Curious about corner case in btrfs code

[xerofoify@gmail.com (Nick)]

On 08/26/2014 08:05 PM, Tobias Boege wrote:
> On Tue, 26 Aug 2014, Nick wrote:
>> On 08/26/2014 06:58 PM, Mandeep Sandhu wrote:
>>> If it's a corner case, it won't be hit often enough right? And if it
>>> was hit often enough, it wouldn't be corner case!? :)
>>>
>>> These 2 are mutually exclusive!
>>>
>>>
>>> On Tue, Aug 26, 2014 at 3:47 PM, Nick <xerofoify@gmail.com> wrote:
>>>> After reading through the code in inode.c today , I am curious about the comment and the following code I will paste
>>>> below. I am curious if this corner case is hit often enough for me to write a patch to improve the speed of this
>>>> corner case. Furthermore , compress_file_range is the function name, in case you can't guess by the pasted code.
>>>> Regards Nick
>>>> 411     /*
>>>> 412      * we don't want to send crud past the end of i_size through
>>>> 413      * compression, that's just a waste of CPU time.  So, if the
>>>> 414      * end of the file is before the start of our current
>>>> 415      * requested range of bytes, we bail out to the uncompressed
>>>> 416      * cleanup code that can deal with all of this.
>>>> 417      *
>>>> 418      * It isn't really the fastest way to fix things, but this is a
>>>> 419      * very uncommon corner.
>>>> 420      */
>>>> 421     if (actual_end <= start)
>>>> 422             goto cleanup_and_bail_uncompressed;
>>>>
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>> I get that my question is if this corner case is hit, enough for me to write a patch to optimize it.
>> In addition the comment states it isn't but want to known for standard compression workloads in btrfs 
>> if it's hit enough for me to work on this and how much speed degradation are me we doing my not writing
>> it better.
>> Nick 
>>
> 
> Here's how I would go about it:
> 
>  1. Understand when the case is met (in theory).
>  2. Try to trigger it on a real system multiple times.
>  3. Try to explore systematically under what circumstances the case is met
>     and rank them by plausibility (if the notion of plausibility makes any
>     sense in a real world scenario -- I don't know).
>  4. Estimate cost vs. benefit.
> 
> I don't know if this is a good way but notice how you can do all this on
> yourself which I think is a plus for everyone. And if you decide in step 4
> to write a patch:
> 
>  5. Use your results from step 3 to create an environment that benefits
>     from your patch (notice how 4 guarantees that there exists such a
>     system with reasonable connection to real needs). Note the numbers.
>  6. Test your patch on as many regular configurations as possible. Note
>     the numbers. If it degrades performance on any of those, abort.
>  7. Do *NOT* send the patch out.
> 
> Regards,
> Tobi
> 

Thanks Tobi,
>From reading the code off the bat, seems to not need to be written as this case is rarely meet for large files
or files that are huge and take a lot of time to write. Was more curious about how to test things like this if 
I need to :).
Nick