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

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Message-ID: <499008B7.3020404@ru.mvista.com>
Date:	Mon, 09 Feb 2009 13:43:03 +0300
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Jeff Garzik <jgarzik@...ox.com>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	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.

Jeff Garzik wrote:

>>   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.
>
> The answer depends on workload.  Though rare, workloads do exist that 
> involve a lot of oddball querying via weird, vendor-specific 
> SCSI[-ish] commands.

   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.

> Moreover, the likelihood and cost of a branch mispredict are both low 
> in this case, IMO.
>
> 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.

   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.

>     Jeff

MBR, Sergei


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