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

[merez@codeaurora.org]

Attached are the trace logs for parallel read and write lmdd operations.

On Tue, July 24, 2012 1:44 am, 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,
> Maya
>
>>
>> Your rest of the arguments anyway depend on this assertion, so can you
> please clarify this.
>>
>>> To overcome this behavior, the solution would be to stop the write
>>> packing
>>> when a read request is fetched, and this is the algorithm suggested by
>>> the
>>> write packing control.
>>> Let's also keep in mind that lmdd benchmarking doesn't fully reflect
> the
>>> real life in which there are not many scenarios that cause massive read
> and write operations. In our user-common-scenarios tests we saw that in
> many cases the write packing decreases the read latency. It can happen
> in
>>> cases where the same amount of write requests is fetched with and
>>> without
>>> packing. In such a case the write packing decreases the transfer time
> of
>>> the write requests and causes the read request to wait for a shorter
>>> time.
>>>> With Maya's patchset ("write packing control"):
>>>> * Venkat thinks that HPI should be used, and the number-of-requests
>>>>   metric is too coarse, and it doesn't let you disable packing at the
>>> right time, and you're essentially implementing a new I/O scheduler
>>> inside
>>> the MMC subsystem without understanding the root cause for why that's
> necessary.
>>> According to our measurements the stop transmission (CMD12) + HPI is a
> heavy operation that may take up to several milliseconds. Therefore, a
> massive usage of HPI can cause a degradation of performance.
>>> In addition, it doesn’t provide a complete solution for read during
>>> write
>>> since it doesn’t solve the problem of “what to do with the interrupted
> write request remainder?”.  That is, a common interrupting read request
> will usually be followed by another one. If we just continue to write
> the
>>> interrupted write request remainder we will probably get another HPI
> due
>>> to the second read request, so eventually we may end up with lots of
>>> HPIs
>>> and write retries. A complete solution will be: stop the current write,
> change packing mode to non-packing, serve the read request, push back
> the
>>> write remainders to the block I/O scheduler and let him schedule them
> again probably after the read burst ends (this requires block layer
> support of course).
>>> Regarding the packing control, there seem to be a confusion since the
> number-of-requests is the trigger for *enabling* the packing (after it
> was
>>> disabled), while a single read request disable packing. Therefore, the
> packing is stopped at the right time.
>>> The packing control doesn't add any scheduling policy to the MMC layer.
> The write packing feature is the one changing the scheduling policy by
> fetching many write requests in a row without a delay that allows read
> requests to come in the middle.
>>> By disabling the write packing, the write packing control returns the
>>> old
>>> scheduling policy. It causes the MMC to fetch the requests one by one,
> thus read requests are served as before.
>>> It is correct that the trigger for enabling the write packing control
> should be adjusted per platform and doesn't give a complete solution.
> As
>>> I
>>> mentioned above, the complete solution will include the usage of write
> packing control, a re-insert of the write packed to the scheduler when
> a
>>> read request is fetched and usage of HPI to stop the packing that is
> already transferred.
>>> To summarize -
>>> We recommend including the write packing in 3.6 due to the following
> reasons:
>>> 1. It significantly improves the write throughput
>>> 2. In some of the cases it even decreases the read latency
>>> 3. The read degradation in simultaneous read-write flows already exist,
> even without this feature
>>> As for the write packing control, it can be included in 3.6 to supply a
> partial solution for the read degradation or we can postpone it to 3.7
> and
>>> integrate it as part of the complete solution.
>>> Thanks,
>>> Maya
>>>> My sense is that there's no way we can solve all of these to
>>>> satisfaction in the next week (which is when the merge window will
>>> open), but that by waiting a cycle we might come up with some good
> answers.
>>>> What do other people think?  If you're excited about these patchsets,
>>> now would be a fine time to come forward with your benchmarking results
> and to help understand the reads-during-writes regression.
>>
>
>
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Sent by consultant of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum