Re: Question about postmaster's CPU usage

Thanks a lot for replies from Kevin, Ken, and Ants Aasma. I really
aappreciate your suggestions and comments.

My server configuration is two physical quad-core CPUs with
hyper-threading enabled.
Each CPU is Intel(R) Xeon(R) CPU E5620(at)2(dot)40GHz(dot) Physical memory is 16GB.
I set shared_buffers as 4GB, effective_cache_size as 10GB and
inventory table is around 500MB.

From the information provided by top command, although the row for
postmaster shows that postmaster is using 100%CPU,
the total CPU user time for the whole server never goes beyond 6.6%us.
I guess it is because postgres only uses a single thread to read
the data or “pushing the data around in RAM” according to Kevin’s statement.
Then my question is actually why postgres can not use the remaining 93.4%CPU.

Btw, I also tried the command suggested by Ants Aasma, but got an error:
explain (analyze on, timing off) select * from inventory;
ERROR: syntax error at or near "analyze"
LINE 1: explain (analyze on, timing off) select * from inventory;

^

Thanks!

Best regards
Kelphet Xiong

On Thu, Mar 28, 2013 at 2:03 PM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:

> kelphet xiong <kelphet(at)gmail(dot)com> wrote:
>
> > When I use postgres and issue a simple sequential scan for a
> > table inventory using query "select * from inventory;", I can see
> > from "top" that postmaster is using 100% CPU, which limits the
> > query execution time. My question is that, why CPU is the
> > bottleneck here and what is postmaster doing? Is there any way to
> > improve the performance? Thanks!
>
> > explain analyze select * from inventory;
> >
> > Seq Scan on inventory (cost=0.00..180937.00 rows=11745000 width=16)
> (actual time=0.005..1030.403 rows=11745000 loops=1)
> > Total runtime: 1750.889 ms
>
> So it is reading and returning 11.7 million rows in about 1 second,
> or about 88 nanoseconds (billionths of a second) per row. You
> can't be waiting for a hard drive for many of those reads, or it
> would take a lot longer, so the bottleneck is the CPU pushing the
> data around in RAM. I'm not sure why 100% CPU usage would surprise
> you. Are you wondering why the CPU works on the query straight
> through until it is done, rather than taking a break periodically
> and letting the unfinished work sit there?
>
> --
> Kevin Grittner
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>