linux-kernel - Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed

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Message-ID: <x49pr6olyco.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 09 Dec 2009 14:51:51 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	"Linux-Kernel" <linux-kernel@...r.kernel.org>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed

Corrado Zoccolo <czoccolo@...il.com> writes:

> On Wed, Dec 9, 2009 at 8:05 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>> On Wed, Dec 09 2009, Jeff Moyer wrote:
>>> OK.  Can we put a comment in there and change the initialization to
>>> cfq_slice_sync * 10?
>>
>> Agree, that would be MUCH easier to understand.
>>
> Sure, we can put a comment there, but I don't like hardcoding a
> constant that depends on how the formula is computed (what if the
> formula is changed, and it doesn't depend on cfq_slice_sync any more,
> or if cfq_slice_sync changes dynamically?).

Then presumably you'd change the initialization of that variable.

> When I wrote it, what I really meant was exactly what you read in the
> C code (assume the last delayed sync happened 1 second ago). Then, the
> effect would be to start with a queue depth of 10 with the current
> formula, but even if we change the formula, 1 second is still
> meaningful (while 10 *cfq_slice_sync, that has the same value, becomes
> misleading). So my proposed fix is just:

Well, given your initial explanation, my suggestion made sense to me.
Given this new explanation, I'm fine with the change below.  Thanks for
clarifying.

Acked-by: Jeff Moyer <jmoyer@...hat.com>

> From f06cd83b45b3a7ee13ae7322197b610085dc70dd Mon Sep 17 00:00:00 2001
> From: Corrado Zoccolo <corrado@...alhost.(none)>
> Date: Wed, 9 Dec 2009 20:40:16 +0100
> Subject: [PATCH] cfq-iosched: commenting non-obvious initialization
>
> Added a comment to explain the initialization of last_delayed_sync.
>
> Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>
> ---
>  block/cfq-iosched.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 98b15b9..69ecee7 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3759,6 +3759,10 @@ static void *cfq_init_queue(struct request_queue *q)
>  	cfqd->cfq_latency = 1;
>  	cfqd->cfq_group_isolation = 0;
>  	cfqd->hw_tag = -1;
> +	/*
> +	 * we optimistically start assuming sync ops weren't delayed in last
> +	 * second, in order to have larger depth for async operations.
> +	 */
>  	cfqd->last_delayed_sync = jiffies - HZ;
>  	INIT_RCU_HEAD(&cfqd->rcu);
>  	return cfqd;

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