Redo threads vs mutiple redo copies

Hi,
Good Day!
One point need to discuss here:
what is the right way to configure redo logs in RAC Instances:
option-1: One Redo Thread Per RAC Instance having multiple redo logfile members sharing the same thread (One ACTIVE Group at a time)
e.g: Thread # 1
Total Log Groups per RAC Instance: 4
LogFile Members per RAC Instance: 3
Option-2: Multiple Redo Threads Per RAC Instance having sigle redo logfile members per log group (multiple ACTIVE Groups at a time)
e.g: Thread# 1 & Thread# 2 for each RAC Instance
Total Log Groups per RAC Instance: 4
LogFile Members per RAC Instance: 1
I have always follows technique 1 (Option-1), if any one can highlight any benifit gain from option-2 over option-1.
pl. share the cons/pros of above two options.
Thanks
Regards,
X

Hi,
I have added 3 groups, with 2 members each on different slices.
group # 14/15/16 were added.SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS
7 2 1909491 52428800 1 NO CURRENT
8 2 1909490 52428800 1 YES ACTIVE
14 1 1834190 4294967296 2 YES ACTIVE
15 1 1834191 4294967296 2 NO CURRENT
16 1 1834189 4294967296 2 YES ACTIVE
how and why oracle internally using previous groups 7 and 8 as in alert log - log switching entries log groups 7/8 are not mentioned.
alert contents;
===============
Thu Jul 28 17:48:06 2011
Thread 1 advanced to log sequence 1834200 (LGWR switch)
Current log# 15 seq# 1834200 mem# 0: /cfs/oracle/redo1/admprod/redo15a.log
Current log# 15 seq# 1834200 mem# 1: /cfs/oracle/redo2/admprod/redo15b.log
Thu Jul 28 17:48:37 2011
Thread 1 advanced to log sequence 1834201 (LGWR switch)
Current log# 16 seq# 1834201 mem# 0: /cfs/oracle/redo1/admprod/redo16a.log
Current log# 16 seq# 1834201 mem# 1: /cfs/oracle/redo2/admprod/redo16b.log
Thu Jul 28 17:48:47 2011
Thread 1 advanced to log sequence 1834202 (LGWR switch)
Current log# 14 seq# 1834202 mem# 0: /cfs/oracle/redo1/admprod/redo14a.log
Current log# 14 seq# 1834202 mem# 1: /cfs/oracle/redo2/admprod/redo14b.log
++++ pl. note archive files generating were not of size 4G (equal to new loggroup file members size0, it's 50M (equeal to loggroup 7/8 sizes) .. pl. comment.
why upon logswitches of loggroups 14/15/16 archive logs generated of 50M and log switching time did'nt increase upon adding big loggroups... it's confirmed that oracle is using previous 50M loggroups in logswitching of newly added groups and generating the archive log of 50M instead of 4G.
i need to drop 50M groups 7/8.
can any one help to share the workaround plus help to understand that confusing oracle behaviour?
Thanks...
Regards,
X
Edited by: Ahmad on Jul 28, 2011 6:17 PM

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Please remember to mark the replies as answers if they help, and unmark the answers if they provide no help. If you have feedback for TechNet Support, contact [email protected]

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Use of volatile modifier

can any one explain me the use of volatile modifier

What does volatile do?
This is probably best explained by comparing the effects that volatile and synchronized have on a method. volatile is a field modifier, while synchronized modifies code blocks and methods. So we can specify three variations of a simple accessors using those two keywords:
int i1; int geti1() {return i1;}
volatile int i2; int geti2() {return i2;}
int i3; synchronized int geti3() {return i3;}
geti1() accesses the value currently stored in i1 in the current thread. Threads can have local copies of variables, and the data does not have to be the same as the data held in other threads. In particular, another thread may have updated i1 in it's thread, but the value in the current thread could be different from that updated value. In fact Java has the idea of a "main" memory, and this is the memory that holds the current "correct" value for variables. Threads can have their own copy of data for variables, and the thread copy can be different from the "main" memory. So in fact, it is possible for the "main" memory to have a value of 1 for i1, for thread1 to have a value of 2 for i1 and for thread2 to have a value of 3 for i1 if thread1 and thread2 have both updated i1 but those updated value has not yet been propagated to "main" memory or other threads.
On the other hand, geti2() effectively accesses the value of i2 from "main" memory. A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Effectively, a variable declared volatile must have it's data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Of course, it is likely that volatile variables have a higher access and update overhead than "plain" variables, since the reason threads can have their own copy of data is for better efficiency.
Well if volatile already synchronizes data across threads, what is synchronized for? Well there are two differences. Firstly synchronized obtains and releases locks on monitors which can force only one thread at a time to execute a code block, if both threads use the same monitor (effectively the same object lock). That's the fairly well known aspect to synchronized. But synchronized also synchronizes memory. In fact synchronized synchronizes the whole of thread memory with "main" memory. So executing geti3() does the following:
1. The thread acquires the lock on the monitor for object this (assuming the monitor is unlocked, otherwise the thread waits until the monitor is unlocked).
2. The thread memory flushes all its variables, i.e. it has all of its variables effectively read from "main" memory (JVMs can use dirty sets to optimize this so that only "dirty" variables are flushed, but conceptually this is the same. See section 17.9 of the Java language specification).
3. The code block is executed (in this case setting the return value to the current value of i3, which may have just been reset from "main" memory).
4. (Any changes to variables would normally now be written out to "main" memory, but for geti3() we have no changes.)
5. The thread releases the lock on the monitor for object this.
So where volatile only synchronizes the value of one variable between thread memory and "main" memory, synchronized synchronizes the value of all variables between thread memory and "main" memory, and locks and releases a monitor to boot. Clearly synchronized is likely to have more overhead than volatile.
I got the above information from the below link......
http://www.javaperformancetuning.com/news/qotm030.shtml
and also we can describe the volatile modifier as
Volatile� A volatile modifier is mainly used in multiple threads. Java allows threads can keep private working copies of the shared variables(caches).These working copies need be updated with the master copies in the main memory.
But possible of data get messed up. To avoid this data corruption use volatile modifier or synchronized .volatile means everything done in the main memory only not in the private working copies (caches).( Volatile primitives cannot be cached ).
So volatile guarantees that any thread will read most recently written value.Because they all in the main memory not in the cache�Also volatile fields can be slower than non volatile.Because they are in main memory not in the cache.But volatile is useful to avoid concurrency problem.
This above information i got from
http://cephas.net/blog/2003/02/17/using-the-volatile-keyword-in-java/
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With Regards,
M.Sudheer.

'synchronised' doesn't seem to work in my code....

I was working on the following code given in Java in 60 minutes a day.
In the program, the final value of balance has to be $1000. But I'm getting $900 as final output even though the methods are declared as synchronized
I put signature of every class in bold for the sake of clarity. Thanks fro the help.
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*}*

First, you should never need to call yield(). It's a throwback to when green threads were common. All yield() does is tell the scheduler that now would be a good time to let another thread have some CPU. The scheduler is free to handle it how it pleases, including ignoring it. Unless you're an expert on multithreading with a very special case, the scheduler is smarter than you at knowing when to context switch, and you're better off letting it do its job.
Additionally, the code has the problem that all access to balance and number needs to by synchronized, not just the writes. (Or, if number is only set at construction and never changed, it could just be declared final). The reason is that syncing affects not only atomicity, but also coherence.
That is, each thread is free to make its own copy of shared member variables, and thread can manipulate those copies independently. Thread T1 can be operating on one copy of balance, and T2 can be operating on a completely different, independent copy with a completely different value. However, entering a sync block forces the local copies to be read from the master copy, and exiting a sync block forces the modified local copies to be written out to the master copy. So, although deposit() and withdraw() are synced and therefore forced to update the master copy of balance, getBalance, not being synchronized, could read the master copy on its first call, and read its local copy forever after, never seeing any changes made by deposit() and withdraw().
On an unrelated note, never do this:
catch (SomeException e) {}That says, "If something goes wrong, I don't want to know about it. Just keep chugging along as if everything is okay, and ignore the fact that the wheels are falling off, the engine is near locked up, and the driver's seat is on fire." At the very least, call e.printStackTrace(); so you can see what went wrong.

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