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
