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    SubjectRe: [RFC PATCH] request: teach the device more intelligent
    From
    2011/8/10 Kyungmin Park <kmpark@infradead.org>:
    > On Wed, Aug 10, 2011 at 5:08 PM, Jens Axboe <axboe@kernel.dk> wrote:
    >> On 2011-08-10 01:43, Kyungmin Park wrote:
    >>> On Wed, Aug 10, 2011 at 3:52 AM, Jens Axboe <axboe@kernel.dk> wrote:
    >>>> On 2011-08-09 05:47, Kyungmin Park wrote:
    >>>>> Hi Jens
    >>>>>
    >>>>> Now eMMC device requires the upper layer information to improve the data
    >>>>> performance and reliability.
    >>>>>
    >>>>> . Context ID
    >>>>> Using the context information, it can sort out the data internally and improve the performance.
    >>>>> The main problem is that it's needed to define "What's the context".
    >>>>> Actually I expect cfq queue has own unique ID but it doesn't so decide to use the pid instead
    >>>>>
    >>>>> . Data Tag
    >>>>> Using the Data Tag (1-bit information), It writes the data at SLC area when it's hot data. So it can make the chip more reliable.
    >>>>> First I expect the REQ_META but current ext4 doesn't pass the WRITE_META. only use the READ_META. so it needs to investigate it.
    >>>>>
    >>>>> With these characteristics, it's helpful to teach the device. After some consideration. it's needed to pass out these information at request data structure.
    >>>>>
    >>>>> Can you give your opinions and does it proper fields at requests?
    >>>>
    >>>> You need this to work on all IO schedulers, not just cfq.
    >>> Of course if the concept is acceptable, I'll add the other IO schedulers also.
    >>>
    >>>> And since that's the case, there's no need to add this field since you can just
    >>>> retrieve it if the driver asks for it. For CFQ, it could look like this:
    >>>>
    >>>> static int cfq_foo(struct request *rq)
    >>>> {
    >>>>        struct cfq_queue *cfqq = rq->elevator_private[1];
    >>>>
    >>>>        if (cfqq)
    >>>>                return cfqq->pid;
    >>>>
    >>>>        return -1;
    >>>> }
    >>>
    >>> The actual user of these information is device driver. e.g.,
    >>> drivers/mmc/card/block.c
    >>> So it's not good to use cfq data structure at D/D. some time later
    >>> these are also used at scsi device drivers.
    >>
    >> No, what I'm suggesting above is the CFQ implementation. You would need
    >> to wire up an elv_ops->get_foo() and have the IO schedulers fill it in.
    >> If you notice, the above function does not take or output anything
    >> related to CFQ in particular, it'll just return you the unique key you
    >> need.
    >>
    >> It's either the above, or a field in the request that the schedulers
    >> fill out. However, it'd be somewhat annoying to grow struct request for
    >> something that has a narrow scope of use. Hence the suggestion to add a
    >> strategy helper for this.
    > Okay, I'll add new elevator function one for getting context or more hints.
    > BTW, does it okay to call elevator function call at D/D?
    >
    > The quick-n-dirty call is like this at "drivers/mmc/card/block.c"
    >
    >                struct elevator_queue *e = md->queue.queue->elevator;
    >                int context = -1;
    >
    >                if (e->ops->elevator_get_req_hint_fn && req) {
    >                        context = e->ops->elevator_get_req_hint_fn(req);
    I'm wondering how the driver deal with elevator switch. A context id from
    one elevator might just be garbage for another elevator.

    Thanks,
    Shaohua
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