linux-kernel - Re: [PATCH] Give kjournald a IOPRIO_CLASS

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Message-ID: <20081002132746.GG19428@kernel.dk>
Date:	Thu, 2 Oct 2008 15:27:47 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

On Thu, Oct 02 2008, Arjan van de Ven wrote:
> On Thu, 2 Oct 2008 11:45:37 +0200
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > 
> > That's a good idea, just bump the priority a little bit. Arjan, did
> > you test that out? I'd suggest just trying prio level 0 and still
> > using best-effort scheduling. Probably still need the sync marking,
> > would be interesting to experiment with though.
> 
> I looked at 0 but it appears the 0 is the default for everyone...
> if everyone just defaulted to > 0 then yes I would have picked 0.

That's not correct, class BE and value 4 is the default (and the code
defaults to that, if you haven't set a value yourself):

#define IOPRIO_NORM     (4)
static inline int task_ioprio(struct io_context *ioc)
{
        if (ioprio_valid(ioc->ioprio))
                return IOPRIO_PRIO_DATA(ioc->ioprio);

        return IOPRIO_NORM;
}

static inline int task_ioprio_class(struct io_context *ioc)
{
        if (ioprio_valid(ioc->ioprio))
                return IOPRIO_PRIO_CLASS(ioc->ioprio);

        return IOPRIO_CLASS_BE;
}

So if you use IOPRIO_CLASS_BE and 0 for the ioprio, you will have the
highest priority of the default scheduling class.

-- 
Jens Axboe

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