Re: [PATCH v1] mmc: fix async request mechanism for sequential read scenarios

[Konstantin Dorfman <kdorfman@codeaurora.org>]

On 10/24/2012 07:07 PM, Per Förlin wrote:
> On 10/24/2012 11:41 AM, Konstantin Dorfman wrote:
>> Hello Per,
>>
>> On Mon, October 22, 2012 1:02 am, Per Forlin wrote:
>>>> When mmcqt reports on completion of a request there should be
>>>> a context switch to allow the insertion of the next read ahead BIOs
>>>> to the block layer. Since the mmcqd tries to fetch another request
>>>> immediately after the completion of the previous request it gets NULL
>>>> and starts waiting for the completion of the previous request.
>>>> This wait on completion gives the FS the opportunity to insert the next
>>>> request but the MMC layer is already blocked on the previous request
>>>> completion and is not aware of the new request waiting to be fetched.
>>> I thought that I could trigger a context switch in order to give
>>> execution time for FS to add the new request to the MMC queue.
>>> I made a simple hack to call yield() in case the request gets NULL. I
>>> thought it may give the FS layer enough time to add a new request to
>>> the MMC queue. This would not delay the MMC transfer since the yield()
>>> is done in parallel with an ongoing transfer. Anyway it was just meant
>>> to be a simple test.
>>>
>>> One yield was not enough. Just for sanity check I added a msleep as
>>> well and that was enough to let FS add a new request,
>>> Would it be possible to gain throughput by delaying the fetch of new
>>> request? Too avoid unnecessary NULL requests
>>>
>>> If (ongoing request is read AND size is max read ahead AND new request
>>> is NULL) yield();
>>>
>>> BR
>>> Per
>> We did the same experiment and it will not give maximum possible
>> performance. There is no guarantee that the context switch which was
>> manually caused by the MMC layer comes just in time: when it was early
>> then next fetch still results in NULL, when it was later, then we miss
>> possibility to fetch/prepare new request.
>>
>> Any delay in fetch of the new request that comes after the new request has
>> arrived hits throughput and latency.
>>
>> The solution we are talking about here will fix not only situation with FS
>> read ahead mechanism, but also it will remove penalty of the MMC context
>> waiting on completion while any new request arrives.
>>
>> Thanks,
>>
> It seems strange that the block layer cannot keep up with relatively slow flash media devices. There must be a limitation on number of outstanding request towards MMC.
> I need to make up my mind if it's the best way to address this issue in the MMC framework or block layer. I have started to look into the block layer code but it will take some time to dig out the relevant parts.
>
> BR
> Per
>
The root cause of the issue in incompletion of the current design with
well known producer-consumer problem solution (producer is block layer,
consumer is mmc layer).
Classic definitions states that the buffer is fix size, in our case we
have queue, so Producer always capable to put new request into the queue.
Consumer context blocked when both buffers (curr and prev) are busy
(first started its execution on the bus, second is fetched and waiting
for the first).
Producer context considered to be blocked when FS (or others bio
sources) has no requests to put into queue.
To maximize performance there are 2 notifications should be used:
1. Producer notifies Consumer about new item to proceed.
2. Consumer notifies Producer about free place.

In our case 2nd notification not need since as I said before - it is
always free space in the queue.
There is no such notification as 1st, i.e. block layer has no way to
notify mmc layer about new request arrived.

What you suggesting is to resolve specific case, when FS READ_AHEAD
mechanism behavior causes delays in producing new requests.
Probably you can resolve this specific case, but do you have guarantee
that this is only case that causes delays between new requests events?
Flash memory devices these days constantly improved on all levels: NAND,
firmware, bus speed and host controller capabilities, this makes any
yield/sleep/timeouts solution only temporary hacks.

Thanks
-- 

Konstantin Dorfman,
QUALCOMM ISRAEL, on behalf of Qualcomm Innovation Center,
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