linux-kernel - Re: aio is unlikely

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 18 May 2007 18:37:26 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Phillip Susi <psusi@....rr.com>, Alex Volkov <avcp-lkmail@....net>,
	'Linux Kernel Mailing List' <linux-kernel@...r.kernel.org>
Subject: Re: aio is unlikely

Andrew Morton wrote:
> On Fri, 18 May 2007 17:54:32 -0400
> Phillip Susi <psusi@....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

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/