Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed | Date | Wed, 09 Dec 2009 14:51:51 -0500 |
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Corrado Zoccolo <czoccolo@gmail.com> writes:
> On Wed, Dec 9, 2009 at 8:05 PM, Jens Axboe <jens.axboe@oracle.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@redhat.com>
> From f06cd83b45b3a7ee13ae7322197b610085dc70dd Mon Sep 17 00:00:00 2001 > From: Corrado Zoccolo <corrado@localhost.(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@gmail.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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