide will yield the same
instructions (on x86). OTOH, if they are signed, then you get different
results. For this rsh, obviously, you get a shift, but for divide, you
get something like this:

; divide edx by 2
mov ecx,edx
lsr ecx,31
asr edx,1
add edx,ecx