Server resources

Question

Hi,
please could you help my understanding of the various outputs pasted below. I am trying to check resources on my database server(which already has 9 database instances) before adding another database instance:
i) Total shared memory used up by existing database instances is roughly *4321M* from outputs pasted below. The value of max shared memory in /etc/system is however approx *4294M* which is less than what the existing instances are using. Does that suggest that already we have exceeded the allowed shared memory limit? And therefore the existing databases run the risk of poor performance?
ii) How does the total value of shared memory relate to the value of total real memory (or RAM) showed by top i.e 8192M . Does it mean that we can increase the value of shminfo_shmmax on the server within the value of available free memory 1473M shown by the top command?
iii) I have researched the explanation of the vmstat output but don't quite understand it, would someone be able to explain it very simply?
Show sga
Total System Global Area 839548928 bytes
Total System Global Area 377487360 bytes
Total System Global Area 377487360 bytes
Total System Global Area 377487360 bytes
Total System Global Area 377487360 bytes
Total System Global Area 377487360 bytes
Total System Global Area 839548928 bytes
Total System Global Area 377487360 bytes
Total System Global Area 377487360 bytes
Total for the 9 databases = 4321509376 bytes
From ipcs -bpm
IPC status from <running system> as of Thu May 13 14:34:41 BST 2010
SEGSZ
Shared Memory:
843071488
377495552
377495552
377495552
843071488
377495552
377495552
377495552
377495552
*4328611840 -----Total*
*/etc/system*
set shmsys:shminfo_shmmax=4294967295
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=100
top
load averages: 0.20, 0.18, 0.18 16:52:18
332 processes: 331 sleeping, 1 on cpu
CPU states: 94.1% idle, 2.8% user, 1.5% kernel, 1.5% iowait, 0.0% swap
Memory: 8192M real, 1473M free, 5629M swap in use, 8664M swap free
vmstat 3 5
kthr memory page disk faults cpu
r b w swap free re mf pi po fr de sr m0 m1 m2 m3 in sy cs us sy id
0 0 0 9409000 2023528 602 205 3413 2 2 0 1 0 0 0 0 374 1051 808 5 2 93
0 0 0 8862120 1493344 19 21 0 0 0 0 0 0 0 0 0 386 567 1005 0 0 100
0 0 0 8862120 1493344 0 0 0 0 0 0 0 0 0 0 0 377 498 993 0 0 100
0 0 0 8850984 1484040 161 3019 0 0 0 0 0 0 0 0 0 478 3935 1183 4 3 93
0 0 0 8844800 1479592 0 0 0 0 0 0 0 0 0 0 0 391 458 1030 0 2 98
Thank you.

Answer 1

The Execution Plan is determined before the query begins executing.
The query cannot stop and restart with a new execution plan.
However, if the same query is executed more than once, even inside a loop
in a PLSQL block, it may get re-parsed and may acquire a different Execution
Plan.
PGA_AGGREGATE_TARGET is a "guide" for Oracle to dtermine the available
memory for an SQL (it looks at PGA memory currently used by other concurrently
executing SQLs). However, considering the dynamics of multiple processes
co-existing on a server, this is not and cannot be a hard limit so the actual
PGA in use at any time can fluctuate.
(consider, for example that other, Non-Oracle, processess might start up and
take memory and Oracle isn't aware of those processes "stealing" available
memory. Or consider a bug in sort / hash algorithms which cause an Oracle
process to take (or request from the OS) much more memory than it "should",
resulting in ORA-4030s !)
However, as a general rule, Oracle uses PGA_AGGREGATE_TARGET
and also provides an advisory view that you can query. (Oracle periodically
collects statistics about PGA usage and SGA usage and updates the
respective advisory views).