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SubjectRe: [PATCH v2 1/1] mmc: block: Add write packing control
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Hi Venkat,

Sorry for the late response. I came back from a long vacation and had many
issues to take care of.
If you still need a rebased version of the packed commands patches, I can
send a rebased version of the write packing control patch once Seungwon
Jeon will send the rebased version of the packing patches.
Please let us know if you ran into conflicts and it is required.

Thanks,
Maya
On Tue, August 28, 2012 10:40 am, S, Venkatraman wrote:
> On Mon, Aug 27, 2012 at 11:58 PM, <merez@codeaurora.org> wrote:
>>
>> On Fri, July 27, 2012 2:07 am, S, Venkatraman wrote:
>>> On Fri, Jul 27, 2012 at 12:24 AM, <merez@codeaurora.org> wrote:
>>>>
>>>> On Thu, July 26, 2012 8:28 am, S, Venkatraman wrote:
>>>>> On Tue, Jul 24, 2012 at 2:14 PM, <merez@codeaurora.org> wrote:
>>>>>> On Mon, July 23, 2012 5:22 am, S, Venkatraman wrote:
>>>>>>> On Mon, Jul 23, 2012 at 5:13 PM, <merez@codeaurora.org> wrote:
>>>>>>>> On Wed, July 18, 2012 12:26 am, Chris Ball wrote:
>>>>>>>>> Hi, [removing Jens and the documentation list, since now we're
>>>>>> talking about the MMC side only]
>>>>>>>>> On Wed, Jul 18 2012, merez@codeaurora.org wrote:
>>>>>>>>>> Is there anything else that holds this patch from being pushed
>>>>>>>>>> to
>>>>>>>> mmc-next?
>>>>>>>>> Yes, I'm still uncomfortable with the write packing patchsets for
>>>>>>>>> a
>>>>>>>> couple of reasons, and I suspect that the sum of those reasons
>>>>>>>> means
>>>>>>>> that
>>>>>>>> we should probably plan on holding off merging it until after 3.6.
>>>>>>>>> Here are the open issues; please correct any misunderstandings:
>>>>>>>>> With
>>>>>> Seungwon's patchset ("Support packed write command"):
>>>>>>>>> * I still don't have a good set of representative benchmarks
>>>>>>>>> showing
>>>>>>>>> what kind of performance changes come with this patchset. It
>>>>>>>>> seems
>>>>>>>> like we've had a small amount of testing on one controller/eMMC
>>>>>>>> part
>>>>>>>> combo
>>>>>>>> from Seungwon, and an entirely different test from Maya, and the
>>>>>> results
>>>>>>>> aren't documented fully anywhere to the level of describing what
>>>>>>>> the
>>>>>> hardware was, what the test was, and what the results were before
>>>>>> and
>>>>>> after the patchset.
>>>>>>>> Currently, there is only one card vendor that supports packed
>>>>>>>> commands.
>>>>>> Following are our sequential write (LMDD) test results on 2 of our
>>>>>> targets
>>>>>>>> (in MB/s):
>>>>>>>> No packing packing
>>>>>>>> Target 1 (SDR 50MHz) 15 25
>>>>>>>> Target 2 (DDR 50MHz) 20 30
>>>>>>>>> With the reads-during-writes regression:
>>>>>>>>> * Venkat still has open questions about the nature of the read
>>>>>>>>> regression, and thinks we should understand it with blktrace
>>>>>>>>> before
>>>>>>>> trying to fix it. Maya has a theory about writes overwhelming
>>>>>>>> reads,
>>>>>>>> but
>>>>>>>> Venkat doesn't understand why this would explain the observed
>>>>>>>> bandwidth drop.
>>>>>>>> The degradation of read due to writes is not a new behavior and
>>>>>>>> exists
>>>>>> also without the write packing feature (which only increases the
>>>>>> degradation). Our investigation of this phenomenon led us to the
>>>>>> Conclusion that a new scheduling policy should be used for mobile
>>>>>> devices,
>>>>>>>> but this is not related to the current discussion of the write
>>>>>>>> packing
>>>>>> feature.
>>>>>>>> The write packing feature increases the degradation of read due to
>>>>>> write
>>>>>>>> since it allows the MMC to fetch many write requests in a row,
>>>>>>>> instead
>>>>>>>> of
>>>>>>>> fetching only one at a time. Therefore some of the read requests
>>>>>>>> will
>>>>>> have to wait for the completion of more write requests before they
>>>>>> can
>>>>>> be
>>>>>>>> issued.
>>>>>>>
>>>>>>> I am a bit puzzled by this claim. One thing I checked carefully
>>>>>>> when
>>>>>> reviewing write packing patches from SJeon was that the code didn't
>>>>>> plough through a mixed list of reads and writes and selected only
>>>>>> writes.
>>>>>>> This section of the code in "mmc_blk_prep_packed_list()", from v8
>>>>>> patchset..
>>>>>>> <Quote>
>>>>>>> + if (rq_data_dir(cur) != rq_data_dir(next)) {
>>>>>>> + put_back = 1;
>>>>>>> + break;
>>>>>>> + }
>>>>>>> </Quote>
>>>>>>>
>>>>>>> means that once a read is encountered in the middle of write
>>>>>>> packing,
>>>>>> the packing is stopped at that point and it is executed. Then the
>>>>>> next
>>>>>> blk_fetch_request should get the next read and continue as before.
>>>>>>>
>>>>>>> IOW, the ordering of reads and writes is _not_ altered when using
>>>>>>> packed
>>>>>> commands.
>>>>>>> For example if there were 5 write requests, followed by 1 read,
>>>>>>> followed by 5 more write requests in the request_queue, the first 5
>>>>>> writes will be executed as one "packed command", then the read will
>>>>>> be
>>>>>> executed, and then the remaining 5 writes will be executed as one
>>>>>> "packed command". So the read does not have to wait any more than it
>>>>>> waited before (packing feature)
>>>>>>
>>>>>> Let me try to better explain with your example.
>>>>>> Without packing the MMC layer will fetch 2 write requests and wait
>>>>>> for
>>>>>> the
>>>>>> first write request completion before fetching another write
>>>>>> request.
>>>>>> During this time the read request could be inserted into the CFQ and
>>>>>> since
>>>>>> it has higher priority than the async write it will be dispatched in
>>>>>> the
>>>>>> next fetch. So, the result would be 2 write requests followed by one
>>>>>> read
>>>>>> request and the read would have to wait for completion of only 2
>>>>>> write
>>>>>> requests.
>>>>>> With packing, all the 5 write requests will be fetched in a row, and
>>>>>> then
>>>>>> the read will arrive and be dispatched in the next fetch. Then the
>>>>>> read
>>>>>> will have to wait for the completion of 5 write requests.
>>>>>>
>>>>>> Few more clarifications:
>>>>>> Due to the plug list mechanism in the block layer the applications
>>>>>> can
>>>>>> "aggregate" several requests to be inserted into the scheduler
>>>>>> before
>>>>>> waking the MMC queue thread.
>>>>>> This leads to a situation where there are several write requests in
>>>>>> the
>>>>>> CFQ queue when MMC starts to do the fetches.
>>>>>>
>>>>>> If the read was inserted while we are building the packed command
>>>>>> then
>>>>>> I
>>>>>> agree that we should have seen less effect on the read performance.
>>>>>> However, the write packing statistics show that in most of the cases
>>>>>> the
>>>>>> packing stopped due to an empty queue, meaning that the read was
>>>>>> inserted
>>>>>> to the CFQ after all the pending write requests were fetched and
>>>>>> packed.
>>>>>>
>>>>>> Following is an example for write packing statistics of a READ/WRITE
>>>>>> parallel scenario:
>>>>>> write packing statistics:
>>>>>> Packed 1 reqs - 448 times
>>>>>> Packed 2 reqs - 38 times
>>>>>> Packed 3 reqs - 23 times
>>>>>> Packed 4 reqs - 30 times
>>>>>> Packed 5 reqs - 14 times
>>>>>> Packed 6 reqs - 8 times
>>>>>> Packed 7 reqs - 4 times
>>>>>> Packed 8 reqs - 1 times
>>>>>> Packed 10 reqs - 1 times
>>>>>> Packed 34 reqs - 1 times
>>>>>> stopped packing due to the following reasons:
>>>>>> 2 times: wrong data direction (meaning a READ was fetched and
>>>>>> stopped
>>>>>> the
>>>>>> packing)
>>>>>> 1 times: flush or discard
>>>>>> 565 times: empty queue (meaning blk_fetch_request returned NULL)
>>>>>>
>>>>>>>
>>>>>>> And I requested blktrace to confirm that this is indeed the
>>>>>>> behaviour.
>>>>>>
>>>>>> The trace logs show that in case of no packing, there are maximum of
>>>>>> 3-4
>>>>>> requests issued before a read request, while with packing there are
>>>>>> also
>>>>>> cases of 6 and 7 requests dispatched before a read request.
>>>>>>
>>>>>> I'm waiting for an approval for sharing the block trace logs.
>>>>>> Since this is a simple test to run you can collect the trace logs
>>>>>> and
>>>>>> let
>>>>>> us know if you reach other conclusions.
>>>>>>
>>>>> Thanks for the brief. I don't have the eMMC4.5 device with me yet, so
>>>>> I can't reproduce the result.
>>>>
>>>> I sent the trace logs of both packing and non packing. Please let me
>>>> know
>>>> if you have additional questions after reviewing them.
>>>>
>>>> The problem you describe is most likely
>>>>> applicable
>>>>> to any block device driver with a large queue depth ( any queue depth
>>>>> >1).
>>>>> I'll check to see what knobs in block affect the result.
>>>>> Speaking of it, what is the host controller you use to test this ?
>>>>
>>>> The controller I use is msm_sdcc.
>>>>
>>>>> I was wondering if host->max_seg_size is taken into account while
>>>>> packed
>>>>> command
>>>>> is in use. If not, shouldn't it be ? - it could act as a better
>>>>> throttle for "packing density".
>>>>
>>>> The max segments (which is calculated from host->max_seg_size) is
>>>> taking
>>>> into account when preparing the packed list (so that the whole packed
>>>> won't exceed the max number of segments).
>>>> I'm not sure I understand how host->max_seg_size can be used as a
>>>> throttle
>>>> for "packing density". Can you please explain?
>>>>
>>> Ok - I overlooked that max_segments is indeed used to limit the number
>>> of requests
>>> that are packed.(And this corresponds to max_seg_size, which is what I
>>> intended)
>>> I should be getting my MMC4.5 test gear in a couple of days - I'll run
>>> it through
>>> on some hosts and can either provide more feedback or Ack this patch.
>>> Regards,
>>> Venkat.
>>
>> Hi Venkat,
>>
>> Do you have additional questions/comments?
>>
> None. I am just running some stress tests on BKOPS patches right now
> and after tomorrow I'll start testing packed command. Will all the
> patches apply on top of current mmc-next ? If not, it would be great
> if
> you can send an updated version.
>
> Thanks,
> Venkat.
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