linux-kernel - Re: [PATCH 2 of 4] Introduce i386 fibril scheduling

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Message-ID: <20070207004443.GE32307@ca-server1.us.oracle.com>
Date:	Tue, 6 Feb 2007 16:44:43 -0800
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Kent Overstreet <kent.overstreet@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Zach Brown <zach.brown@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-aio@...ck.org, Suparna Bhattacharya <suparna@...ibm.com>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [PATCH 2 of 4] Introduce i386 fibril scheduling

On Tue, Feb 06, 2007 at 04:23:52PM -0800, Davide Libenzi wrote:
> To how many "sessions" those 1000 *parallel* I/O operations refer to? 
> Because, if you batch them in an async fashion, they have to be parallel.

	They're independant.  Of course they have to be parallel, that's
what I/O wants.

> Without the per-async operation status code, you'll need to wait a result 
> *for each* submitted syscall, even the ones that completed syncronously.

	You are right, but it's more efficient in some cases.

> Open questions are:
> 
> - Is the 1000 *parallel* syscall vectored submission case common?

	Sure is for I/O.  It's the majority of the case.  If you have
1000 blocks to send out, you want them all down at the request queue at
once, where they can merge.

> - Is it more expensive to forcibly have to wait and fetch a result even 
>   for in-cache syscalls, or it's faster to walk the submission array?

	Not everything is in-cache.  Databases will be doing O_DIRECT
and will expect that 90% of their I/O calls will block.  Why should they
have to iterate this list every time?  If this is the API, they *have*
to.  If there's an efficient way to get "just the ones that didn't
block", then it's not a problem.

Joel


-- 

"The real reason GNU ls is 8-bit-clean is so that they can
 start using ISO-8859-1 option characters."
	- Christopher Davis (ckd@...osh.kei.com)

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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