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    SubjectRe: [RFC PATCH] request: teach the device more intelligent
    On Wed, Aug 10, 2011 at 5:08 PM, Jens Axboe <> wrote:
    > On 2011-08-10 01:43, Kyungmin Park wrote:
    >> On Wed, Aug 10, 2011 at 3:52 AM, Jens Axboe <> 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);

    >>> As to the hot part, I think that would be better as just a request flag
    >>> like eg the meta flag.
    >> Yes it can use the JBD_flags at cfq. but same reason it's not proper
    >> reference at device drivers. that's reason to make a filed at request.
    >> Device driver should or must see the request data structure and don't
    >> refer the upper layer data structures.
    > The device driver sees the struct request, which is where this flag ends
    > up.
    It's ext{3,4} series specific detection method. if user use the
    another filesystem, it's difficult to detect hot/cold.
    I'll more think about it.

    Thank you for suggestion.
    Kyungmin Park
    > --
    > Jens Axboe
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