CPU queuing and library cache: mutex X waits

I had a funny thing happen today.  I had recently participated in a forum thread about  library cache: mutex X waits.  In that discussion the library cache: mutex X waits were on an AWR report but when we dug deeper we realized it was because the database server’s CPU was totally pegged out and overloaded.

Today, I saw the same thing on one of my own servers.  Here are the top 5 events:

Event Waits Time(s) Avg wait (ms) % DB time Wait Class
DB CPU 53,149 8.67
db file sequential read 3,188,834 21,758 7 3.55 User I/O
library cache: mutex X 4,260 8,943 2099 1.46 Concurrency
log file sync 1,226,545 4,562 4 0.74 Commit
latch: cache buffers chains 21,097 4,281 203 0.70 Concurrency

Notice how small a percentage of the DB time these top events are.  This is because most of the time is spent waiting on the CPU queue.  Here was the load when I checked it:

$ uptime
11:35am  up 268 days, 13:27,  4 users,  load average: 14.40, 14.52, 14.76

This server has 2 cores so a load of 14 means a lot of queuing.

The operating system statistics on the AWR report show the same thing – a load of 15 and a lot of OS_CPU_WAIT_TIME.

Operating System Statistics

  • *TIME statistic values are diffed. All others display actual values. End Value is displayed if different
  • ordered by statistic type (CPU Use, Virtual Memory, Hardware Config), Name
Statistic Value End Value
AVG_BUSY_TIME 3,925,986
AVG_IDLE_TIME 1,115,182
AVG_SYS_TIME 383,368
AVG_USER_TIME 3,541,505
BUSY_TIME 7,853,118
IDLE_TIME 2,231,218
IOWAIT_TIME 1,214,686
SYS_TIME 768,403
USER_TIME 7,084,715
LOAD 1 15
OS_CPU_WAIT_TIME 17,375,799,211,900
VM_IN_BYTES 442,109,952
VM_OUT_BYTES 540,237,824
TCP_RECEIVE_SIZE_MAX 1,073,725,440
TCP_SEND_SIZE_MAX 2,147,483,647

So, I thought I would post this so people could see what an AWR report looks like when the cpu it totally pegged out and queuing big time.  Note that the AWR report was from 1 am to 3 pm so the starting load was low but the ending load during the afternoon was at 15.

– Bobby

About Bobby

I live in Chandler, Arizona with my wife and three daughters. I work for US Foods, the second largest food distribution company in the United States. I have worked in the Information Technology field since 1989. I have a passion for Oracle database performance tuning because I enjoy challenging technical problems that require an understanding of computer science. I enjoy communicating with people about my work.
This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply