[Bizgres-general] [PERFORM] Introducing a new linux readahead framework

[Luke Lonergan]

Jim,

I¹m thinking about it, we¹re already using a fixed read-ahead of 16MB using
blockdev on the stock Redhat 2.6.9 kernel, it would be nice to not have to
set this so we may try it.

- Luke 


On 4/26/06 3:28 PM, "Jim C. Nasby" <jnasby at pervasive.com> wrote:

> (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.com
> Pervasive Software      http://pervasive.com    work: 512-231-6117
> vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461
> _______________________________________________
> Bizgres-general mailing list
> Bizgres-general at pgfoundry.org
> http://pgfoundry.org/mailman/listinfo/bizgres-general
> 
> 


-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://pgfoundry.org/pipermail/bizgres-general/attachments/20060426/d12d7413/attachment.html