linux-kernel - Re: [PATCH RFC 00/14] Add the BFQ I/O Scheduler to blk-mq

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Message-ID: <1489509154.2676.6.camel@sandisk.com>
Date:   Tue, 14 Mar 2017 16:32:47 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "paolo.valente@...aro.org" <paolo.valente@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "fchecconi@...il.com" <fchecconi@...il.com>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "avanzini.arianna@...il.com" <avanzini.arianna@...il.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "tj@...nel.org" <tj@...nel.org>,
        "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>
Subject: Re: [PATCH RFC 00/14] Add the BFQ I/O Scheduler to blk-mq

On Tue, 2017-03-14 at 16:35 +0100, Paolo Valente wrote:
> > Il giorno 07 mar 2017, alle ore 02:00, Bart Van Assche <bart.vanassche@...disk.com> ha scritto:
> > 
> > Additionally, the complexity of the code is huge. Just like for CFQ,
> > sooner or later someone will run into a bug or a performance issue
> > and will post a patch to fix it. However, the complexity of BFQ is
> > such that a source code review alone won't be sufficient to verify
> > whether or not such a patch negatively affects a workload or device
> > that has not been tested by the author of the patch. This makes me
> > wonder what process should be followed to verify future BFQ patches?
> 
> Third and last, a proposal: why don't we discuss this issue at LSF
> too?  In particular, we could talk about the parts of BFQ that seem
> more complex to understand, until they become clearer to you.  Then I
> could try to understand what helped make them clearer, and translate
> it into extra comments in the code or into other, more radical
> changes.

Hello Paolo,

Sorry if my comment was not clear enough. Suppose that e.g. someone would
like to modify the following code:

static int bfq_min_budget(struct bfq_data *bfqd)
{
       if (bfqd->budgets_assigned < bfq_stats_min_budgets)
               return bfq_default_max_budget / 32;
       else
               return bfqd->bfq_max_budget / 32;
}

How to predict the performance impact of any changes in e.g. this function?
It is really great that a performance benchmark is available. But what should
a developer do who only has access to a small subset of all the storage
devices that are supported by the Linux kernel and hence who can not run the
benchmark against every supported storage device? Do developers who do not
fully understand the BFQ algorithms and who run into a performance problem
have any other option than trial and error for fixing such performance issues?

Thanks,

Bart.