Andrew Morton wrote:
On Fri, 18 May 2007 17:54:32 -0400
Phillip Susi <[email protected]> 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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