Re: Multi Core Support for compression in compression.c

[Nick Krause]

On Tue, Jul 29, 2014 at 1:14 PM, Austin S Hemmelgarn
<ahferroin7@xxxxxxxxx> wrote:
> 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.
>

Seems better then my ideas , I will need to work on this later as for now I have
some reading on the Linux networking stack.
Regards Nick
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