linux-kernel - Re: Scatter-gather list constraints

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Message-ID: <Pine.LNX.4.44L0.0806221025220.3610-100000@netrider.rowland.org>
Date:	Sun, 22 Jun 2008 10:35:41 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	AntonioLin <antonio.lin@...ormicro.com>,
	David Vrabel <david.vrabel@....com>
Subject: Re: Scatter-gather list constraints

On Sun, 22 Jun 2008, Andi Kleen wrote:

> 
> >> - Is it performance critical?
> > 
> > For people using wireless USB drives, yes.
> 
> But only if there is a lot of 512 byte block IO? The only case I can think
> of right now would be XFS log IO and perhaps some O_DIRECT/raw device
> accesses.

Sorry, I misunderstood your question.  There probably will not be a lot 
of 512-byte block I/O -- not in the workloads I'm acquainted with.  But 
there will be some.

It isn't performance-critical, in the sense that slowing down the odd
512-byte block transfers won't hurt performance much.  But it is
critical in the sense that the transfers must work properly when they 
do occur.

> If it's only an relative oddball just copying is fine imho.

You mean, have the USB stack allocate bounce buffers and copy the data 
between the S-G buffers (which may be in high memory) and the bounce 
buffers?  We're talking about a potentially fairly large amount of 
data, say up to 100 KB.  Is that really easier than splitting up an I/O 
request?

Alan Stern

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