Re: [PATCH 3/6 v3] cfq-iosched: Introduce vdisktime and io weight for CFQ queue

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

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
>