Re: Multi Core Support for compression in compression.c

[Nick Krause]

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
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