linux-kernel - Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

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Message-ID: <538C8A47.1050502@kernel.dk>
Date:	Mon, 02 Jun 2014 08:29:27 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Paolo Valente <paolo.valente@...more.it>,
	Li Zefan <lizefan@...wei.com>,
	Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	Paolo Valente <posta_paolo@...oo.it>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

On 2014-05-30 23:16, Tejun Heo wrote:
>> for turning patch #2 into a series of changes for CFQ instead. We need to
>> end up with something where we can potentially bisect our way down to
>> whatever caused any given regression. The worst possible situation is "CFQ
>> works fine for this workload, but BFQ does not" or vice versa. Or hangs, or
>> whatever it might be.
>
> It's likely that there will be some workloads out there which may be
> affected adversely, which is true for any change really but with both
> the core scheduling and heuristics properly characterized at least
> finding a reasonable trade-off should be much less of a crapshoot and
> the expected benefits seem to easily outweigh the risks as long as we
> can properly sequence the changes.

Exactly, I think we are pretty much on the same page here. As I said in 
the previous email, the biggest thing I care about is not adding a new 
IO scheduler wholesale. If Paolo can turn the "add BFQ" patch into a 
series of patches against CFQ, then I would have no issue merging it for 
testing (and inclusion, when it's stable enough).

One thing I've neglected to bring up but have been thinking about - 
we're quickly getting to the point where the old request_fn IO path will 
become a legacy thing, mostly in maintenance mode. That isn't a problem 
for morphing bfq and cfq, but it does mean that development efforts in 
this area would be a lot better spent writing an IO scheduler that fits 
into the blk-mq framework instead.

I realize this is a tall order right now, as I haven't included any sort 
of framework for that in blk-mq yet. So what I envision happening is 
that I will write a basic deadline (ish) scheduler for blk-mq, and 
hopefully others can then pitch in and we can get the ball rolling on 
that side as well.

-- 
Jens Axboe

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