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

Since the original trace logs are to big to be sent, I'm sending a parsed
version of the files.
Please let me know if the original trace logs are still required.

On Tue, July 24, 2012 1:23 pm, merez@codeaurora.org wrote:

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

On Tue, July 24, 2012 1:44 am, merez@codeaurora.org wrote:

quoted

On Mon, July 23, 2012 5:22 am, S, Venkatraman wrote:

quoted

On Mon, Jul 23, 2012 at 5:13 PM,  [off-list ref] wrote:

quoted

On Wed, July 18, 2012 12:26 am, Chris Ball wrote:

quoted

Hi,  [removing Jens and the documentation list, since now we're

talking about the MMC side only]

quoted

quoted

quoted

On Wed, Jul 18 2012, merez@codeaurora.org wrote:

quoted

Is there anything else that holds this patch from being pushed to

mmc-next?

quoted

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.

quoted

Here are the open issues; please correct any misunderstandings: With

Seungwon's patchset ("Support packed write command"):

quoted

quoted

quoted

* 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

quoted

quoted

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.

quoted

quoted

Currently, there is only one card vendor that supports packed
commands.

Following are our sequential write (LMDD) test results on 2 of our
targets

quoted

quoted

(in MB/s):
                       No packing        packing
Target 1 (SDR 50MHz)     15               25
Target 2 (DDR 50MHz)     20               30

quoted

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,

quoted

quoted

but this is not related to the current discussion of the write packing

feature.

quoted

quoted

The write packing feature increases the degradation of read due to

write

quoted

quoted

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

quoted

quoted

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.

quoted

This section of the code in "mmc_blk_prep_packed_list()", from v8

patchset..

quoted

<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.

quoted

IOW, the ordering of reads and writes is _not_ altered when using
packed

commands.

quoted

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)

quoted

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

quoted

Your rest of the arguments anyway depend on this assertion, so can you

please clarify this.

quoted

quoted

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

quoted

quoted

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

quoted

quoted

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

quoted

quoted

the write requests and causes the read request to wait for a shorter
time.

quoted

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.

quoted

quoted

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.

quoted

quoted

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

quoted

quoted

interrupted write request remainder we will probably get another HPI

due

quoted

quoted

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

quoted

quoted

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).

quoted

quoted

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

quoted

quoted

disabled), while a single read request disable packing. Therefore, the

packing is stopped at the right time.

quoted

quoted

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.

quoted

quoted

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.

quoted

quoted

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

quoted

quoted

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

quoted

quoted

read request is fetched and usage of HPI to stop the packing that is

already transferred.

quoted

quoted

To summarize -
We recommend including the write packing in 3.6 due to the following

reasons:

quoted

quoted

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

quoted

quoted

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

quoted

quoted

integrate it as part of the complete solution.
Thanks,
Maya

quoted

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.

quoted

quoted

quoted

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.

quoted

--
Sent by consultant of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum



--
To unsubscribe from this list: send the line "unsubscribe linux-mmc" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

--
Sent by consultant of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum

-- 
Sent by consultant of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum