оЕПЕЯШКЙЮ: ESQL/C - Critical Section?

["Shulzhenko Vasyl" <vasilis@softline.kiev.ua>]

-----хЯУНДМНЕ ЯННАЫЕМХЕ-----
нР: Madison Pruet <mpruet@informix.com>
цПСООШ: comp.databases.informix
йНЛС: Jack O'Hara <johara@writeme.com>
дЮРЮ: 1 ЛЮЪ 1999 Ц. 10:02
рЕЛЮ: Re: ESQL/C - Critical Section?


>When a transaction enters a critical section, it is doing somthing that if
>it fails, then the integrety of the engine is at risk.  The most obvious
>thing would be somthing like when the log entry for an update is in the
>process of being put into the log buffer.  Obviously the entire entry can't
>be put into the buffer as a single machine instruction.  Also, we can't
>allow the checkpoint to occur while transactions are putting an entry into
>the log buffer since one of the things done during a checkpoint is to flush
>the log buffer.
>
>So, if a transaction is about to do somthing as sensitive as putting a entry
>into the log buffer, we mark the transaction as being in a critical state.
>When that activity is complete, we remove the critical state.
>
>The checkpoint waits until all transactions are out of a critical section.
>By the way, when the checkpoint begins, it causes a lock on any transcation
>going into a critical section.  Obviously we don't want to hold the critical
>state for long.
>
>Jack O'Hara wrote:
>
>> Hi All,
>>
>> Running OLTP, Informix SQL Server 7.22, Solaris platform.
>>
>> Am trying to get a handle on understanding exactly when a transaction is
>> in a  "critical section" and its effect on a checkpoint.  So far, I
>> understand a checkpoint blocks any new transactions, but it must also
>> wait for all transactions that are in (or have reached??) a critical
>> section which results in longer checkpoint durations.
>>
>> When EXACTLY has a transaction entered/reached a "critical section"?
>>     ...is it immediately after a "begin work"?
>>     ...or after a "begin work" and some record has been
>> inserted/modified (created a dirty buffer page)?
>>     ...or ???
>>
>> Once the transaction is in a critical section, must the transaction
>> continue to the point of a commit/rollback before the checkpoint can
>> continue, or can the checkpoint continue at some earlier juncture (a
>> pause in transaction)?
>>
>> Applying answers for the above questions to an ESQL/C program then:
>>
>> If an ESQL/C program has done a "EXEC SQL BEGIN WORK" followed
>> immediately by "sleep(5)", is this transaction considered in a critical
>> section for the entire 5 second sleep causing the checkpoint to wait
>> during this time?
>>
>Nope.
>
>>
>>
>> If not, what if after the "EXEC SQL BEGIN WORK" an insert/update of a
>> record was performed before the "sleep(5)".  Would this transaction be
>> considered in a critical section for the entire 5 second sleep causing
>> the checkpoint to wait during this time?
>
>Again nope.  In both of these cases, the sleep(5) can only occur while
>control is in the esql program.  Thus the sqlexec thread in the engine would
>be on a SMREAD condition or NETNORM.   If it's there, it can't be in a
>critical section.
>
>>
>>
>> I've read past postings and Informix manuals on this topic, but still
>> find rather vague when attempting to apply to the above questions.
>>
>> Any enlightenment would be greatly appreciated.
>>
>> Thanks,
>> Jack O'Hara
>> johara@writeme.com
>
>
>