Re: [PATCH] cfq-iosched: fix ncq detection code

[Corrado Zoccolo]

Hi Vivek,
On Sat, Nov 14, 2009 at 8:09 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Sat, Nov 14, 2009 at 02:33:40PM +0100, Corrado Zoccolo wrote:
>> CFQ's detection of queueing devices assumes a non-queuing device and detects
>> if the queue depth reaches a certain threshold.
>
> Hi Corrado,
>
> In cfq_init_queue(), we init hw_tag = 1. So I think we start with the
> assumption that device supports NCQ.
Yes, it is a cut&paste typo. It should have been:
CFQ's detection of queueing devices assumes a queuing device and detects

>
>> ÂUnder some workloads (e.g.
>> synchronous reads), CFQ effectively forces a unit queue depth, thus defeating
>> the detection logic. ÂThis leads to poor performance on queuing hardware,
>> since the idle window remains enabled.
>
> Is it always the case that NCQ hardware performs better if we don't idle.
> I got a rotational disk which supports NCQ but if we don't idle, than it
> becomes seek bound and throughput drops.

I think this is not the main point. The real point of this patch is to
detect NCQ correctly.
After the detection is correct, we can have a sound policy that
decides when and how much to idle.
For the how-much part, I'm working on measuring the cost of a seek, so
we can wait proportionally for that cost.
This will imply a reasonable idle for rotational (even NCQ) media, and
small or no idle for NCQ SSD.

> On SSD also, seeks are cheap but not free. So if workload is less, then
> we might prefer to idle and I think that's the purpose of this dynamic
> hw_tag thing. If non-idling kind of workload is sufficient, then disable
> idling otherwise in case of sync workloads, continue to idle.
I don't think so. The dynamic hw_tag purpose was to determine if for
some reason the depth was dynamically altered, but it doesn't work
very well, so we can't rely on it.

>
> In fact, whether device supports NCQ or not, isn't this information
> statically available? I do a cat on "/sys/block/<sdd>/device/queue_depth"
> and on non queuing hardware it has been set to 1 and on queuing hardware
> it has been set to 31 by default after system boot. So I am assuming we
> statically have the information about device queuing capability? At least
> for the locally connected devices.
It is only for devices that plug into the SCSI subsystem.
>>
>> Given this premise, switching to hw_tag = 0 after we have proved at
>> least once that the device is NCQ capable is not a good choice.
>
> If it is proven that NCQ always means that idling is not good.
No. The hidden assumption is 'if we trust in our policy'.
If the policy is not good, maybe it is because, due to the fluctuating
NCQ detection, we could never test it thoroughly.
> Not true
> for atleast locally connected media. But is probably true many a times
> for SSD and fast storage arrays.

Thanks,
Corrado
>
> Thanks
> Vivek
>
>>
>> The new detection code starts in an indeterminate state, in which CFQ behaves
>> as if hw_tag = 1, and then, if for a long observation period we never saw
>> large depth, we switch to hw_tag = 0, otherwise we stick to hw_tag = 1,
>> without reconsidering it again.
>>
>> Signed-off-by: Corrado Zoccolo <czoccolo@xxxxxxxxx>
>> ---
>> Âblock/cfq-iosched.c | Â 24 +++++++++++++++---------
>> Â1 files changed, 15 insertions(+), 9 deletions(-)
>>
>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>> index 1bcbd8c..6925ab9 100644
>> --- a/block/cfq-iosched.c
>> +++ b/block/cfq-iosched.c
>> @@ -191,8 +191,14 @@ struct cfq_data {
>> Â Â Â Â*/
>> Â Â Â int rq_queued;
>> Â Â Â int hw_tag;
>> - Â Â int hw_tag_samples;
>> - Â Â int rq_in_driver_peak;
>> + Â Â /*
>> + Â Â Â* hw_tag can be
>> + Â Â Â* -1 => indeterminate, (cfq will behave as if NCQ is present, to allow better detection)
>> + Â Â Â* Â1 => NCQ is present (hw_tag_est_depth is the estimated max depth)
>> + Â Â Â* Â0 => no NCQ
>> + Â Â Â*/
>> + Â Â int hw_tag_est_depth;
>> + Â Â unsigned int hw_tag_samples;
>>
>> Â Â Â /*
>> Â Â Â Â* idle window management
>> @@ -2527,8 +2533,11 @@ static void cfq_update_hw_tag(struct cfq_data *cfqd)
>> Â{
>> Â Â Â struct cfq_queue *cfqq = cfqd->active_queue;
>>
>> - Â Â if (rq_in_driver(cfqd) > cfqd->rq_in_driver_peak)
>> - Â Â Â Â Â Â cfqd->rq_in_driver_peak = rq_in_driver(cfqd);
>> + Â Â if (rq_in_driver(cfqd) > cfqd->hw_tag_est_depth)
>> + Â Â Â Â Â Â cfqd->hw_tag_est_depth = rq_in_driver(cfqd);
>> +
>> + Â Â if (cfqd->hw_tag == 1)
>> + Â Â Â Â Â Â return;
>>
>> Â Â Â if (cfqd->rq_queued <= CFQ_HW_QUEUE_MIN &&
>> Â Â Â Â Â rq_in_driver(cfqd) <= CFQ_HW_QUEUE_MIN)
>> @@ -2547,13 +2556,10 @@ static void cfq_update_hw_tag(struct cfq_data *cfqd)
>> Â Â Â if (cfqd->hw_tag_samples++ < 50)
>> Â Â Â Â Â Â Â return;
>>
>> - Â Â if (cfqd->rq_in_driver_peak >= CFQ_HW_QUEUE_MIN)
>> + Â Â if (cfqd->hw_tag_est_depth >= CFQ_HW_QUEUE_MIN)
>> Â Â Â Â Â Â Â cfqd->hw_tag = 1;
>> Â Â Â else
>> Â Â Â Â Â Â Â cfqd->hw_tag = 0;
>> -
>> - Â Â cfqd->hw_tag_samples = 0;
>> - Â Â cfqd->rq_in_driver_peak = 0;
>> Â}
>>
>> Âstatic void cfq_completed_request(struct request_queue *q, struct request *rq)
>> @@ -2960,7 +2966,7 @@ static void *cfq_init_queue(struct request_queue *q)
>> Â Â Â cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
>> Â Â Â cfqd->cfq_slice_idle = cfq_slice_idle;
>> Â Â Â cfqd->cfq_latency = 1;
>> - Â Â cfqd->hw_tag = 1;
>> + Â Â cfqd->hw_tag = -1;
>> Â Â Â cfqd->last_end_sync_rq = jiffies;
>> Â Â Â return cfqd;
>> Â}
>> --
>> 1.6.2.5
>>
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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