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Re: Introducing a new linux readahead framework

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: Steve Poe <steve(dot)poe(at)gmail(dot)com>, bizgres-general(at)pgfoundry(dot)org
Cc: Wu Fengguang <wfg(at)mail(dot)ustc(dot)edu(dot)cn>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Introducing a new linux readahead framework
Date: 2006-04-26 22:28:56
Message-ID: 20060426222855.GC97354@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-performance
(including bizgres-general)

Has anyone done any testing on bizgres? It's got some patches that
eliminate a lot of IO bottlenecks, so it might present even larger
gains.

On Wed, Apr 26, 2006 at 03:08:59PM -0500, Steve Poe wrote:
> I found an average 14% improvement Using Pg 7.4.11 with odbc-bench as my
> test bed with Wu's kernel patch. I have not tried version 8.x yet.
> 
> Thanks Wu.  
> 
> Steve Poe
> 
> Using Postgresql 7.4.11, on an dual Opteron with 4GB
> 
> On Fri, 2006-04-21 at 09:38 +0800, Wu Fengguang wrote:
> > Greetings,
> > 
> > I'd like to introduce a new readahead framework for the linux kernel:
> > http://www.ussg.iu.edu/hypermail/linux/kernel/0603.2/1021.html
> > 
> > HOW IT WORKS
> > 
> > In adaptive readahead, the context based method may be of particular
> > interest to postgresql users. It works by peeking into the file cache
> > and check if there are any history pages present or accessed. In this
> > way it can detect almost all forms of sequential / semi-sequential read
> > patterns, e.g.
> > 	- parallel / interleaved sequential scans on one file
> > 	- sequential reads across file open/close
> > 	- mixed sequential / random accesses
> > 	- sparse / skimming sequential read
> > 
> > It also have methods to detect some less common cases:
> > 	- reading backward
> > 	- seeking all over reading N pages
> > 
> > WAYS TO BENEFIT FROM IT
> > 
> > As we know, postgresql relies on the kernel to do proper readahead.
> > The adaptive readahead might help performance in the following cases:
> > 	- concurrent sequential scans
> > 	- sequential scan on a fragmented table
> > 	  (some DBs suffer from this problem, not sure for pgsql)
> > 	- index scan with clustered matches
> > 	- index scan on majority rows (in case the planner goes wrong)
> > 
> > TUNABLE PARAMETERS
> > 
> > There are two parameters which are described in this email:
> > http://www.ussg.iu.edu/hypermail/linux/kernel/0603.2/1024.html
> > 
> > Here are the more oriented guidelines for postgresql users:
> > 
> > - /proc/sys/vm/readahead_ratio
> > Since most DB servers are bounty of memory, the danger of readahead
> > thrashing is near to zero. In this case, you can set readahead_ratio to
> > 100(or even 200:), which helps the readahead window to scale up rapidly.
> > 
> > - /proc/sys/vm/readahead_hit_rate
> > Sparse sequential reads are read patterns like {0, 2, 4, 5, 8, 11, ...}.
> > In this case we might prefer to do readahead to get good I/O performance
> > with the overhead of some useless pages. But if you prefer not to do so,
> > set readahead_hit_rate to 1 will disable this feature.
> > 
> > - /sys/block/sd<X>/queue/read_ahead_kb
> > Set it to a large value(e.g. 4096) as you used to do.
> > RAID users might want to use a bigger number.
> > 
> > TRYING IT OUT
> > 
> > The latest patch for stable kernels can be downloaded here:
> > http://www.vanheusden.com/ara/
> > 
> > Before compiling, make sure that the following options are enabled:
> > Processor type and features -> Adaptive file readahead
> > Processor type and features ->   Readahead debug and accounting
> > 
> > HELPING AND CONTRIBUTING
> > 
> > The patch is open to fine-tuning advices :)
> > Comments and benchmarking results are highly appreciated.
> > 
> > Thanks,
> > Wu
> > 
> > ---------------------------(end of broadcast)---------------------------
> > TIP 4: Have you searched our list archives?
> > 
> >                http://archives.postgresql.org
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Have you searched our list archives?
> 
>                http://archives.postgresql.org
> 

-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

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