Re: High Context-Switches on Linux 8.1.4 Server

Tom Lane wrote:
>> We're seeing an average of 30,000 context-switches a sec.  This problem
>> was much worse w/8.0 and got bearable with 8.1 but slowly resurfaced.
>
> Is this from LWLock or spinlock contention?  strace'ing a few backends
> could tell the difference: look to see how many select(0,...) you see
> compared to semop()s.  Also, how many of these compared to real work
> (such as read/write calls)?

Over a 20 second interval, I've got about 85 select()s and 6,230
semop()s. 2604 read()s vs 16 write()s.

> Do you have any long-running transactions, and if so does shutting
> them down help?  There's been some discussion about thrashing of the
> pg_subtrans buffers being a problem, and that's mainly a function of
> the age of the oldest open transaction.

Not long-running.  We do have a badly behaving legacy app that is
leaving some backends "idle in transaction"  They're gone pretty quickly
so I can't kill them fast enough, but running a pg_stat_activity  will
always show at least a handful.  Could this be contributing?

Based on the number of semop's we're getting it does look like
shared_memory may be getting thrased - any suggestions?  We did try
lowering shared_memory usage in half the previous day, but that did
little to help (it didn't make performance any worse and we still saw
the high context-switches, but it didn't make it any better either).

--
Sumbry][