linux-kernel - Re: Multi Core Support for compression in compression.c

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Date:	Mon, 28 Jul 2014 12:19:52 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Nick Krause <xerofoify@...il.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-btrfs@...r.kernel.org SYSTEM list:BTRFS FILE" 
	<linux-btrfs@...r.kernel.org>
Subject: Re: Multi Core Support for compression in compression.c

On 2014-07-28 11:57, Nick Krause wrote:
> On Mon, Jul 28, 2014 at 11:13 AM, Nick Krause <xerofoify@...il.com>
> wrote:
>> On Mon, Jul 28, 2014 at 6:10 AM, Austin S Hemmelgarn 
>> <ahferroin7@...il.com> wrote:
>>> On 07/27/2014 11:21 PM, Nick Krause wrote:
>>>> On Sun, Jul 27, 2014 at 10:56 PM, Austin S Hemmelgarn 
>>>> <ahferroin7@...il.com> 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).


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