linux-kernel - Re: [patch] epoll use a single inode ...

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Date:	Wed, 7 Mar 2007 21:48:43 -0500
From:	Kyle Moffett <mrmacman_g4@....com>
To:	"Michael K. Edwards" <medwards.linux@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Anton Blanchard <anton@...ba.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Avi Kivity <avi@...o.co.il>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [patch] epoll use a single inode ...

On Mar 07, 2007, at 20:25:14, Michael K. Edwards wrote:
> On 3/7/07, Linus Torvalds <torvalds@...ux-foundation.org> wrote
>> In general, using software prefetching is just a stupid idea, unless
>>
>>  - the prefetch really is very strict (ie for a linked list you do  
>> exactly the above kinds of things to make sure that you don't try  
>> to prefetch the non-existent end entry)
>> AND
>>  - the CPU is stupid (in-order in particular).
>>
>> I think Intel even suggests in their optimization manuals to *not*  
>> do software prefetching, because hw can usually simply do better  
>> without it.
>
> Not the XScale -- it performs quite poorly without prefetch, as  
> people who have run ARMv5-optimized binaries on it can testify.
>
> The Intel XScale(r) core prefetch load instruction is a true  
> prefetch instruction because the load destination is the data or  
> mini-data cache and not a register. Compilers for processors which  
> have data caches, but do not support prefetch, sometimes use a load  
> instruction to preload the data cache. This technique has the  
> disadvantages of using a register to load data and requiring  
> additional registers for
> subsequent preloads and thus increasing register pressure. By  
> contrast, the prefetch can be used to reduce register pressure  
> instead of increasing it.
>
> The prefetch load is a hint instruction and does not guarantee that  
> the data will be loaded. Whenever the load would cause a fault or a  
> table walk, then the processor will ignore the prefetch  
> instruction, the fault or table walk, and continue processing the  
> next instruction. This is particularly advantageous in the case  
> where a linked list or recursive data structure is terminated by a  
> NULL pointer. Prefetching the NULL pointer will not fault program  
> flow.

Prefetching is also fairly critical on a Power4 or G5 PowerPC system  
as they have a long memory latency; an L2-cache miss can cost 200+  
cycles.  On such systems the "dcbt" prefetch instruction brings in a  
single 128-byte cacheline and has no serializing effects whatsoever,  
making it ideal for use in a linked-list-traversal inner loop.

Cheers,
Kyle Moffett

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