Message-ID: <CANfBPZ_cgOvfW1oCLCjmnbXNZKWotbtwJ+4r5MetctTDb7jXWg@mail.gmail.com>
Date:	Tue, 12 Jun 2012 09:46:04 +0530
From:	"S, Venkatraman" <svenkatr@...com>
To:	merez@...eaurora.org
Cc:	Seungwon Jeon <tgih.jun@...sung.com>, linux-mmc@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	"DOCUMENTATION'" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] mmc: block: Add write packing control

On Tue, Jun 12, 2012 at 1:40 AM,  <merez@...eaurora.org> wrote:
>
>> On Mon, Jun 11, 2012 at 7:25 PM,  <merez@...eaurora.org> wrote:
>>>
>>>> Maya Erez <merez@...eaurora.org> wrote:
>>>>>
>>>>> > Hi,
>>>>> >
>>>>> > How can we check the effect?
>>>>> > Do you have any result?
>>>>> We ran parallel lmdd read and write operations and found out that the
>>>>> write packing causes the read throughput to drop from 24MB/s to
>>>>> 12MB/s.
>>>>> The write packing control managed to increase the read throughput back
>>>>> to
>>>>> the original value.
>>>>> We also examined "real life" scenarios, such as performing a big push
>>>>> operation in parallel to launching several applications. We measured
>>>>> the
>>>>> read latency and found out that with the write packing control the
>>>>> worst
>>>>> case of the read latency was smaller.
>>>>>
>>>>> > Please check the several comment below.
>>>>> >
>>>>> > Maya Erez <merez@...eaurora.org> wrote:
>>>>> >> The write packing control will ensure that read requests latency is
>>>>> >> not increased due to long write packed commands.
>>>>> >>
>>>>> >> The trigger for enabling the write packing is managing to pack
>>>>> several
>>>>> >> write requests. The number of potential packed requests that will
>>>>> >> trigger
>>>>> >> the packing can be configured via sysfs by writing the required
>>>>> value
>>>>> >> to:
>>>>> >> /sys/block/<block_dev_name>/num_wr_reqs_to_start_packing.
>>>>> >> The trigger for disabling the write packing is fetching a read
>>>>> request.
>>>>> >>
>>>>> >> ---
>>>>> >>  Documentation/mmc/mmc-dev-attrs.txt |   17 ++++++
>>>>> >>  drivers/mmc/card/block.c            |  100
>>>>> >> ++++++++++++++++++++++++++++++++++-
>>>>> >>  drivers/mmc/card/queue.c            |    8 +++
>>>>> >>  drivers/mmc/card/queue.h            |    3 +
>>>>> >>  include/linux/mmc/host.h            |    1 +
>>>>> >>  5 files changed, 128 insertions(+), 1 deletions(-)
>>>>> >>
>>>>> >> diff --git a/Documentation/mmc/mmc-dev-attrs.txt
>>>>> >> b/Documentation/mmc/mmc-dev-attrs.txt
>>>>> >> index 22ae844..08f7312 100644
>>>>> >> --- a/Documentation/mmc/mmc-dev-attrs.txt
>>>>> >> +++ b/Documentation/mmc/mmc-dev-attrs.txt
>>>>> >> @@ -8,6 +8,23 @@ The following attributes are read/write.
>>>>> >>
>>>>> >>   force_ro                Enforce read-only access even if write
>>>>> protect switch is
>>>>> >> off.
>>>>> >>
>>>>> >> + num_wr_reqs_to_start_packing    This attribute is used to
>>>>> determine
>>>>> >> + the trigger for activating the write packing, in case the write
>>>>> >> + packing control feature is enabled.
>>>>> >> +
>>>>> >> + When the MMC manages to reach a point where
>>>>> >> num_wr_reqs_to_start_packing
>>>>> >> + write requests could be packed, it enables the write packing
>>>>> feature.
>>>>> >> + This allows us to start the write packing only when it is
>>>>> beneficial
>>>>> >> + and has minimum affect on the read latency.
>>>>> >> +
>>>>> >> + The number of potential packed requests that will trigger the
>>>>> packing
>>>>> >> + can be configured via sysfs by writing the required value to:
>>>>> >> + /sys/block/<block_dev_name>/num_wr_reqs_to_start_packing.
>>>>> >> +
>>>>> >> + The default value of num_wr_reqs_to_start_packing was determined
>>>>> by
>>>>> >> + running parallel lmdd write and lmdd read operations and
>>>>> calculating
>>>>> >> + the max number of packed writes requests.
>>>>> >> +
>>>>> >>  SD and MMC Device Attributes
>>>>> >>  ============================
>>>>> >>
>>>>> >> diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
>>>>> >> index 2785fd4..ef192fb 100644
>>>>> >> --- a/drivers/mmc/card/block.c
>>>>> >> +++ b/drivers/mmc/card/block.c
>>>>> >> @@ -114,6 +114,7 @@ struct mmc_blk_data {
>>>>> >>   struct device_attribute force_ro;
>>>>> >>   struct device_attribute power_ro_lock;
>>>>> >>   int     area_type;
>>>>> >> + struct device_attribute num_wr_reqs_to_start_packing;
>>>>> >>  };
>>>>> >>
>>>>> >>  static DEFINE_MUTEX(open_lock);
>>>>> >> @@ -281,6 +282,38 @@ out:
>>>>> >>   return ret;
>>>>> >>  }
>>>>> >>
>>>>> >> +static ssize_t
>>>>> >> +num_wr_reqs_to_start_packing_show(struct device *dev,
>>>>> >> +                           struct device_attribute *attr, char
>>>>> *buf)
>>>>> >> +{
>>>>> >> + struct mmc_blk_data *md = mmc_blk_get(dev_to_disk(dev));
>>>>> >> + int num_wr_reqs_to_start_packing;
>>>>> >> + int ret;
>>>>> >> +
>>>>> >> + num_wr_reqs_to_start_packing =
>>>>> md->queue.num_wr_reqs_to_start_packing;
>>>>> >> +
>>>>> >> + ret = snprintf(buf, PAGE_SIZE, "%d\n",
>>>>> num_wr_reqs_to_start_packing);
>>>>> >> +
>>>>> >> + mmc_blk_put(md);
>>>>> >> + return ret;
>>>>> >> +}
>>>>> >> +
>>>>> >> +static ssize_t
>>>>> >> +num_wr_reqs_to_start_packing_store(struct device *dev,
>>>>> >> +                          struct device_attribute *attr,
>>>>> >> +                          const char *buf, size_t count)
>>>>> >> +{
>>>>> >> + int value;
>>>>> >> + struct mmc_blk_data *md = mmc_blk_get(dev_to_disk(dev));
>>>>> >> +
>>>>> >> + sscanf(buf, "%d", &value);
>>>>> >> + if (value >= 0)
>>>>> >> +         md->queue.num_wr_reqs_to_start_packing = value;
>>>>> >> +
>>>>> >> + mmc_blk_put(md);
>>>>> >> + return count;
>>>>> >> +}
>>>>> >> +
>>>>> >>  static int mmc_blk_open(struct block_device *bdev, fmode_t mode)
>>>>> >>  {
>>>>> >>   struct mmc_blk_data *md = mmc_blk_get(bdev->bd_disk);
>>>>> >> @@ -1313,6 +1346,48 @@ static void mmc_blk_rw_rq_prep(struct
>>>>> >> mmc_queue_req *mqrq,
>>>>> >>   mmc_queue_bounce_pre(mqrq);
>>>>> >>  }
>>>>> >>
>>>>> >> +static void mmc_blk_write_packing_control(struct mmc_queue *mq,
>>>>> >> +                                   struct request *req)
>>>>> >> +{
>>>>> >> + struct mmc_host *host = mq->card->host;
>>>>> >> + int data_dir;
>>>>> >> +
>>>>> >> + if (!(host->caps2 & MMC_CAP2_PACKED_WR))
>>>>> >> +         return;
>>>>> >> +
>>>>> >> + /*
>>>>> >> +  * In case the packing control is not supported by the host, it
>>>>> should
>>>>> >> +  * not have an effect on the write packing. Therefore we have to
>>>>> >> enable
>>>>> >> +  * the write packing
>>>>> >> +  */
>>>>> >> + if (!(host->caps2 & MMC_CAP2_PACKED_WR_CONTROL)) {
>>>>> >> +         mq->wr_packing_enabled = true;
>>>>> >> +         return;
>>>>> >> + }
>>>>> >> +
>>>>> >> + if (!req || (req && (req->cmd_flags & REQ_FLUSH))) {
>>>>> >> +         if (mq->num_of_potential_packed_wr_reqs >
>>>>> >> +                         mq->num_wr_reqs_to_start_packing)
>>>>> >> +                 mq->wr_packing_enabled = true;
>>>>> >> +         return;
>>>>> >> + }
>>>>> >> +
>>>>> >> + data_dir = rq_data_dir(req);
>>>>> >> +
>>>>> >> + if (data_dir == READ) {
>>>>> >> +         mq->num_of_potential_packed_wr_reqs = 0;
>>>>> >> +         mq->wr_packing_enabled = false;
>>>>> >> +         return;
>>>>> >> + } else if (data_dir == WRITE) {
>>>>> >> +         mq->num_of_potential_packed_wr_reqs++;
>>>>> >> + }
>>>>> >> +
>>>>> >> + if (mq->num_of_potential_packed_wr_reqs >
>>>>> >> +                 mq->num_wr_reqs_to_start_packing)
>>>>> >> +         mq->wr_packing_enabled = true;
>>>>> > Write Packing is available only if continuing write requests are
>>>>> over
>>>>> > num_wr_reqs_to_start_packing?
>>>>> > That means individual request(1...17) will be issued with
>>>>> non-packing.
>>>>> > Could you explain your policy more?
>>>>> We try to identify the case where there is parallel read and write
>>>>> operations. In our experiments we found out that the number of write
>>>>> requests between read requests in parallel read and write operations
>>>>> doesn't exceed 17 requests. Therefore, we can assume that fetching
>>>>> more
>>>>> than 17 write requests without hitting a read request can indicate
>>>>> that
>>>>> there is no read activity.
>>>> We can apply this experiment regardless I/O scheduler?
>>>> Which I/O scheduler was used with this experiment?
>>> The experiment was performed with the CFQ scheduler. Since the deadline
>>> uses a batch of 16 requests it should also fit the deadline scheduler.
>>> In case another value is required, this value can be changed via sysfs.
>>>>
>>>>> You are right that this affects the write throughput a bit but the
>>>>> goal
>>>>> of
>>>>> this algorithm is to make sure the read throughput and latency are not
>>>>> decreased due to write. If this is not the desired result, this
>>>>> algorithm
>>>>> can be disabled.
>>>>> >> +
>>>>> >> +}
>>>>> >> +
>>>>> >>  static u8 mmc_blk_prep_packed_list(struct mmc_queue *mq, struct
>>>>> request
>>>>> >> *req)
>>>>> >>  {
>>>>> >>   struct request_queue *q = mq->queue;
>>>>> >> @@ -1332,6 +1407,9 @@ static u8 mmc_blk_prep_packed_list(struct
>>>>> >> mmc_queue *mq, struct request *req)
>>>>> >>                   !card->ext_csd.packed_event_en)
>>>>> >>           goto no_packed;
>>>>> >>
>>>>> >> + if (!mq->wr_packing_enabled)
>>>>> >> +         goto no_packed;
>>>>> > If wr_packing_enabled is set to true, several write requests can be
>>>>> > packed.
>>>>> > We don't need to consider read request since packed write?
>>>>> I'm not sure I understand the question. We check if there was a read
>>>>> request in the mmc_blk_write_packing_control, and in such a case set
>>>>> mq->wr_packing_enabled to false.
>>>>> If I didn't answer the question, please explain it again.
>>>> Packed write can be possible after exceeding 17 requests.
>>>> Is it assured that read request doesn't follow immediately after packed
>>>> write?
>>>> I wonder this case.
>>> Currently in such a case we will send the packed command followed by the
>>> read request. The latency of this read request will be high due to
>>> waiting
>>> for the completion of the packed write. However, since we will disable
>>> the
>>> write packing, the latency of the following read requests will be low.
>>> We are working on a solution where the read request will bypass the
>>> write
>>> requests in such a case. This change requires modification of the
>>> scheduler in order to re-insert the write requests to the scheduler.
>>>>
>>
>> Thats the precise reason for using foreground HPI (shameless plug :-))
>> I understand the intent of write packing control, but using the number
>> of requests
>> as a metric is too coarse. Some writes could be for only one sector
>> (512B) and others
>> could be in 512KB or more, giving a 1000x variance.
>>
>> Foreground HPI solves this problem by interrupting only on a wait
>> threshold.
>>
>> Another aspect is that if a packed write is in progress, and you have
>> a read request,
>> you will most likely disable packing for the _next_ write, not the
>> ongoing one, right ?
>> That's too late an intervention IMHO.
>>
> If a write request is in progress and a read is fetched we pln to use HPI
> to stop it and re-insert the remider of the write packed command back to
> the scheduler for a later dispatch.
IIUC, there were 2 reasons mentioned by you for introducing write
packing control -
1) Read bandwidth drop
2) Use case "latency" or if I were to guess, "sluggish UI".

So if (2) is solved by HPI, we can investigate the reason for (1) and
fix that, rather
than adding another functionality (which belongs in the I/O scheduler
anyway) to MMC.

> Regarding the packing control trigger, we also tried using a trigger of an
> amount of write bytes between read. However, the number of potential
> packed requests seemed like the reasonable trigger since we would like to
> activate the packing only when it will be beneficial, regardless of the
> write requests sizes.
>
Why ? How do you know "when it will be beneficial" ? As I mentioned,
the number of
blocks per request would vary over time, and also depends on the
filesystem. OTOH, even small
writes could take a lot longer than usual (>500ms) due to garbage
collection etc.
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