Richard Huxton wrote:
[EMAIL PROTECTED] wrote:
Hi,
We've migrated one of our servers from pg 7.4 to 8.1 and from times
to times (4 hours) the server start doing a lot of context switching
and all transactions become very slow.
The average context switching for this server as vmstat shows is 1
but when the problem occurs it goes to 250000.
You'll tend to see it when you have multiple clients and most queries
can use RAM rather than disk I/O. My understanding of what happens is
that PG requests data from RAM - it's not in cache so the process gets
suspended to wait. The next process does the same, with the same result.
You end up with lots of processes all fighting over what data is in
the cache and no-one gets much work done.
Does this happen also with 8.0, or is specific to 8.1 ?
I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server
when I raise `shared_buffers' from 8192 to 40000.
I would expect an increase in tps/concurrent clients, but I see an average
performance below a certain threshold of users, and when concurrent users
get above that level, performance starts to drop, no matter what I do.
Server logs and io/vm statistics seem to indicate that there is little
or no disk activity but machine loads increases to 7.0/8.0.
After some minutes, the problem goes away, and performance returns
to acceptable levels.
When the load increases, *random* database queries show this "slowness",
even if they are perfectly planned and indexed.
Is there anything we can do?
--
Cosimo
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