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Re: Reasoning behind process instead of thread based

From: Martijn van Oosterhout <kleptog(at)svana(dot)org>
To: Thomas Hallgren <thhal(at)mailblocks(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Reasoning behind process instead of thread based
Date: 2004-10-27 20:36:16
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On Wed, Oct 27, 2004 at 10:07:48PM +0200, Thomas Hallgren wrote:
> >Threaded servers have one main advantate:
> >Threads are lightweight processes and starting a new thread is faster
> >than starting a new executable.
> >
> A few more from the top of my head:

A lot of these advantages are due to sharing an address space, right?
Well, the processes in PostgreSQL share address space, just not *all*
of it. They communicate via this shared memory.

> 1. Threads communicate much faster than processes (applies to locking 
> and parallel query processing).
> 2. All threads in a process can share a common set of optimized query plans.

PostgreSQL could do this too, but I don't think anyone's looked into
sharing query plans, probably quite difficult.

> 3. All threads can share lots of data cached in memory (static but 
> frequently accessed tables etc.).

Table data is already shared. If two backends are manipulating the same
table, they can lock directly via shared memory rather than some OS

> 4. In environments built using garbage collection, all threads can share 
> the same heap of garbage collected data.
> 5. A multi-threaded system can apply in-memory heuristics for self 
> adjusting heaps and other optimizations.
> 6. And lastly, my favorite; a multi-threaded system can be easily 
> integrated with, and make full use of, a multi-threaded virtual 
> execution environment such as a Java VM.

I can't really comment on these.

I think PostgreSQL has nicely combined the benefits of shared memory
with the robustness of multiple processes...
Martijn van Oosterhout   <kleptog(at)svana(dot)org>
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

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