Re: Multi Core Support for compression in compression.c

[Austin S Hemmelgarn]

On 2014-07-29 13:08, Nick Krause wrote:
> On Mon, Jul 28, 2014 at 2:36 PM, Nick Krause <xerofoify@xxxxxxxxx> wrote:
>> On Mon, Jul 28, 2014 at 12:19 PM, Austin S Hemmelgarn
>> <ahferroin7@xxxxxxxxx> wrote:
>>> On 2014-07-28 11:57, Nick Krause wrote:
>>>> On Mon, Jul 28, 2014 at 11:13 AM, Nick Krause <xerofoify@xxxxxxxxx>
>>>> wrote:
>>>>> On Mon, Jul 28, 2014 at 6:10 AM, Austin S Hemmelgarn
>>>>> <ahferroin7@xxxxxxxxx> wrote:
>>>>>> On 07/27/2014 11:21 PM, Nick Krause wrote:
>>>>>>> On Sun, Jul 27, 2014 at 10:56 PM, Austin S Hemmelgarn
>>>>>>> <ahferroin7@xxxxxxxxx> wrote:
>>>>>>>> On 07/27/2014 04:47 PM, Nick Krause wrote:
>>>>>>>>> This may be a bad idea , but compression in brtfs seems
>>>>>>>>> to be only using one core to compress. Depending on the
>>>>>>>>> CPU used and the amount of cores in the CPU we can make
>>>>>>>>> this much faster with multiple cores. This seems bad by
>>>>>>>>> my reading at least I would recommend for writing
>>>>>>>>> compression we write a function to use a certain amount
>>>>>>>>> of cores based on the load of the system's CPU not using
>>>>>>>>> more then 75% of the system's CPU resources as my system
>>>>>>>>> when idle has never needed more then one core of my i5
>>>>>>>>> 2500k to run when with interrupts for opening eclipse are
>>>>>>>>> running. For reading compression on good core seems fine
>>>>>>>>> to me as testing other compression software for reads ,
>>>>>>>>> it's way less CPU intensive. Cheers Nick
>>>>>>>> We would probably get a bigger benefit from taking an
>>>>>>>> approach like SquashFS has recently added, that is,
>>>>>>>> allowing multi-threaded decompression fro reads, and
>>>>>>>> decompressing directly into the pagecache. Such an approach
>>>>>>>> would likely make zlib compression much more scalable on
>>>>>>>> large systems.
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> Austin, That seems better then my idea as you seem to be more
>>>>>>> up to date on brtfs devolopment. If you and the other
>>>>>>> developers of brtfs are interested in adding this as a
>>>>>>> feature please let me known as I would like to help improve
>>>>>>> brtfs as the file system as an idea is great just seems like
>>>>>>> it needs a lot of work :). Nick
>>>>>> I wouldn't say that I am a BTRFS developer (power user maybe?),
>>>>>> but I would definitely say that parallelizing compression on
>>>>>> writes would be a good idea too (especially for things like
>>>>>> lz4, which IIRC is either in 3.16 or in the queue for 3.17).
>>>>>> Both options would be a lot of work, but almost any performance
>>>>>> optimization would.  I would almost say that it would provide a
>>>>>> bigger performance improvement to get BTRFS to intelligently
>>>>>> stripe reads and writes (at the moment, any given worker thread
>>>>>> only dispatches one write or read to a single device at a
>>>>>> time, and any given write() or read() syscall gets handled by
>>>>>> only one worker).
>>>>>>
>>>>>
>>>>> I will look into this idea and see if I can do this for writes.
>>>>> Regards Nick
>>>>
>>>> Austin, Seems since we don't want to release the cache for inodes
>>>> in order to improve writes if are going to use the page cache. We
>>>> seem to be doing this for writes in end_compressed_bio_write for
>>>> standard pages and in end_compressed_bio_write. If we want to cache
>>>> write pages why are we removing then ? Seems like this needs to be
>>>> removed in order to start off. Regards Nick
>>>>
>>> I'm not entirely sure, it's been a while since I went exploring in the
>>> page-cache code.  My guess is that there is some reason that you and I
>>> aren't seeing that we are trying for write-around semantics, maybe one
>>> of the people who originally wrote this code could weigh in?  Part of
>>> this might be to do with the fact that normal page-cache semantics
>>> don't always work as expected with COW filesystems (cause a write goes
>>> to a different block on the device than a read before the write would
>>> have gone to).  It might be easier to parallelize reads first, and
>>> then work from that (and most workloads would probably benefit more
>>> from the parallelized reads).
>>>
>> I will look into this later today and work on it then.
>> Regards Nick
> 
> Seems the best way to do is to create a kernel thread per core like in NFS and
> depending on the load of the system use these threads.
> Regards Nick
> 
It might be more work now, but it would probably be better in the long
run to do it using kernel workqueues, as they would provide better
support for suspend/hibernate/resume, and then you wouldn't need to
worry about scheduling or how many CPU cores are in the system.

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