Ok - so I ran the same test on my system and get a total speed of113MB/sec. Why is this? Why is the system so limited to around just110MB/sec? I tuned read ahead up a bit, and my results improve abit.. Alex On 11/18/05, Luke Lonergan <llonergan@xxxxxxxxxxxxx> wrote:> Dave,>> On 11/18/05 5:00 AM, "Dave Cramer" <pg@xxxxxxxxxxxxx> wrote:> >> > Now there's an interesting line drawn in the sand. I presume you have> > numbers to back this up ?> >> > This should draw some interesting posts.>> Part 2: The answer>> System A:>> This system is running RedHat 3 Update 4, with a Fedora 2.6.10 Linux kernel.>> On a single table with 15 columns (the Bizgres IVP) at a size double memory> (2.12GB), Postgres 8.0.3 with Bizgres enhancements takes 32 seconds to scan> the table: that's 66 MB/s. Not the efficiency I'd hope from the onboard> SATA controller that I'd like, I would have expected to get 85% of the> 100MB/s raw read performance.>> So that's $1,200 / 66 MB/s (without adjusting for 2003 price versus now) => 18.2 $/MB/s>> Raw data:> [llonergan@kite4 IVP]$ cat scan.sh> #!/bin/bash>> time psql -c "select count(*) from ivp.bigtable1" dgtestdb> [llonergan@kite4 IVP]$ cat sysout1> count> ----------> 10000000> (1 row)>>> real 0m32.565s> user 0m0.002s> sys 0m0.003s>> Size of the table data:> [llonergan@kite4 IVP]$ du -sk dgtestdb/base> 2121648 dgtestdb/base>> System B:>> This system is running an XFS filesystem, and has been tuned to use very> large (16MB) readahead. It's running the Centos 4.1 distro, which uses a> Linux 2.6.9 kernel.>> Same test as above, but with 17GB of data takes 69.7 seconds to scan (!)> That's 244.2MB/s, which is obviously double my earlier point of 110-120MB/s.> This system is running with a 16MB Linux readahead setting, let's try it> with the default (I think) setting of 256KB – AHA! Now we get 171.4 seconds> or 99.3MB/s.>> So, using the tuned setting of "blockdev —setra 16384" we get $6,000 /> 244MB/s = 24.6 $/MB/s> If we use the default Linux setting it's 2.5x worse.>> Raw data:> [llonergan@modena2 IVP]$ cat scan.sh> #!/bin/bash>> time psql -c "select count(*) from ivp.bigtable1" dgtestdb> [llonergan@modena2 IVP]$ cat sysout3> count> ----------> 80000000> (1 row)>>> real 1m9.875s> user 0m0.000s> sys 0m0.004s> [llonergan@modena2 IVP]$ !du> du -sk dgtestdb/base> 17021260 dgtestdb/base>> Summary:>> <cough, cough> OK – you can get more I/O bandwidth out of the current I/O> path for sequential scan if you tune the filesystem for large readahead.> This is a cheap alternative to overhauling the executor to use asynch I/O.>> Still, there is a CPU limit here – this is not I/O bound, it is CPU limited> as evidenced by the sensitivity to readahead settings. If the filesystem> could do 1GB/s, you wouldn't go any faster than 244MB/s.>> - Luke ---------------------------(end of broadcast)---------------------------TIP 2: Don't 'kill -9' the postmaster
