Re: Introducing a new linux readahead framework - Mailing list pgsql-performance

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
>
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