All,
I realize the excessive-context-switching-on-xeon issue has been
discussed at length in the past, but I wanted to follow up and verify my
conclusion from those discussions:
On a 2-way or 4-way Xeon box, there is no way to avoid excessive
(30,000-60,000 per second) context switches when using PostgreSQL 7.4.5
to query a data set small enough to fit into main memory under a
significant load.
I am experiencing said symptom on two different dual-Xeon boxes, both
Dells with ServerWorks chipsets, running the latest RH9 and RHEL3
kernels, respectively. The databases are 90% read, 10% write, and are
small enough to fit entirely into main memory, between pg shared buffers
and kernel buffers.
We recently invested in an solid-state storage device
(http://www.superssd.com/products/ramsan-320/) to help write
performance. Our entire pg data directory is stored on it. Regrettably
(and in retrospect, unsurprisingly) we found that opening up the I/O
bottleneck does little for write performance when the server is under
load, due to the bottleneck created by excessive context switching. Is
the only solution then to move to a different SMP architecture such as
Itanium 2 or Opteron? If so, should we expect to see an additional
benefit from running PostgreSQL on a 64-bit architecture, versus 32-bit,
context switching aside? Alternatively, are there good 32-bit SMP
architectures to consider other than Xeon, given the high cost of
Itanium 2 and Opteron systems?
More generally, how have others scaled "up" their PostgreSQL
environments? We will eventually have to invent some "outward"
scalability within the logic of our application (e.g. do read-only
transactions against a pool of Slony-I subscribers), but in the short
term we still have an urgent need to scale upward. Thoughts? General wisdom?
Best Regards,
Bill Montgomery
