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parallel query and latch waits

Posted on 2001-06-07
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Last Modified: 2007-11-27
I have a table set up with parallel degree 4
the table is spread out across 4 physical disk mounts.

Everything run great except if I have a subquery
 
i.e.

select sum(impressions)
from fact_table --parallel degree 4
where user_id in (select user_id
                   from users
                   where  external_role_type IN('Named User', 'Registered User')
                 )

latch waits are 60% of non-idle wait time (from spotlight)
What can I do to reduce this latch problem?

If I remove parallel from the fact table I do not have a problem with latches?

Thanks Rob
0
Comment
Question by:rkatofiasc
8 Comments
 
LVL 2

Expert Comment

by:sefa
ID: 6168671
Latch waits may occur from many different cases.
Also there some types of latches.
Identify where the latch contention occurs.

According to your select and if I understood correctly that there is no problem when you remove parallel clause from statement; it may be Buffer Cache LRU latch .
Increase the parameter
DB_BLOCK_LRU_LATCHES
refer to documentation of your oracle version how to set this parameter.

0
 

Author Comment

by:rkatofiasc
ID: 6168741
It looks like the wait is on the cache buffers chains
stats look like this
latches 1,024
gets/sec 126,085
 %gets 91.64
sleeps /sec 15.47 %
%sleeps 100
misses/sec 4,540
%misses 99.94
0
 
LVL 2

Expert Comment

by:sefa
ID: 6171224
I think you have got these statistics from spotlight.
It will be better to use UTLBSTAT and UTLESTAT while running the query.
Can you post the follwing line values:

Total section - CPU used by this session,DBRW checkpoints,background checkpoints completed , background checkpoint started,physical read, db block gets, consistent gets

Wait events - buffer busy waits

Latch statistics - cache buffers chains ,cache buffers lru chains

Also in spotlight you may get some hints to solve the problem.

Please post some additional information as Oracle verison , platform, number of CPU's ?
Is there parallel server configuration ?
Is the system DSS or OLTP ?
Also some information about the tables in the query their usage statistics ?

0
 

Author Comment

by:rkatofiasc
ID: 6178943
STATSPACK report for

DB Name         DB Id    Instance     Inst Num Release     OPS Host
------------ ----------- ------------ -------- ----------- --- ------------
ACTP2          863268966 actp2               1 8.1.7.0.0   NO  cvdw2

Cache Sizes
~~~~~~~~~~~
           db_block_buffers:      36000          log_buffer:     163840
              db_block_size:       8192    shared_pool_size:  120000000

Load Profile
~~~~~~~~~~~~                            Per Second       Per Transaction
                                   ---------------       ---------------
                  Redo size:              5,037.11            816,012.00
              Logical reads:             66,650.98         10,797,459.00
              Block changes:                  6.96              1,128.00
             Physical reads:              1,321.01            214,004.00
            Physical writes:                 32.44              5,255.00
                 User calls:                  8.68              1,406.00
                     Parses:                  1.80                292.00
                Hard parses:                  0.11                 18.00
                      Sorts:                  2.75                446.00
                     Logons:                  0.25                 40.00
                   Executes:                  3.53                572.00
               Transactions:                  0.01

  % Blocks changed per Read:    0.01    Recursive Call %:   47.14
 Rollback per transaction %:    0.00       Rows per Sort: #######

Instance Efficiency Percentages (Target 100%)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            Buffer Nowait %:  100.00       Redo NoWait %:  100.00
            Buffer  Hit   %:   98.02    In-memory Sort %:   99.33
            Library Hit   %:   97.86        Soft Parse %:   93.84
         Execute to Parse %:   48.95         Latch Hit %:   93.13
Parse CPU to Parse Elapsd %:   75.00     % Non-Parse CPU:  100.00

 Shared Pool Statistics        Begin   End
                               ------  ------
             Memory Usage %:   92.00   92.29
    % SQL with executions>1:   77.61   77.49
  % Memory for SQL w/exec>1:   64.29   64.35

Top 5 Wait Events
~~~~~~~~~~~~~~~~~                                             Wait     % Total
Event                                               Waits  Time (cs)   Wt Time
-------------------------------------------- ------------ ------------ -------
PX Deq: Execution Msg                                 805       69,460   79.20
PX Deq: Execute Reply                               1,120        9,125   10.41
latch free                                          1,681        3,447    3.93
PX Deq: Table Q Normal                             11,620        3,020    3.44
PX Deq Credit: send blkd                              877          530     .60
          -------------------------------------------------------------
Wait Events for DB: ACTP2  Instance: actp2  Snaps: 7 -8
-> cs - centisecond -  100th of a second
-> ms - millisecond - 1000th of a second
-> ordered by wait time desc, waits desc (idle events last)

                                                                    Avg
                                                     Total Wait    wait  Waits
Event                               Waits   Timeouts  Time (cs)    (ms)   /txn
---------------------------- ------------ ---------- ----------- ------ ------
PX Deq: Execution Msg                 805        331      69,460    863  805.0
PX Deq: Execute Reply               1,120          2       9,125     81 ######
latch free                          1,681      1,650       3,447     21 ######
PX Deq: Table Q Normal             11,620          3       3,020      3 ######
PX Deq Credit: send blkd              877          2         530      6  877.0
direct path write                      30          0         498    166   30.0
direct path read                      623          0         474      8  623.0
db file parallel write                 23          0         381    166   23.0
db file sequential read               886          0         262      3  886.0
control file parallel write            52          0         223     43   52.0
PX Deq Credit: need buffer            770          0         127      2  770.0
PX Deq: Signal ACK                     42         16          74     18   42.0
log file parallel write                36          0          27      8   36.0
db file scattered read                 42          0          15      4   42.0
library cache pin                      26          0          11      4   26.0
refresh controlfile command            31          0           6      2   31.0
log file sync                           5          0           6     12    5.0
enqueue                                44          0           5      1   44.0
PX Deq: Parse Reply                    28          0           5      2   28.0
control file sequential read          245          0           1      0  245.0
PX Deq: Join ACK                       43          0           1      0   43.0
file open                              16          0           0      0   16.0
SQL*Net more data to client             9          0           0      0    9.0
PX Idle Wait                          439        395      85,974   1958  439.0
SQL*Net message from client         1,103          0      30,064    273 ######
SQL*Net message to client           1,103          0           0      0 ######
SQL*Net more data from clien            8          0           0      0    8.0
          -------------------------------------------------------------
Background Wait Events for DB: ACTP2  Instance: actp2  Snaps: 7 -8
-> ordered by wait time desc, waits desc (idle events last)

                                                                    Avg
                                                     Total Wait    wait  Waits
Event                               Waits   Timeouts  Time (cs)    (ms)   /txn
---------------------------- ------------ ---------- ----------- ------ ------
db file parallel write                 22          0         366    166   22.0
control file parallel write            52          0         223     43   52.0
log file parallel write                37          0          30      8   37.0
control file sequential read           93          0           1      0   93.0
LGWR wait for redo copy                 1          0           0      0    1.0
rdbms ipc message                     284        155      47,714   1680  284.0
smon timer                              1          1      30,721 ######    1.0
pmon timer                             52         52      16,016   3080   52.0
          -------------------------------------------------------------

Instance Activity Stats for DB: ACTP2  Instance: actp2  Snaps: 7 -8

Statistic                                    Total   per Second    per Trans
--------------------------------- ---------------- ------------ ------------
CPU used by this session                 1,298,542      8,015.7  1,298,542.0
CPU used when call started                  41,398        255.5     41,398.0
CR blocks created                               14          0.1         14.0
DBWR buffers scanned                           124          0.8        124.0
DBWR checkpoint buffers written                203          1.3        203.0
DBWR checkpoints                                 0          0.0          0.0
DBWR free buffers found                        124          0.8        124.0
DBWR lru scans                                   2          0.0          2.0
DBWR make free requests                          2          0.0          2.0
DBWR revisited being-written buff                0          0.0          0.0
DBWR summed scan depth                         124          0.8        124.0
DBWR transaction table writes                    8          0.1          8.0
DBWR undo block writes                          53          0.3         53.0
DDL statements parallelized                      0          0.0          0.0
DFO trees parallelized                           6          0.0          6.0
DML statements parallelized                      0          0.0          0.0
PX local messages recv'd                    14,751         91.1     14,751.0
PX local messages sent                      14,749         91.0     14,749.0
Parallel operations downgraded 1                 0          0.0          0.0
Parallel operations downgraded 25                0          0.0          0.0
Parallel operations downgraded 50                0          0.0          0.0
Parallel operations downgraded to                0          0.0          0.0
Parallel operations not downgrade                6          0.0          6.0
SQL*Net roundtrips to/from client            1,031          6.4      1,031.0
SQL*Net roundtrips to/from dblink                0          0.0          0.0
background checkpoints completed                 0          0.0          0.0
background checkpoints started                   0          0.0          0.0
background timeouts                            163          1.0        163.0
branch node splits                               0          0.0          0.0
buffer is not pinned count               7,100,956     43,833.1  7,100,956.0
buffer is pinned count                      49,994        308.6     49,994.0
bytes received via SQL*Net from c          133,248        822.5    133,248.0
bytes received via SQL*Net from d                0          0.0          0.0
bytes sent via SQL*Net to client           326,728      2,016.8    326,728.0
bytes sent via SQL*Net to dblink                 0          0.0          0.0
calls to get snapshot scn: kcmgss              853          5.3        853.0
calls to kcmgas                                 38          0.2         38.0
calls to kcmgcs                                 68          0.4         68.0
change write time                                2          0.0          2.0
cleanouts and rollbacks - consist                0          0.0          0.0
cleanouts only - consistent read                28          0.2         28.0
cluster key scan block gets                  5,543         34.2      5,543.0
cluster key scans                            2,237         13.8      2,237.0
commit cleanout failures: block l                6          0.0          6.0
commit cleanout failures: buffer                 1          0.0          1.0
commit cleanout failures: callbac                0          0.0          0.0
commit cleanout failures: cannot                 0          0.0          0.0
commit cleanouts                               165          1.0        165.0
commit cleanouts successfully com              158          1.0        158.0
consistent changes                              23          0.1         23.0
consistent gets                         10,998,589     67,892.5 ############
current blocks converted for CR
cursor authentications                           4          0.0          4.0
data blocks consistent reads - un               23          0.1         23.0
db block changes                             1,128          7.0      1,128.0
db block gets                                6,555         40.5      6,555.0
Instance Activity Stats for DB: ACTP2  Instance: actp2  Snaps: 7 -8

Statistic                                    Total   per Second    per Trans
--------------------------------- ---------------- ------------ ------------
deferred (CURRENT) block cleanout               59          0.4         59.0
dirty buffers inspected                          2          0.0          2.0
enqueue conversions                             84          0.5         84.0
enqueue releases                               282          1.7        282.0
enqueue requests                               310          1.9        310.0
enqueue timeouts                                20          0.1         20.0
enqueue waits                                    0          0.0          0.0
exchange deadlocks                               0          0.0          0.0
execute count                                  572          3.5        572.0
free buffer inspected                            3          0.0          3.0
free buffer requested                        1,488          9.2      1,488.0
hot buffers moved to head of LRU               500          3.1        500.0
immediate (CR) block cleanout app               28          0.2         28.0
immediate (CURRENT) block cleanou               91          0.6         91.0
index fast full scans (direct rea                0          0.0          0.0
index fast full scans (full)                     0          0.0          0.0
index fast full scans (rowid rang                0          0.0          0.0
leaf node splits                                19          0.1         19.0
logons cumulative                               40          0.3         40.0
logons current
messages received                               96          0.6         96.0
messages sent                                   96          0.6         96.0
no buffer to keep pinned count           3,576,035     22,074.3  3,576,035.0
no work - consistent read gets             686,246      4,236.1    686,246.0
opened cursors cumulative                      298          1.8        298.0
opened cursors current
parse count (hard)                              18          0.1         18.0
parse count (total)                            292          1.8        292.0
parse time cpu                                  48          0.3         48.0
parse time elapsed                              64          0.4         64.0
physical reads                             214,004      1,321.0    214,004.0
physical reads direct                      212,642      1,312.6    212,642.0
physical writes                              5,255         32.4      5,255.0
physical writes direct                       5,050         31.2      5,050.0
physical writes non checkpoint               5,133         31.7      5,133.0
pinned buffers inspected                         0          0.0          0.0
prefetched blocks                              343          2.1        343.0
prefetched blocks aged out before                0          0.0          0.0
process last non-idle time          24,526,027,324 ############ ############
queries parallelized                             6          0.0          6.0
recovery array reads                             0          0.0          0.0
recovery blocks read                             0          0.0          0.0
recursive calls                              1,254          7.7      1,254.0
recursive cpu usage                            404          2.5        404.0
redo blocks written                          1,661         10.3      1,661.0
redo buffer allocation retries                   0          0.0          0.0
redo entries                                   608          3.8        608.0
redo log space requests                          0          0.0          0.0
redo log space wait time                         0          0.0          0.0
redo ordering marks                              0          0.0          0.0
redo size                                  816,012      5,037.1    816,012.0
redo synch time                                  6          0.0          6.0
redo synch writes                                5          0.0          5.0
redo wastage                                 9,648         59.6      9,648.0
redo write time                                150          0.9        150.0
redo writer latching time                        0          0.0          0.0
Instance Activity Stats for DB: ACTP2  Instance: actp2  Snaps: 7 -8

Statistic                                    Total   per Second    per Trans
--------------------------------- ---------------- ------------ ------------
redo writes                                     38          0.2         38.0
rollback changes - undo records a                0          0.0          0.0
rollbacks only - consistent read                14          0.1         14.0
rows fetched via callback                3,373,018     20,821.1  3,373,018.0
session connect time                67,475,700,284 ############ ############
session logical reads                   10,797,459     66,651.0 ############
session pga memory                     922,197,028  5,692,574.3 ############
session pga memory max                 934,130,920  5,766,240.3 ############
session uga memory                      33,992,412    209,829.7 ############
session uga memory max                 202,048,820  1,247,214.9 ############
sorts (disk)                                     3          0.0          3.0
sorts (memory)                                 443          2.7        443.0
sorts (rows)                               987,201      6,093.8    987,201.0
summed dirty queue length                        0          0.0          0.0
switch current to new buffer
table fetch by rowid                     3,408,178     21,038.1  3,408,178.0
table fetch continued row                    2,710         16.7      2,710.0
table scan blocks gotten                   311,324      1,921.8    311,324.0
table scan rows gotten                  29,462,221    181,865.6 ############
table scans (direct read)                      228          1.4        228.0
table scans (long tables)                      229          1.4        229.0
table scans (rowid ranges)                     228          1.4        228.0
table scans (short tables)                     556          3.4        556.0
total file opens                                16          0.1         16.0
transaction rollbacks                            0          0.0          0.0
transaction tables consistent rea                0          0.0          0.0
transaction tables consistent rea                0          0.0          0.0
user calls                                   1,406          8.7      1,406.0
user commits                                     1          0.0          1.0
user rollbacks                                   0          0.0          0.0
write clones created in backgroun                0          0.0          0.0
write clones created in foregroun               29          0.2         29.0
          -------------------------------------------------------------

                                           Pct    Avg                 Pct
                                   Get          Get   Slps       NoWait NoWait
Latch Name                       Requests      Miss  /Miss     Requests   Miss
----------------------------- -------------- ------ ------ ------------ ------
Token Manager                             16    0.0                   0
active checkpoint queue latch             76    0.0                   0
archive control                           60    0.0                   0
cache buffer handles                     278    0.0                   0
cache buffers chains              11,295,272    7.1    0.0        1,933    0.1
cache buffers lru chain                  515    0.0               1,478    0.1
channel handle pool latch                 24    0.0                  24    0.0
channel operations parent lat             48    0.0                  24    0.0
checkpoint queue latch                 2,024    0.0                   0
dml lock allocation                       62    0.0                   0
enqueue hash chains                      722    1.1    0.0            0
enqueues                               1,169    0.2    0.0            0
error message lists                       86    3.5    0.0            0
event group latch                         24    0.0                   0
file number translation table              9    0.0                   0
job_queue_processes parameter              2    0.0                   0
ktm global data                            1    0.0                   0
latch wait list                            9    0.0                  11    9.1
library cache                         20,395    1.5    0.0            0
list of block allocation                  82    0.0                   0
loader state object freelist             464    0.2    0.0            0
longop free list                           6    0.0                   0
messages                                 729    0.0                   0
multiblock read objects                  200    0.0                   0
ncodef allocation latch                    2    0.0                   0
parallel query alloc buffer              844   12.7    0.0            0
parallel query stats                      58   37.9    0.2            0
process allocation                        24    0.0                  24    0.0
process group creation                    48    0.0                   0
process queue                            316    1.6    0.0            0
process queue reference              384,474    0.0    0.0       16,780    0.2
query server freelists                   474    2.1    0.0            0
redo allocation                          742    0.0                   0
redo writing                             328    0.0                   0
row cache objects                      6,356    0.0    0.0            0
sequence cache                            83    0.0                   0
session allocation                       804    3.6    0.6            0
session idle bit                       3,005    0.0                   0
session switching                          2    0.0                   0
shared pool                            2,182    0.4    0.0            0
sort extent pool                          16    0.0                   0
transaction allocation                   121    0.0                   0
transaction branch allocation              2    0.0                   0
undo global data                       1,182    0.3    0.0            0
user lock                                 96    0.0                   0
          -------------------------------------------------------------
Latch Sleep breakdown for DB: ACTP2  Instance: actp2  Snaps: 7 -8
-> ordered by misses desc

                                Get                                  Spin &
Latch Name                    Requests         Misses      Sleeps Sleeps 1->4
-------------------------- -------------- ----------- ----------- ------------
cache buffers chains           11,295,272     805,333       1,647 803775/1474/
                                                                  79/5/0
library cache                      20,395         312          12 301/10/1/0/0
session allocation                    804          29          17 16/9/4/0/0
parallel query stats                   58          22           5 17/5/0/0/0
          -------------------------------------------------------------
Latch Miss Sources for DB: ACTP2  Instance: actp2  Snaps: 7 -8
-> only latches with sleeps are shown
-> ordered by name, sleeps desc

                                                     NoWait             Waiter
Latch Name               Where                       Misses     Sleeps  Sleeps
------------------------ -------------------------- ------- ---------- -------
cache buffers chains     kcbgtcr: kslbegin                0      1,639   1,641
library cache            kglpnal: child: alloc spac       0          8       3
library cache            kglhdgn: child:                  0          3       5
library cache            kglhdgc: child:                  0          1       3
parallel query stats     kxfprst: KSLBEGIN                0          5       5
session allocation       ksuxds: KSUSFCLC not set         0         15      10
session allocation       ksursi                           0          2       7
          -------------------------------------------------------------

SGA Memory Summary for DB: ACTP2  Instance: actp2  Snaps: 7 -8

SGA regions                       Size in Bytes
------------------------------ ----------------
Database Buffers                    294,912,000
Fixed Size                               73,888
Redo Buffers                            180,224
Variable Size                       153,415,680
                               ----------------
sum                                 448,581,792


SGA breakdown difference for DB: ACTP2  Instance: actp2  Snaps: 7 -8

Pool        Name                        Begin value      End value  Difference
----------- ------------------------ -------------- -------------- -----------
java pool   free memory                  20,000,768     20,000,768           0
large pool  PX msg pool                   4,128,780      4,128,780           0
large pool  free memory                     299,988        299,988           0
shared pool Checkpoint queue                360,180        360,180           0
shared pool KGFF heap                       143,644        143,644           0
shared pool KGK heap                         21,788         21,788           0
shared pool KQLS heap                     1,962,980      1,962,980           0
shared pool OUTLNCACHE                       18,252         18,252           0
shared pool PL/SQL DIANA                  2,901,996      2,901,996           0
shared pool PL/SQL MPCODE                 1,465,456      1,477,404      11,948
shared pool PL/SQL SOURCE                   130,652        130,652           0
shared pool PLS non-lib hp                    2,096          2,096           0
shared pool PX subheap                      173,976        173,976           0
shared pool State objects                   310,064        310,064           0
shared pool db_block_buffers              4,896,000      4,896,000           0
shared pool db_block_hash_buckets           707,224        707,224           0
shared pool db_files                        362,296        362,296           0
shared pool dictionary cache             16,320,284     16,320,500         216
shared pool event statistics per ses        781,200        781,200           0
shared pool fixed allocation callbac          1,280          1,280           0
shared pool free memory                  10,321,804      9,941,132    -380,672
shared pool library cache                15,968,808     16,009,388      40,580
shared pool miscellaneous                 8,807,216      8,823,528      16,312
shared pool partitioning d                2,466,264      2,466,264           0
shared pool sessions                        485,100        485,100           0
shared pool sql area                     60,307,220     60,618,836     311,616
shared pool table columns                    22,864         22,864           0
shared pool table definiti                    4,984          4,984           0
shared pool temp table age                    2,296          2,296           0
shared pool trigger inform                      480            480           0
shared pool view columns d                    1,072          1,072           0
            db_block_buffers            294,912,000    294,912,000           0
            fixed_sga                        73,888         73,888           0
            log_buffer                      163,840        163,840           0
          -------------------------------------------------------------
init.ora Parameters for DB: ACTP2  Instance: actp2  Snaps: 7 -8

                                                                  End value
Parameter Name                Begin value                       (if different)
----------------------------- --------------------------------- --------------
background_dump_dest          /u01/app/oracle/admin/actp2/bdump
bitmap_merge_area_size        20480000
compatible                    8.1.7.0
control_files                 /dbf01/oradata/actp2/control01.ct
core_dump_dest                /u01/app/oracle/admin/actp2/cdump
db_block_buffers              36000
db_block_lru_latches          5
db_block_size                 8192
db_file_multiblock_read_count 32
db_files                      1000
db_name                       actp2
dml_locks                     500
global_names                  TRUE
hash_area_size                33554432
hash_multiblock_io_count      0
ifile                         /u01/app/oracle/admin/actp2/pfile
job_queue_interval            60
job_queue_processes           2
log_archive_dest              /orarch/actp2
log_archive_format            actp2_log%s.arc
log_buffer                    163840
log_checkpoint_interval       100000000
max_dump_file_size            10240
nls_date_format               DD-MON-RR
open_cursors                  500
parallel_automatic_tuning     TRUE
processes                     200
rollback_segments             r01, r02, r03, r04, rlg
shared_pool_size              120000000
sort_area_retained_size       16777216
sort_area_size                16777216
sort_multiblock_read_count    32
timed_statistics              TRUE
user_dump_dest                /u01/app/oracle/admin/actp2/udump
utl_file_dir                  *
          -------------------------------------------------------------
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Author Comment

by:rkatofiasc
ID: 6178954
This is a DSS system
the fact_table is a composite partition
hashed into 4 buckets. Each hast bucket resides on a different file mount. The table is set to parallel degree 4.
The users table is a dimension table noparallel.
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Accepted Solution

by:
sefa earned 100 total points
ID: 6190148
Sorry for long period.
As I see;
Everything looks fine except,

parallel query alloc buffer              844   12.7    0.0            0
parallel query stats                      58   37.9    0.2            0

Misses are very high beacuse of...

                              cache buffers chains           11,295,272     805,333       1,647 803775/1474/
                                                                 79/5/0

It is not clear but I think the above line is belonging to immediate latch and misses value is very high.

There is a heavy load over a single or a group of database block caused by paralelism. This causes cache buffers chains.

My suggestion is to set _db_block_hash_buckets parameter but it is a special parameter and must be offered from Oracle Support. So please please refer to them if possible or check metalink for the usage and other cautions about this parameter.

The other suggestion is change the spin count value but also it must not be done before talking Oracle Support again.

Regards.
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by:Moondancer
ID: 7033734
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Expert Comment

by:Mindphaser
ID: 7043357
Force accepted

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