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SubjectRe: [PATCH v2 1/1] mmc: block: Add write packing control
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.


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