PostgreSQL 7.1 on SMP: FreeBSD 4.3 || Linux 2.4.x? - Mailing list pgsql-general

I have a dual-CPU (PIII 600MHz), 512MB, IDE-HD box.  A couple of months ago I was using Linux 2.4.3 on this box and was
gettinggood performance on a custom network-server/db application I am running.  Basically, I am getting constant
networktraffic to the machine, storing transmitted information in memory until some threshold size, and then forking
offa process to transfer the shared memory to PostgreSQL database.  I saw better performance doing this than dumping to
thedatabase right away as the data was coming in.  The application takes advantage of optimized SQL statements, vacuum
analyze,indexes, persistent db connections, bound values, delayed transaction commits, etc.. 

Recently, I moved the box to FreeBSD 4.3.  There were no hardware changes to the SMP box, no changes to the application
software,and no changes to the database schema or configuration (no fsync, buffers, shared memory, etc).  Soft updates
areon for the FFS, and under Linux I was using the standard ext fs (not a journaling one).   

However, I am seeing a degredation in performance. Sadly, I didn't take detailed measurements under Linux 2.4.3, but I
amseeing the postgres processes that do the insert/updates (on the order of 1000 rows every couple of minutes) take a
lotlonger than before.  With Linux I had been seeing the postgres process that did those inserts/updates last for about
20seconds (say 50-100tps).  With FreeBSD I am seeing the process last for 1.5 - 2 minutes (say approx. 10-20tps). 

Whew.  Now for the question part.  I understand that SMP support under Linux is a lot more fine-grained than FreeBSD in
the2.4.x kernels.  Can this be causing my database performance degredation?  When the postgres process is writing those
thousandsof rows to various tables, I am still getting a constant flow of network traffic coming into the ethernet
ports. The amount of data transfer is very low per connection, but the frequency of connections is high (for example,
100connections per second, but 100 bytes per connection). I don't have a clear understanding of kernel space issues,
butcould the net traffic be generating interrupts that cause one of the CPUs to lock in kernel space, effectively
prohibitingthe other CPU (or other processes) from concurrently working the postgres stuff?  My understanding is that
underFreeBSD, if a kernel system call happens on one CPU, it gets a big kernel lock and the other CPU spins and
waits...? Whereas under Linux, might both processes (postgres and network server) be truly working concurrently?
Ideally,it would be nice to bind CPU0 to handle the network stuff while CPU1 could work on the disk I/O for the
postgresstuff.  I know it's not as simple as that, but is Linux 2.4.x better suited to doing this application than
FreeBSD4.x? 

On a side note, my intention is not to instigate the *BSD vs. Linux arguments.  I have been very pleased with FreeBSD
(security,minimalism, extremely robust & stable, etc.).  However, I have a very specific application where their SMP
differencesmight be exploited for better db performance. 

Thanks for any info/help you might be able to provide.

--Ben

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