Re: Postgres refusing to use >1 core

Just wanted to again say thanks for everyone's help.

The main problem was that my program was running in serial, not parallel,
even though I thought I used a textbook example of PLINQ. Your assistance
helped me get to the point where I could conclusively determine everything
was running in serial. It was more obvious than I realized.

Thanks to help through
http://stackoverflow.com/questions/6086111/plinq-on-concurrentqueue-isnt-multithreading,
I have switched to the .NET Framework's Task Parallel Library, and it's
slamming the 8 cores hard now! And there's a bunch of concurrent connections
to Postgres. :-)

Aren

On Mon, May 9, 2011 at 4:23 PM, Aren Cambre <aren(at)arencambre(dot)com> wrote:

> I have a multi-threaded app. It uses ~22 threads to query Postgres.
>
> Postgres won't use more than 1 CPU core. The 22-threaded app only has 3%
> CPU utilization because it's mostly waiting on Postgres.
>
> Here's the details:
>
> The app has a "main" thread that reads table A's 11,000,000 rows, one at a
> time. The main thread spawns a new thread for each row in table A's data.
> This new thread:
>
> 1. Opens a connection to the DB.
> 2. Does some calculations on the data, including 1 to 102 SELECTs on
> table B.
> 3. With an INSERT query, writes a new row to table C.
> 4. Closes the connection.
> 5. Thread dies. Its data is garbage collected eventually.
>
> Physical/software details:
>
> - Core i7 processor--4 physical cores, but OS sees 8 cores
> via hyper-threading
> - 7200 RPM 500 GB HDD
> - About 1/2 total RAM is free during app execution
> - Windows 7 x64
> - Postgres 9.0.4 32-bit (32-bit required for PostGIS)
> - App is C# w/ .NET 4.0. PLINQ dispatches threads. Npgsql is Postgres
> connection tool.
>
> At first, the app pounds all 8 cores. But it quickly tapers off, and only 1
> core that's busy. The other 7 cores are barely doing a thing.
>
> Postgres has 9 open processes. 1 process was slamming that 1 busy core. The
> other 8 Postgres processes were alive but idle.
>
> Each thread creates its own connection. It's not concurrently shared with
> the main thread or any other threads. I haven't disabled connection pooling;
> when a thread closes a connection, it's technically releasing it into a pool
> for later threads to use.
>
> Disk utilization is low. The HDD light is off much more than it is on, and
> a review of total HDD activity put it between 0% and 10% of total capacity.
> The HDD busy indicator LED would regularly flicker every 0.1 to 0.3 seconds.
>
> The app runs 2 different queries on table B. The 1st query is run once, the
> 2nd query can be run up to 101 times. Table C has redundant indexes: every
> column referenced in the SQL WHERE clauses for both queries are indexed
> separately and jointly. E.g., if query X references columns Y and Z, there
> are 3 indexes:
>
> 1. An index for Y
> 2. An index for Z
> 3. An index for Y and Z
>
> Table C is simple. It has four columns: two integers, a varchar(18), and a
> boolean. It has no indexes. A primary key on the varchar(18) column is its
> only constraint.
>
> A generalized version of my INSERT command for table C is:
> *INSERT INTO raw.C VALUES (:L, :M, :N, :P)*
>
> I am using parameters to fill in the 4 values.
>
> I have verified table C manually, and correct data is being stored in it.
>
> Several Google searches suggest Postgres should use multiple cores
> automatically. I've consulted with Npgsql's developer, and he didn't see how
> Npgsql itself could force Postgres to one core. (See
> http://pgfoundry.org/pipermail/npgsql-devel/2011-May/001123.html.)
>
> What can I do to improve this? Could I be inadvertently limiting Postgres
> to one core?
>
> Aren Cambre
>