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

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Message-ID: <51C77344.2040907@gmail.com>
Date:	Sun, 23 Jun 2013 15:14:28 -0700
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Matthew Wilcox <willy@...ux.intel.com>,
	Jens Axboe <axboe@...nel.dk>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.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 6/23/13 3:09 AM, Ingo Molnar wrote:
> If an IO driver is implemented properly then it will batch up requests for
> the controller, and gets IRQ-notified on a (sub-)batch of buffers
> completed.
>
> If there's any spinning done then it should be NAPI-alike polling: a
> single "is stuff completed" polling pass per new block of work submitted,
> to opportunistically interleave completion with submission work.
>
> I don't see where active spinning brings would improve performance
> compared to a NAPI-alike technique. Your numbers obviously show a speedup
> we'd like to have, I'm just wondering whether the same speedup (or even
> more) could be implemented via:
>
>   - smart batching that rate-limits completion IRQs in essence
>   + NAPI-alike polling
>
> ... which would almost never result in IRQ driven completion when we are
> close to CPU-bound and while not yet saturating the IO controller's
> capacity.
>
> The spinning approach you add has the disadvantage of actively wasting CPU
> time, which could be used to run other tasks. In general it's much better
> to make sure the completion IRQs are rate-limited and just schedule. This
> (combined with a metric ton of fine details) is what the networking code
> does in essence, and they have no trouble reaching very high throughput.

Networking code has a similar proposal for low latency sockets using 
polling: https://lwn.net/Articles/540281/

David
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