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SubjectRe: aio is unlikely
Andrew Morton wrote:
> On Fri, 18 May 2007 17:54:32 -0400
> Phillip Susi <psusi@cfl.rr.com> wrote:
>
>> Andrew Morton wrote:
>>> Yes, if you agree with Jeff's original point.
>>>
>>> But I don't, actually. Sure, on some machines+workloads, AIO is more
>>> common than sync IO. But I expect that when we sum across all the
>>> machines+workloads in the world, sync IO is more common and is hence the
>>> case we should optimise for.
>>>
>>> That's assuming that the unlikely() actually does something.
>> But as Jeff said, that's not what unlikely is for. It should only be
>> used when it is unlikely for everybody, all the time, because when it is
>> right, it helps rather little, but when it is wrong, it hurts a lot.
>
> It does? Tell us more.

It is difficult to quantify either way. The details are both
CPU-specific and compiler-specific. The best information can be culled
from the gcc list archives, which is where I obtained my knowledge on
the subject (which is now ~2 years old).

Under the hood, likely() and unlikely() are implemented as percentage
predictions. likely() is implemented in the kernel as a 99-100% chance
of success, and unlikely() is implemented as a 0-1% chance of success.

As such, for our purposes, likely() and unlikely() should only be used
when a situation is [likely | unlikely] across all runtime
configurations. So if you mark a branch unlikely() when it is hit often
by 1% of your users, that is an incorrect usage.

The effects are probably most dramatic on older CPUs. Repeatedly
hitting an unlikely() can cause a pipeline stall on every single access.
Branch delay slots are filled improperly, with obvious implications.

But on modern hardware, I would /guess/ that the effect of repeatedly
hitting an unlikely() would be mitigated by smarter branch prediction.

We really need a GCC expert to answer this question in any more detail.

Jeff


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