Re: [PATCH] x86 rwsem optimization extreme

Zachary Amsden
2010-02-18T04:26:14Z

>
> On 02/17/2010 05:53 PM, Linus Torvalds wrote:
>
>> - but adc _throughput_ is also typically much higher, which indicates
>> that even if you do flag renaming, the 'adc' quite likely only
>> schedules in a single ALU unit.
>>
>> For example, on a Pentium, adc/sbb can only go in the U pipe, and I think
>> the same is true of 'stc'. Now, nobody likely cares about Pentiums any
>> more, but the point is, 'adc' does often have constraints that a regular
>> 'add' does not, and there's an example of a 'stc+adc' pair would at the
>> very least have to be scheduled with an instruction in between.
>>
> No doubt. I doubt it much matters in this context, but either way I
> think the patch is probably a bad idea... much for the same as my incl
> hack was - since the code isn't actually inline, saving a handful bytes
> is not the right tradeoff.
>
> -hpa
>
>

Incidentally, the cost of putting all the rwsem code inline, using the
straightforward approach, for git-tip, using defconfig on x86_64 is 3565
bytes / 20971778 bytes total, or 0.0168%, using gcc 4.4.3.

That's small enough to actually consider it.

Even smaller if you leave trylock as a function... actually no, that
didn't work, size increased. I'm guessing many call sites also end up
calling the explicit form as a fallback.

If you inline only read_lock functions and write release, nope, that
didn't work either.

If you inline only read_lock functions, that still isn't it. Many other
permutations are possible, but I've wasted enough time.

Although, with a more clever inline implementation, if some of the
constraints to %rdx go away...

Zach