linux-kernel - Re: aio is unlikely

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Message-Id: <20070518140624.1a4db517.akpm@linux-foundation.org>
Date:	Fri, 18 May 2007 14:06:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Alex Volkov" <avcp-lkmail@....net>
Cc:	"'Jeff Garzik'" <jeff@...zik.org>,
	"'Linux Kernel Mailing List'" <linux-kernel@...r.kernel.org>
Subject: Re: aio is unlikely

On Fri, 18 May 2007 16:49:49 -0400
"Alex Volkov" <avcp-lkmail@....net> wrote:

> 
> Andrew Morton wrote:
> >     aio is unlikely
> >     Stick an unlikely() around is_aio(): I assert that most IO is
> synchronous.
> >
> > -#define in_aio() !is_sync_wait(current->io_wait)
> > +#define in_aio() (unlikely(!is_sync_wait(current->io_wait)))
> 
> > Jeff Garzik <jeff@...zik.org> wrote:
> > 
> > > > -#define in_aio() !is_sync_wait(current->io_wait)
> > > > +#define in_aio() (unlikely(!is_sync_wait(current->io_wait)))
> > > 
> > > Please revert.  Workload-dependent "likelihood" should not cause 
> > > programmers to add such markers.
> > a) disagree with the above
> > 
> > b) if in_aio() ever returns true we do
> > 
> > 	printk(KERN_ERR "%s(%s:%d) called in async context!\n",
> > 		__FUNCTION__, __FILE__, __LINE__);
> > 
> >    so I sure hope it's unlikely for all workloads. 
> 
> Shouldn't unlikely() go where in_aio() is actually used, if we printk(error)
> there?
> Isn't putting likely/unlikely into a boolean function-like macro itself
> asking for later trouble?
> 

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.
-
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