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Thursday, June 23, 2016

Oracle ADDM shows Virtual Memory Paging on 10gR2 11gR2

 

Problem description

I’ve recently notices a common issue on Oracle Enterprise Linux 5.x with Oracle database 10gR2 and 11gR2. The Oracle ADDM Performance Analysis showed the following alert as a top event  in both OEM console – Virtual Memory Paging and in the ADDM report – Significant virtual memory paging was detected on the host operating system.

Problem Analysis

From the first look I thought this was a virtual memory issue on the boxes but on the other hand I could not find any evidence at OS level of any problem with virtual memory paging. Additionally I was surprised that the issue occurred at the same time on different servers and Oracle versions. So, below I did some checks to prove there was no memory issue on these Linux boxes.

$ vmstat 5 5
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
0  1 471576 1246004 1508736 52959096    0    0   395    77    0    0  3  2 92  3  0
5  1 471576 1241264 1508736 52959104    0    0  2157   478 3902 12026  2  1 94  3  0
0  0 471576 1258284 1508736 52959120    0    0  2013   914 3954 12746  2  1 94  3  0
1  0 471576 1258532 1508736 52959140    0    0  1415  3188 5151 14083  2  1 95  2  0
6    2 471576 1216776 1508736 52959344    0    0  3210  1384 4901 13155  3  1 92  4  0

* There is no activity with respect to swap (si/so). It is 0 only.

$ sar -B
Linux 2.6.18-164.el5 (host)    03/09/2012
12:00:01 AM  pgpgin/s pgpgout/s   fault/s  majflt/s
12:10:01 AM    337.13    908.20  12189.45      0.01
12:20:01 AM   2764.88   2947.61  14416.18      0.00
12:30:02 AM   2947.02   1580.62  14432.52      0.00
12:40:01 AM   1019.20    703.31  13251.20      0.00


* Paging is usual on Linux systems and you should be fine as long as you have no considerable majflt/s.

$ free -m
total       used       free     shared    buffers     cached
Mem:         64459      63368       1090          0       1473      51724
-/+ buffers/cache:      10170      54288
Swap:        25023        460      24563

* Swap file is almost not used.
Linux, like many other versions of UNIX, no longer swaps an entire process from memory to disk. The virtual memory system is said to be a paging system, where parts of a process are paged to and from disk. When a process starts up, at least some pages of information are brought into memory: a page of text (code), a page of data, and an empty/zero page. Once the instructions or data the process is using are no longer contained in those pages, the virtual memory system will page-in more data and code as is necessary. This is known as a demand-paged virtual memory system. Page-ins is a normal feature of the system. Considerable page-outs happen as a consequence of not having enough memory.

Cause and Solution

If indeed no paging occurs on the OS level, there is no need for concern and the ADDM or OEM warning can safely be ignored on Oracle database 10gR2 (10.2.0.5) and 11gR2. In fact the real cause of the issue was the incorrect paging statistics showed through V$OSSTAT dictionary view.
If you want to suppress the wrong ADDM / OEM finding you have to apply following Oracle bug fixes:
for 10.2.0.5 – apply patch for Bug 12347332 for your Operating System.
for 11.2. –  apply Patch 10220118

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