linux-kernel - Re: RFC: Allow block drivers to poll for I/O instead of sleeping

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Date:	Tue, 25 Jun 2013 17:00:21 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-nvme@...ts.infradead.org,
	Linux SCSI List <linux-scsi@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC: Allow block drivers to poll for I/O instead of sleeping

On Mon, Jun 24 2013, Matthew Wilcox wrote:
> On Mon, Jun 24, 2013 at 10:07:51AM +0200, Ingo Molnar wrote:
> > I'm wondering, how will this scheme work if the IO completion latency is a 
> > lot more than the 5 usecs in the testcase? What if it takes 20 usecs or 
> > 100 usecs or more?
> 
> There's clearly a threshold at which it stops making sense, and our
> current NAND-based SSDs are almost certainly on the wrong side of that
> threshold!  I can't wait for one of the "post-NAND" technologies to make
> it to market in some form that makes it economical to use in an SSD.
> 
> The problem is that some of the people who are looking at those
> technologies are crazy.  They want to "bypass the kernel" and "do user
> space I/O" because "the kernel is too slow".  This patch is part of an
> effort to show them how crazy they are.  And even if it doesn't convince
> them, at least users who refuse to rewrite their applications to take
> advantage of magical userspace I/O libraries will see real performance
> benefits.

Fully concur with that. At least on the read side, nand is just getting
crappier and polled completions is usually not going to be great. On the
write side, however, there are definite gains. Completions in the
10-15usec range aren't unusual. And once we hit PCM, well, it'll be fun.

On the write side, there are plenty of super latency customers out there
who would LOVE to poll when/if it's useful. Most often also the same
kind of people who talk the crazy of putting everything in user space.
Which is why I like the polling. If we can get sufficiently close, then
we can shut some of that up.

-- 
Jens Axboe

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