Re: [PATCH] add likely around access_ok for uaccess

[Manfred Spraul]

Pavel Machek wrote:

> Hi!
>
>  
>
> > > > while trying to figure out why sysv msg is around 30% slower than pipes 
> > > > for data transfers I noticed that gcc's autodetection (3.2.2) guesses 
> > > > the "if(access_ok())" tests in uaccess.h wrong and puts the error memset 
> > > > into the direct path and the copy out of line.
> > > >
> > > > The attached patch adds likely to the tests - any objections? What about 
> > > > the archs except i386?
> > > >        
> > > How much speedup did you gain?
> > >      
> > How much can it hurt?
> >    
>
> The change is obviously okay, I just wanted to know... If it gains 30%
> on sysv messages.. that would be pretty big surprise.
No.
I didn't benchmark it at all. I'd expect one or two cycles.
The 30% difference are due to misaligned buffers: The sysvmsg ABI uses
struct msgbuf {
   unsigned long type;
   char data[];
};
msgbuf was aligned, thus data on an not 8-byte aligned address. Thus the 
"rep;movsl" microcode fastpath didn't kick in, and that caused the 
30-50% slowdown.
After manually misaligning the msgbuf structure, and properly aligning 
the kernel buffers, the performance of sysvmsg is now identical to 
pipes: Around 4 k cycles for a one-byte ping-pong, and around 10k cycles 
for 4 kB ping-pong, with a Celeron mobile 1.13 GHz.

I haven't decided yet if a patch to align the kernel buffers is a good 
thing or not - it only helps my benchmark, I'm not aware of a real-world 
app that uses sysvmsg for bulk data transfers.

--
   Manfred

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