Oracle, Ultra SPARC T4 & High SYS CPU Usage

Suspect this is not exclusive to T4, servers with lots of RAM may be susceptible.

Customer moved to a new T4 based server and started having major problems once more than a few dozen users were connected to the system.

Database was largely ok, some excessive redo which we addressed, but after that statspack reports showing top wait event was 95%+ CPU Time (normally a good sign).

Unfortunately performance of the whole server was sluggish and the application bordering on the unusable, and whilst the server had 96GB RAM and 96 cores it was struggling badly when still 60% idle.

Only clue was that the CPU usage reported by sar and mpstat was SYS rather than user – so something was going wrong at the OS level.

mpstat suggested that something was spinning waiting on a mutex (high counts on smtx were correlated with sys cpu usage).

Using DTrace to understand mpstat output [ID 1278725.1]

lockstat indicated that the caller functions where it was spending most time were page_trylock_contig_pages & page_trylock.

A Primer On Lockstat [ID 1005868.1]

That led us to

E-PERF: Poor Performance on Solaris Server Possibly Associated with PSRUNRMT [ID 663487.1]

which suggests

it appears that page_coalescing, caused by the Sol10 LPOOB (Large Page Out Of Box) feature, may be contributing to the issue

So let’s get that turned off for a start – it’s a kernel setting so needs to go in /etc/system but you can also change on the fly with

echo 'pg_contig_disable/W 1' | mdb -kw

Things improved for half an hour or so and then started to deteriorate in the same way again, but this time lockstat was telling us the main culprit was page_freelist_lock.

Which took us to

page_freelist_coalesce() could cause system performance issues [ID 1486130.1]

which suggested that we should also disable

echo 'mpss_coalesce_disable/W 1' | mdb -kw

with those two changes made on the fly (and recorded in /etc/system for the next reboot)

* Oracle memory issues
set mpss_coalesce_disable=1
set pg_contig_disable=1

the run queue has dropped from over 30 to around 3, server is 95% idle, sys cpu usage has all but disappeared and performance is now as good with 400 users as it was with 40.

Advertisements