Re: HW - contention
- From: Tanel Põder <tanel.poder.003@xxxxxxx>
- To: <att755@xxxxxxxxxxx>, <Oracle-L@xxxxxxxxxxxxx>
- Date: Tue, 17 Jan 2006 14:07:25 -0600
Hi,
10gR2, RHEL4, OCFS2, DL585 AMD Opteron 64bit
Non-RAC on OCFS2?
Im running a benchmark on the above hardware using hammerora (GPL TPC like
tool I saw a presention on at UK OUG 2005). All runs very fast to begin
with but then slows to a crawl. AWR/ADDM shows the reason for the slow
down to be HWM contention presumably do to a large number of multiple
concurrent inserts happening (once the initial HWM is breeched). I did
some googling
Can you verify it by setting 10046 trace on one of your inserting sessions?
and found lots of references to the _bump_highwatre_mark_count parameter
to "fix" this. My question is as I am using ASSM tablspaces surely this no
longer applies - there are no freelists/freelist groups on such
tablespaces
That parameter should be obsolete for ASSM (but you never know..)
if my understanding is correct. If this is so then why am I getting this
contention and how can it be fixed when using ASSM. I will try the
parameter next time I hit the issue.
How big extents you have? ASSM high-HWM extensions are not done over extent
boundaries IIRC, thus with small extents you'd have lots of extensions to
do, each requiring HW-enqueue get. ASSM relieves HW-enqueue contention by
increasing HHWM in large sizes (depending on extent and current segment
size), so less HW operations have to be done.
The LHWM extensions can be done without having HW-enqueue as they involve
only changing few records in a BMB block and the BMB is pinned for that
exclusively anyway. The actual batch formatting of datablocks is done while
having FB enqueue, allowing finer granularity of locking block ranges in
case of multiple parallel inserts.
If extent sizes are large enough, ASSM should be able to cope with heavy
concurrent inserting well. There are few ASSM HWM related hidden parameters
introduced in 9.2 and 10.2 but as I haven't tested those, I'll not mention
them ;)
Tanel.
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