Tim, I should have mentioned that and you're exactly right in that AMM can
be a problem. We don't use either ASMM or AMM and manage the pools
manually because of the resize operation penalties if the floors aren't
setup correctly.
So, no there are no resize operations except where I manually added 5GB of
free space to the shared pool just to see if it improved the latch
contention. It actually seemed to make it worse which leads me to believe
the size of the shared_pool is causing thrashing.
Chris
On Thu, May 31, 2018 at 1:17 PM, Tim Gorman <tim.evdbt@xxxxxxxxx> wrote:
Chris,
Is auto SGA management enabled? That is, is either SGA_TARGET > 0 or
MEMORY_TARGET > 0?
If so, please consider examining the log of auto SGA management in
V$SGA_RESIZE_OPS view?
As described in my presentation
<http://evdbt.com/download/presentation-rdbms-forensics-troubleshooting-using-ash/>
and white paper
<http://evdbt.com/download/paper-rdbms-forensics-troubleshooting-using-ash/>
on "RDBMS Forensics: Troubleshooting Using ASH", there is a situation in
some applications, particularly analytic or data mart applications, where
huge complex SQL or PL/SQL cursors cause rapid expansion of the Shared Pool
during parse, and then equally rapid expansion of the Buffer Cache during
subsequent execution and fetch. Adjusting the size of the Shared Pool
requires the Shared Pool latch, so thrashing space between the Buffer Cache
and the Shared Pool is almost certain to result in that latch becoming a
bottleneck, as described in the presentation.
Jonathan Lewis also discusses this phenomenon in a related posting HERE
<https://jonathanlewis.wordpress.com/2007/04/16/sga-resizing/>.
Anyway, my solution for the thrashing was to determine a "floor" value for
the parameter SHARED_POOL_SIZE based on the historic average size of
"__shared_pool_size" as recorded in the AWR view DBA_HIST_PARAMETERS, and
set SHARED_POOL_SIZE to that "floor" value, preventing the Shared Pool from
being made smaller than that value. The basic idea is to permit auto SGA
management to adjust occasionally, but not thrash.
Please let us know what you find, and I hope this helps?
Thanks!
-Tim
On 5/31/18 11:39, Dominic Brooks wrote:
Am I out of touch or is that an exceedingly large shared pool?
Sent from my iPhone
On 31 May 2018, at 18:05, Chris Taylor <christopherdtaylor1994@xxxxxxxxx>
wrote:
version 12.1
Linux x86-64
Event Waits Total Wait Time (sec) Wait Avg(ms) % DB time Wait Class
latch: shared pool 402,269 103.4K 256.95 29.5 Concurrency
DB CPU 100.3K 28.6
latch free 362,927 42.2K 116.26 12.0 Other
Background:
We've got a database with 4TB of RAM and a 260GB shared pool. Everything
was running normally until 2 nights ago when a regularly scheduled job that
does TONS of DDL suddenly blew up the shared pool.
Trying to query x$ksmsp for shared pool sizes is also causing huge amounts
of latch waits.
When these jobs aren't running, the db is working normally. Starting just
one of the jobs brings back the shared pool latching problem immediately.
I'm thinking the shared pool is heavily fragmented but am looking for
additional queries or views I can use to diagnose shared pool problems.
We're not running into any shared pool errors as far as memory allocations.
This query hangs and causes huge contention:
select ksmchcls "ChnkClass",
sum(ksmchsiz) "SumChunkTypeMem",
Max(ksmchsiz) "LargstChkofThisTyp",
count(1) "NumOfChksThisType",
round((sum(ksmchsiz)/tot_sp_mem.totspmem),2)*100||'%' "PctTotSPMem"
from x$ksmsp,
(select sum(ksmchsiz) TotSPMem from x$ksmsp) tot_sp_mem
group by ksmchcls, tot_sp_mem.TotSPMem
order by 5;
This query runs fine and seems to indicate there is plenty of reserve
space:
select free_space, round(avg_free_size,2) as
avg_free_size,free_count,max_free_size,used_space,
round(avg_used_size,2) as avg_used_size, used_count, max_used_size,
requests, request_misses, last_miss_size,
request_failures, last_failure_size, aborted_request_threshold,
aborted_requests
from v$shared_pool_reserved
/
FREE_SPACE AVG_FREE_SIZE FREE_COUNT MAX_FREE_SIZE USED_SPACE AVG_USED_SIZE
USED_COUNT MAX_USED_SIZE REQUESTS REQUEST_MISSES LAST_MISS_SIZE
REQUEST_FAILURES LAST_FAILURE_SIZE ABORTED_REQUEST_THRESHOLD
ABORTED_REQUESTS
13435398168 236538.7 16023 27316040 319665968 5627.92 40777 1048552
10903867 0 0 0 0 2147483647 0
The Jobs in question do PARTITION EXCHANGES for many, many sites with temp
tables that are built as part of the job.
The DB has been up since: 04/03/2018 07:12:23 and the last 2 days are the
first time this has been seen and is repeatable by restarting any one of
the jobs (there are 8 total)
Thinking about flushing the shared pool (or restarting db off hours) as a
test to see if it resolves the issue (like throwing a grenade into a
anthill resolves a problem with ants...not the best approach but might be
effective)
Any thoughts/comments/insults?
Thanks,
Chris
