Ok - so I ran the same test on my system and get a total speed of
113MB/sec. Why is this? Why is the system so limited to around just
110MB/sec? I tuned read ahead up a bit, and my results improve a
bit..
Alex
On 11/18/05, Luke Lonergan <llonergan@greenplum.com> wrote:
> Dave,
>
> On 11/18/05 5:00 AM, "Dave Cramer" <pg@fastcrypt.com> 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
