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

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Date:	Tue, 6 Feb 2007 16:23:52 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Joel Becker <Joel.Becker@...cle.com>
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, 6 Feb 2007, Joel Becker wrote:

> On Tue, Feb 06, 2007 at 03:56:14PM -0800, Davide Libenzi wrote:
> > Async syscall submissions are a _one time_ things. It's not like a live fd 
> > that you can push inside epoll and avoid the multiple O(N) passes.
> > First of all, the amount of syscalls that you'd submit in a vectored way 
> > are limited. They do not depend on the total number of connections, but on 
> 
> 	I regularly see apps that want to submit 1000 I/Os at once.
> Every submit.  But it's all against one or two file descriptors.  So, if
> you return to userspace, they have to walk all 1000 async_results every
> time, just to see which completed and which didn't.  And *then* go wait
> for the ones that didn't.  If they just wait for them all, they aren't
> spinning cpu on the -EASYNC operations.
> 	I'm not saying that "don't return a completion if we can
> non-block it" is inherently wrong or not a good idea.  I'm saying that
> we need a way to flag them efficiently.

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

- Is the 1000 *parallel* syscall vectored submission case common?

- 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?



- Davide


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