linux-kernel - Re: [PATCH] libata-sff: fix 32-bit PIO regression

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Date:	Mon, 09 Feb 2009 13:56:17 +0300
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Jeff Garzik <jgarzik@...ox.com>, Mikael Pettersson <mikpe@...uu.se>
Cc:	Hugh Dickins <hugh@...itas.com>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk, rjw@...k.pl
Subject: Re: [PATCH] libata-sff: fix 32-bit PIO regression

Hello, I wrote:

>> The answer depends on workload.  Though rare, workloads do exist that 
>> involve a lot of oddball querying via weird, vendor-specific 
>> SCSI[-ish] commands.
>   Do you really think that the transfers having lengths non-divisible 
> by 4 make any *significant* percentage even on the ATAPI devices? I 
> think it's you who is really wrong.
>
>   Can you give an example of a *continous* querying with the data 
> transferring commands?
>   Hm, it just occured to me that the typical ATAPI command packet is 
> 12 bytes long.

   Haha, I even can't count! 12 divides by 4, of course. :-D

>> Or a more human version of the rule:  if you have to have a long 
>> email thread about unlikely() placement, it is best just to avoid 
>> using unlikely() in that case at all.  Branch prediction units in 
>> modern CPUs are damned good anyways, and there is always the 
>> likelihood that a human-placed unlikely() becomes wrong in the 
>> future. Moreover, the likelihood and cost of a branch mispredict are 
>> both low in this case, IMO.
>
>   There are still CPUs without the branch prediction, you know -- 
> Linux runs not only on x86.
>
>> Plus the code is more readable without unlikely(), IMO.
>
>   I tend to disagree. However, the packet command transfer is not 
> unlikely at all, so I'll remove that unlikely() in the respun patch.

   No, I'll keep it now. This case is indeed unlikely.

>>     Jeff

MBR, Sergei


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