Message-ID: <5c49b0ed0701160113i8866879y3a134461c4ae7974@mail.gmail.com>
Date:	Tue, 16 Jan 2007 01:13:45 -0800
From:	"Nate Diller" <nate.diller@...il.com>
To:	"David Brownell" <david-b@...bell.net>
Cc:	"Nate Diller" <nate@...mi.com>, "Andrew Morton" <akpm@...l.org>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Trond Myklebust" <trond.myklebust@....uio.no>,
	"Benjamin LaHaise" <bcrl@...ck.org>,
	"Alexander Viro" <viro@...iv.linux.org.uk>,
	"Suparna Bhattacharya" <suparna@...ibm.com>,
	"Kenneth W Chen" <kenneth.w.chen@...el.com>,
	"David Brownell" <dbrownell@...rs.sourceforge.net>,
	"Christoph Hellwig" <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	netdev@...r.kernel.org, ocfs2-devel@....oracle.com,
	linux-aio@...ck.org, xfs-masters@....sgi.com
Subject: Re: [PATCH -mm 9/10][RFC] aio: usb gadget remove aio file ops

On 1/15/07, David Brownell <david-b@...bell.net> wrote:
> On Monday 15 January 2007 5:54 pm, Nate Diller wrote:
> > This removes the aio implementation from the usb gadget file system.
>
> NAK.  I see a deep mis-understanding here.
>
>
> > Aside
> > from making very creative (!) use of the aio retry path, it can't be of any
> > use performance-wise
>
> Other than the basic win of letting one userspace thread keep an I/O
> stream active while at the same time processing the data it reads or
> writes??  That's the "async" part of AIO.
>
> There's a not-so-little thing called "I/O overlap" ... which is the only
> way to prevent wasting bandwidth between (non-cacheable) I/O requests,
> and thus is the only way to let userspace code achieve anything close
> to the maximum I/O bandwidth the hardware can achieve.
>
> We want to see the host side "usbfs" evolve to support AIO like this
> too, for the same reasons.  (Currently it has fairly ugly AIO code
> that looks unlike any other AIO code in Linux.  Recent updates to
> support a file-per-endpoint device model are a necessary precursor
> to switching over to standard AIO syscalls.)
>
>
> > because it always kmalloc()s a bounce buffer for the
> > *whole* I/O size.
>
> By and large that's a negligible factor compared to being able to
> achieve I/O overlap.  ISTR the reason for not doing fancy DMA magic
> was that the cost of this style AIO was under 1 KByte object code
> on ARM, which was easy to justify ... while DMA magic to do that
> sort of stuff would be much fatter, as well as more error prone.
>
> (And that's why the "creative" use of the retry path.  As I've
> observed before, "retry" is a misnomer in the general sense of
> an async I/O framework.  It's more of a semi-completion callback;
> I/O can't in general be "retried" on error or fault, and even in
> the current usage it's not really a "retry".)
>
>
> Now that high speed peripheral hardware is becoming more common on
> embedded Linuxes -- TI has DaVinci, OMAP 2430, TUSB6010 (as found
> in the new Nokia 800 tablets); Atmel AVR32 AP7000; at least a couple
> parts that should be able to use the same musb_hdrc driver as those
> TI parts; and a few other chips I've heard of -- there may be some
> virtue in eliminating the memcpy, since those CPUs don't have many
> MIPS to waste.  (Iff the memcpy turns out to be a real issue...)
>
>
> > Perhaps the only reason to keep it around is the ability
> > to cancel I/O requests, which only applies when using the user space async
> > I/O interface.
>
> It's good to have almost the complete kernel API functionality
> exposed to userspace, and having I/O cancelation is an inevitable
> consequence of a complete AIO framework ... but that particular
> issue was not a driving concern.
>
>
> The reason for AIO is to have a *STANDARD* userspace interface
> for *ASYNC I/O* which otherwise can't exist.  You know, the kind
> of I/O interface that can't be implemented with read() and write()
> syscalls, which for non-buffered I/O necessarily preclude all I/O
> overlap.  AIO itself is a direct match to most I/O frameworks'
> primitives.  (AIOCB being directly analagous to peripheral side
> "struct usb_request" and host side "struct urb".)
>
>
> You know, I've always thought that one reason the AIO discussions
> seemed strange is that they weren't really focussed on I/O (the
> lowlevel after-the-caches stuff) so much as filesystems (several
> layers up in the stack, with intervening caching frameworks).
>
> The first several implementations of AIO that I saw were restricted
> to "real" I/O and not applicable to disk backed files.  So while I
> was glad the Linux approach didn't make that mistake, it's seemed
> that it might be wanting to make a converse mistake: neglecting I/O
> that isn't aimed at data stored on disks.
>
>
> > I highly doubt that is enough incentive to justify the extra
> > complexity here or in user-space, so I think it's a safe bet to remove this.
> > If that feature still desired, it would be possible to implement a sync
> > interface that does an interruptible sleep.
>
> What's needed is an async, non-sleeeping, interface ... with I/O
> overlap.  That's antithetical to using read()/write() calls, so
> your proposed approach couldn't possibly work.

haha, wow ok you convinced me :)

I got a bit impatient when I was working on this, it took some time
just to figure out the intention of the code, and I'm trying to hold
to a bit of a schedule here.  Without any clear (to me) reason, I
didn't want to spend a lot of effort fixing this up.

There's really no big difference between the usb drivers here and the
disk I/O scheduler queue, AFAICT, so it seems like the solution I want
is to do a kmap() on the user buffer and then do the I/O straight out
of that.  That will eliminate the need for the bounce buffer.  I'll
post a new version along with the iodesc changes later this week.

NATE
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