linux-kernel - Re: [rfc] direct IO submission and completion scalability issues

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Date:	Sun, 03 Feb 2008 20:14:45 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	David Chinner <dgc@....com>
CC:	Nick Piggin <npiggin@...e.de>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...e.de,
	jens.axboe@...cle.com, James.Bottomley@...elEye.com,
	andrea@...e.de, clameter@....com, akpm@...ux-foundation.org,
	andrew.vasquez@...gic.com, willy@...ux.intel.com,
	Zach Brown <zach.brown@...cle.com>
Subject: Re: [rfc] direct IO submission and completion scalability issues

David Chinner wrote:
> Hi Nick,
> 
> When Matthew was describing this work at an LCA presentation (not
> sure whether you were at that presentation or not), Zach came up
> with the idea that allowing the submitting application control the
> CPU that the io completion processing was occurring would be a good
> approach to try.  That is, we submit a "completion cookie" with the
> bio that indicates where we want completion to run, rather than
> dictating that completion runs on the submission CPU.
> 
> The reasoning is that only the higher level context really knows
> what is optimal, and that changes from application to application.

well.. kinda. One of the really hard parts of the submit/completion stuff is that
the slab/slob/slub/slib allocator ends up basically "cycling" memory through the system;
there's a sink of free memory on all the submission cpus and a source of free memory
on the completion cpu. I don't think applications are capable of working out what is
best in this scenario..


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