Curious about corner case in btrfs code

Curious about corner case in btrfs code

[Nick]

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;

Curious about corner case in btrfs code

[Mandeep Sandhu]

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;
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Curious about corner case in btrfs code

[Nick]

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;
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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 

Curious about corner case in btrfs code

[Mandeep Sandhu]

Then setup a test system which will reproduce hitting this corner case
and instrument your code to see is there's any gain at all.



On Tue, Aug 26, 2014 at 4:14 PM, Nick <xerofoify@gmail.com> 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;
>>>
>>> _______________________________________________
>>> Kernelnewbies mailing list
>>> Kernelnewbies at kernelnewbies.org
>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> 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

Curious about corner case in btrfs code

[Tobias Boege]

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;
> >>
> >> _______________________________________________
> >> Kernelnewbies mailing list
> >> Kernelnewbies at kernelnewbies.org
> >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> 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

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk

Curious about corner case in btrfs code

[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;
>>>>
>>>> _______________________________________________
>>>> Kernelnewbies mailing list
>>>> Kernelnewbies at kernelnewbies.org
>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>> 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 

Curious about corner case in btrfs code

[Valdis.Kletnieks at vt.edu]

On Tue, 26 Aug 2014 20:13:10 -0400, Nick said:

> 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.

Nope.  Same exact logic for small files.  The code is doing "Don't bother
trying to decompress if we're off the end of the file entirely".  Doesn't
matter if the file is 43K in size or 43G.
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Curious about corner case in btrfs code

[Tobias Boege]

On Tue, 26 Aug 2014, Nick wrote:
> 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;
> >>>>
> >>>> _______________________________________________
> >>>> Kernelnewbies mailing list
> >>>> Kernelnewbies at kernelnewbies.org
> >>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> >> 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.
>

Thanks for letting me compose my mail before you take a closer look at your
matter and decide it's not worth it.

> Was more curious about how to test things like this if 
> I need to :).

Then you need to phrase that -- in the *first* mail of a thread. There is no
need to hide your real questions behind different ones.

I think the steps 1 and 2 above can still be used to answer your question.
Basically, to test something you set up and run your system and then you do
things. What your "system" is constituted of and what "things" are depends
upon the subject and 1 and 2 might help you clear that up.

Regards,
Tobi

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk

Curious about corner case in btrfs code

[Nick]

On 08/26/2014 08:37 PM, Tobias Boege wrote:
> On Tue, 26 Aug 2014, Nick wrote:
>> 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;
>>>>>>
>>>>>> _______________________________________________
>>>>>> Kernelnewbies mailing list
>>>>>> Kernelnewbies at kernelnewbies.org
>>>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>> 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.
>>
> 
> Thanks for letting me compose my mail before you take a closer look at your
> matter and decide it's not worth it.
> 
>> Was more curious about how to test things like this if 
>> I need to :).
> 
> Then you need to phrase that -- in the *first* mail of a thread. There is no
> need to hide your real questions behind different ones.
> 
> I think the steps 1 and 2 above can still be used to answer your question.
> Basically, to test something you set up and run your system and then you do
> things. What your "system" is constituted of and what "things" are depends
> upon the subject and 1 and 2 might help you clear that up.
> 
> Regards,
> Tobi
> 
Sorry , thought it was a stupid question. Will note for next time.
Regards Nick 

Curious about corner case in btrfs code

[Greg Freemyer]

On August 26, 2014 8:13:10 PM EDT, Nick <xerofoify@gmail.com> wrote:
>
>
>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;
>>>>>
>>>>> _______________________________________________
>>>>> Kernelnewbies mailing list
>>>>> Kernelnewbies at kernelnewbies.org
>>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>> 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 

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.

Curious about corner case in btrfs code

[Valdis.Kletnieks at vt.edu]

On Tue, 26 Aug 2014 21:49:51 -0400, Greg Freemyer said:
> 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.

Never underestimate the value of that.  Clued users who submit good
bug reports and who can replicate/test patches or provide good test
cases are worth their weight in gold.
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Curious about corner case in btrfs code

[nick]

On 14-08-27 12:08 AM, Valdis.Kletnieks at vt.edu wrote:
> On Tue, 26 Aug 2014 21:49:51 -0400, Greg Freemyer said:
>> 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.
> 
> Never underestimate the value of that.  Clued users who submit good
> bug reports and who can replicate/test patches or provide good test
> cases are worth their weight in gold.
> 
Good to known after testing file systems and what tools are the standard in the community.
Regards Nick 

Curious about corner case in btrfs code

[Aruna Hewapathirane]

Nick,

Have a look here please: http://www.brendangregg.com/linuxperf.html

Aruna


On Wed, Aug 27, 2014 at 12:47 AM, nick <xerofoify@gmail.com> wrote:

>
>
> On 14-08-27 12:08 AM, Valdis.Kletnieks at vt.edu wrote:
> > On Tue, 26 Aug 2014 21:49:51 -0400, Greg Freemyer said:
> >> 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.
> >
> > Never underestimate the value of that.  Clued users who submit good
> > bug reports and who can replicate/test patches or provide good test
> > cases are worth their weight in gold.
> >
> Good to known after testing file systems and what tools are the standard
> in the community.
> Regards Nick
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
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Curious about corner case in btrfs code

[nick]

On 14-08-27 01:09 AM, Aruna Hewapathirane wrote:
> Nick,
> 
> Have a look here please: http://www.brendangregg.com/linuxperf.html
> 
> Aruna
> 
> 
> On Wed, Aug 27, 2014 at 12:47 AM, nick <xerofoify@gmail.com> wrote:
> 
>>
>>
>> On 14-08-27 12:08 AM, Valdis.Kletnieks at vt.edu wrote:
>>> On Tue, 26 Aug 2014 21:49:51 -0400, Greg Freemyer said:
>>>> 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.
>>>
>>> Never underestimate the value of that.  Clued users who submit good
>>> bug reports and who can replicate/test patches or provide good test
>>> cases are worth their weight in gold.
>>>
>> Good to known after testing file systems and what tools are the standard
>> in the community.
>> Regards Nick
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
> 
Thanks  for the web page I will start reading it later.
Cheers Nick 

Curious about corner case in btrfs code

[Valdis.Kletnieks at vt.edu]

On Tue, 26 Aug 2014 18:47:41 -0400, Nick said:
> 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.

    421         if (actual_end <= start)
    422                 goto cleanup_and_bail_uncompressed;

Wow, that's a heck of a small optimization target. How to approach it?

Step 3: Become proficient enough to instrument the code and actually measure it.
Because upstream will never accept a patch optimizing a "corner case" unless
you have actual numbers that show an actual improvement under production
workloads as opposed to microbenchmarks.

Step 2: Read the code, and ask yourself how the hell you intend to make the
code any faster than the relatively cheap if/goto that's currently in place.
Note that the combination of superscalar and speculative execution on modern
processors and good optimizing compilers means that it's possible that the 2
lines of code will execute in effectively *zero* cycles.  Hard to make it
faster than that. :)

Step 1: Note that the zero-cycles branch is in a loop that includes a
call to btrfs_compress_pages(), which is going to take *at least* 4096
cycles to process a 4K page (simply because you have to look at each byte
in the buffer at least once).  So the percentage savings is pretty low here,
because it's swamped by calls to compress and memset and other stuff that
chews a lot more cycles.

Step 0: Ask yourself how often programs either *ignore* an EOF condition
on a file they are reading, and keep trying to read past the end of the file,
or do an explicit lseek()/read() beyond the end of the file, and whether it's
worth optimizing that case@the expense of making the code more complex and
probalby slower in the non-corner-case case.

The tl;dr:  You're barking up the wrong tree here.
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