Messages in this thread | | | Date | Thu, 20 Jan 2011 11:58:18 +0800 | From | Gui Jianfeng <> | Subject | Re: [PATCH 3/6 v3] cfq-iosched: Introduce vdisktime and io weight for CFQ queue |
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Vivek Goyal wrote: > On Mon, Dec 27, 2010 at 04:51:00PM +0800, Gui Jianfeng wrote: >> Introduce vdisktime and io weight for CFQ queue scheduling. Currently, io priority >> maps to a range [100,1000]. It also gets rid of cfq_slice_offset() logic and makes >> use the same scheduling algorithm as CFQ group does. This helps for CFQ queue and >> group scheduling on the same service tree. >> >> Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com> > > [..] >> @@ -1246,47 +1278,71 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq, >> >> service_tree = service_tree_for(cfqq->cfqg, cfqq_prio(cfqq), >> cfqq_type(cfqq)); >> + /* >> + * For the time being, put the newly added CFQ queue at the end of the >> + * service tree. >> + */ >> + if (RB_EMPTY_NODE(&cfqe->rb_node)) { >> + /* >> + * If this CFQ queue moves to another group, the original >> + * vdisktime makes no sense any more, reset the vdisktime >> + * here. >> + */ >> + parent = rb_last(&service_tree->rb); >> + if (parent) { >> + u64 boost; >> + s64 __vdisktime; >> + >> + __cfqe = rb_entry_entity(parent); >> + cfqe->vdisktime = __cfqe->vdisktime + CFQ_IDLE_DELAY; >> + >> + /* Give some vdisktime boost according to its weight */ >> + boost = cfq_get_boost(cfqd, cfqe); >> + __vdisktime = cfqe->vdisktime - boost; >> + if (__vdisktime > service_tree->min_vdisktime) >> + cfqe->vdisktime = __vdisktime; >> + else >> + cfqe->vdisktime = service_tree->min_vdisktime; >> + } else >> + cfqe->vdisktime = service_tree->min_vdisktime; > > Hi Gui, > > Is above logic actually working? I suspect that most of the time all the > new queues will end up getting same vdisktime and that is st->min_vdisktime > and there will be no service differentiation across various prio. > > Reason being, on SSD, idle is disabled. So very few/no queue will consume > its slice and follow reque path. So every queue will be new. Now you are > doing following. > > cfqd->vdisktime = vdisktime_of_parent + IDLE_DELAY - boost > > assume vdisktime_of_parent=st->min_vdisktime, then (IDLE_DELAY - boost) > is always going to be a -ve number and hence cfqd->vdisktime will > default to st->min_vdisktime. (IDLE_DELAY=200 while boost should be a huge > value due to SERVICE_SHIFT thing).
Vivek,
Actually, I tested on rotational disk with idling disabled, I saw service differentiation between two tasks with different ioprio. I don't have a SSD on hand, But I'll get one and do more tests.
> > I think this logic needs refining. Maybe instead of subtracting the boost > we can instead place entities further away from st->min_vdisktime and > offset is higher for lower ioprio queues. > > cfqe->vdisktime = st->min_vdisktime + offset > > here offset is something similar to boost but reversed in nature in the > sense that lower weight has got lower offset and vice-versa.
I'll consider this idea and try it.
> > The important test here will be to run bunch of cfqq queues of different > ioprio on a SSD with queue depth 1 and see if you can see the service > differentiation. If yes, then you can increase the queue depth a bit > and also number of competing queues and see what's the result. Also > monitor the blktrace and vdisktime and make sure higher prio queues > get to run more than lower prio queues. > > This is the most critical piece of converting cfqq scheduling logic, > so lets make sure that we get it right.
Yes, of course. :)
Thanks, Gui
> > > Thanks > Vivek >
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