Re: Seeking understanding of my "gc cr multi block request" waits

Richard Piasecki wrote:
> Hello everyone.
>
> I am the DBA for a three-node RAC database that is suffering from an intermittent performance problem. The Oracle release is 10.2.0.1 on RedHat linux.
> Periodically, a particular job that normally runs in less than a second takes several seconds to complete, sometimes requiring as much as 20 seconds.
> An examination of raw trace files created during the job execution has revealed that the wait event causing the slowdown is "gc cr multi block
> request". This particular wait event is not well-documented by Oracle, and I have found little information on it on the internet. I am posting this
> message to try to get confirmation of my understanding of the event from the other DBA's who may have experienced it.
>
> I'm thinking the cause of the wait event is several possibilities, and I want to get everyone's opinion on the subject. Mr. Gopalakrishnan's wonderful
> book on RAC does not mention "gc cr multi block request". It does, however, mention the event "gc cr request" as a "Place Holder" event.
>
> 1. If "gc cr multi block request" is also a place holder event (which would seem logical, implying multi-block IO), I would think I should never see
> it as a major event. I would think it should be substituted by one of the "gc*2-way" or "gc*3-way" events, as the Gopalakrishnan book implies. So, I'm
> confused as to why I am getting so much "gc cr multi block request" waits without any "2-way" or "3-way" waits.
>
> 2. On the other hand, if the "gc cr multi block request" event is not a place holder event, does it indicate the wait time experienced by the instance
> while trying to get a lock request from the master of the resource? A few of these waits are over a second, according to the raw trace file. That
> seems like an awfully long time to just get a lock request from the master. Any idea what could cause that?
>
> 3. Could the "gc cr multi block request" event be the total time spent obtaining the block from another instance? This would make sense given the
> length of some of these events and the total lack of any "2-way" or "3-way" events. But, that doesn't jive with the information in the Gopalakrishnan
> book or in any other resource I have read.
>
> 4. There is an Oracle bug #3951017, but it is supposedly fixed in 10gR2, and it causes a widespread slowdown, not the intermittent, specific to a
> single session, type of slowdown that I am experiencing. So, I doubt I have this problem.
>
>
> Can anyone confirm if this wait event is one of the four possibilities above or one I haven't mentioned?
>
>
>
> All these possibilities share the same set of possible solutions (except for #4), according to my understanding. Please correct me if my list of
> solutions is erroneous or incomplete.
>
> 1. Use application/data partitioning techniques to try to remove the inter-instance contention for the blocks in question.
> 2. Use jumbo frames on the interconnect (the current interconnect is configured with an MTU of 1500)
> 3. I think disk-IO is sometimes involved with these cache fusion operations to flush redo log buffers, so improving disk speed may help as well. We
> are currently on RAID-5 but plan to implement a series of RAID-1 arrays under ASM control in the near future.
> 4. Tweaking the number of LMS processes on the holding instance, but no CPU spikes have been noticed during these slowdowns, so I question having to
> do this.
>
>
>
> Does anyone have any thoughts, comments, suggestions, explanations or solutions that may help me to decipher the reasons for these waits and the means
> by which to eliminate them. Any help would be gratefully accepted. Thanks.
>
>
>
> --- Rich

Run these:

• Current block transfer statistics

col "AVG RECVD TIME (ms)" format 9999999.9 col inst_id format 9999
prompt GCS CURRENT BLOCKS

SELECT b1.inst_id, b2.value RECEIVED, b1.value "RECEIVE TIME", ((b1.value/b2.value)*10) "AVG RECEIVE TIME (ms)" FROM gv$sysstat b1, gv$sysstat b2
WHERE b1.name = 'global cache current block receive time' AND b2.name = 'global cache current blocks received' AND b1.inst_id = b2.inst_id;

• block contention measured by using block transfer time

col "AVG RECVD TIME (ms)" format 9999999.9 col inst_id format 9999

SELECT b1.inst_id, b2.value RECEIVED, b1.value "RECEIVE TIME", ((b1.value/b2.value)*10) "AVG RECEIVE TIME (ms)" FROM gv$sysstat b1, gv$sysstat b2
WHERE b1.name = 'global cache cr block receive time' AND b2.name = 'global cache cr blocks received' AND b1.inst_id = b2.inst_id;

They may point in the right direction.

-- 
Daniel A. Morgan
University of Washington
damorgan_at_x.washington.edu (replace x with u to respond)
Puget Sound Oracle Users Group
www.psoug.org