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SubjectRe: MD-RAID: Use seq_putc() in three status functions?
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On 10/17/2016 09:39 AM, SF Markus Elfring wrote:
>>>> Does it improve code? Does it improve anything?
>>>
>>> Yes. - I got such an impression.
>>>
>>> * Is it more efficient to call the function "seq_printf" for the desired data processing
>>> for a single character than to pass it to the function "" in a string?
>>>
>>> * Will the required data transfer shrink a bit for the affected functions because of
>>> such a change?
>>>
>> Which are questions _you_ should be able to answer.
>
> I wonder that the answers are not obvious for you already.
>
> Calling the function "seq_putc" will be more efficient than "seq_printf"
> in this case because of the following reasons.
>
> 1. How does the distribution look like for supported processor architectures
> where the data transfer for bytes (as a function call parameter)
> is faster than for (string) pointers?
>
How would I know? I would assume that _you_ did some measurements here;
after all, _you_ are trying to push this patch.
I could easily claim that seq_printf() is more efficient than
seq_putc(), and won't apply your patch.
So _you_ have to prove that your patch is more efficient.

> 2. Did anybody measure already how many the execution times can vary
> for these functions?
>
Probably not.
But referring to the previous topic:
Unless _you_ prove that _your_ patch is more efficient it won't get
applied. _You_ want us to apply your patch, so the burden is on _you_ to
provide the required data.


> Where do you get doubts about its efficiency for the data processing
> of a single character?
>
Because it's being called at the end of a function calling seq_printf()
already. So exchanging a single call is probably not helping anything,
as the compiler will optimize it anyway.
Case in point: with your patch the x86_64 compiler generates nearly
identical code for driver/md/raid1.c, but with one instruction _more_
after your patch has been applied.

So it's not immediately obvious that your patch is an improvement.

Cheers,

Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@suse.de +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)

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