On August 26, 2014 8:13:10 PM EDT, Nick [off-list ref] wrote:

On 08/26/2014 08:05 PM, Tobias Boege wrote:

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On Tue, 26 Aug 2014, Nick wrote:

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On 08/26/2014 06:58 PM, Mandeep Sandhu wrote:

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If it's a corner case, it won't be hit often enough right? And if

it

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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 [off-list ref] wrote:

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After reading through the code in inode.c today , I am curious

about the comment and the following code I will paste

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below. I am curious if this corner case is hit often enough for me

to write a patch to improve the speed of this

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corner case. Furthermore , compress_file_range is the function

name, in case you can't guess by the pasted code.

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Regards Nick
411     /*
412      * we don't want to send crud past the end of i_size

through

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413      * compression, that's just a waste of CPU time.  So, if

the

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414      * end of the file is before the start of our current
415      * requested range of bytes, we bail out to the

uncompressed

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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

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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.

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In addition the comment states it isn't but want to known for

standard compression workloads in btrfs 

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if it's hit enough for me to work on this and how much speed

degradation are me we doing my not writing

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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

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    and rank them by plausibility (if the notion of plausibility

makes any

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    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

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yourself which I think is a plus for everyone. And if you decide in

step 4

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to write a patch:

 5. Use your results from step 3 to create an environment that

benefits

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    from your patch (notice how 4 guarantees that there exists such a
    system with reasonable connection to real needs). Note the

numbers.

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 6. Test your patch on as many regular configurations as possible.

Note

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    the numbers. If it degrades performance on any of those, abort.
 7. Do *NOT* send the patch out.

Regards,
Tobi

Thanks Tobi,

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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 

If you want to do anything more than code beautification for btrfs, you absolutely have to learn how to write xfstest scripts.  Dispute the name, the Linux kernel now uses xfstests to exercise all kernel file systems.

I don't follow btrfs, but xfs as an obvious example is pretty strict about only accepting meaningful patches if they are accompanied by a user space xfstest tool to exercise it.  Surprisingly that seems to be less true of new functionality.  Bug fix patches have an almost mandatory request for an accompanying xfstest.

The other side of that is if you read thru the kernel bugzilla and find a filesystem bug you want to fix, a good first step is to focus on writing a xfstest script that will show the bug and fail appropriately.

A decade plus ago when I first got interested in using xfs for a production system, the freeze feature would randomly fail for me.  I was able to update the xfstest that exercised that code and get a reproducer.  I didn't do the actual kernel fix, but I still think of that as my first significant contribution to the kernel.

Greg
-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.