linux-kernel - Re: MD-RAID: Use seq_putc() in three status functions?

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Date:   Mon, 17 Oct 2016 11:00:42 +0200
From:   SF Markus Elfring <elfring@...rs.sourceforge.net>
To:     Hannes Reinecke <hare@...e.de>
Cc:     linux-raid@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Guoqing Jiang <gqjiang@...e.com>, Jens Axboe <axboe@...com>,
        Joe Perches <coupons@...ches.com>,
        Mike Christie <mchristi@...hat.com>,
        Neil Brown <neilb@...e.com>, Shaohua Li <shli@...nel.org>,
        Tomasz Majchrzak <tomasz.majchrzak@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org, kbuild-all@...org,
        ltp@...ts.linux.it
Subject: Re: MD-RAID: Use seq_putc() in three status functions?

>> 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?

How many processor architecture characteristics do you know already?

* Is a string pointer often longer than a byte?

* I imagine that it can become also interesting to check byte level data access
  under constraints of machine word sizes and alignment.

> I would assume that _you_ did some measurements here;

How much would you trust in any concrete numbers I could present
for this use case?

Do you give more trust to a reference testing platform?

> I could easily claim that seq_printf() is more efficient than
> seq_putc(), and won't apply your patch.

This is also possible in principle.

> So _you_ have to prove that your patch is more efficient.

How many results would we like to clarify from various hardware
and software combinations?

>> 2. Did anybody measure already how many the execution times can vary
>>    for these functions?
>>
> Probably not.

Thanks for this information.

How important are the mentioned functions for you within the Linux
programming interface so far?

> Unless _you_ prove that _your_ patch is more efficient it won't get applied.

Which data would you accept as a "prove" in this case?

>>    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.

Interesting view …

> So exchanging a single call is probably not helping anything,
> as the compiler will optimize it anyway.

How common is the discussed software transformation between implementations
for optimising compilers?

> 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.

Which software versions and command parameters did you try out
for this information (from an unspecified run time environment)?

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

I agree that there are system properties and constraints which can be
considered further.

Regards,
Markus