Re: aio is unlikely

[Jeff Garzik <jeff@garzik.org>]

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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