On Mon, May 14, 2012 at 9:56 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
> On Mon, May 14, 2012 at 9:06 AM, Bruce Momjian <bruce@momjian.us> wrote:
>
>> This is the git commit message:
>>
>> Make group commit more effective.
>>
>> When a backend needs to flush the WAL, and someone else is already flushing
>> the WAL, wait until it releases the WALInsertLock and check if we still need
>> to do the flush or if the other backend already did the work for us, before
>> acquiring WALInsertLock. This helps group commit, because when the WAL flush
>> finishes, all the backends that were waiting for it can be woken up in one
>> go, and the can all concurrently observe that they're done, rather than
>> waking them up one by one in a cascading fashion.
>>
>> This is based on a new LWLock function, LWLockWaitUntilFree(), which has
>> peculiar semantics. If the lock is immediately free, it grabs the lock and
>> returns true. If it's not free, it waits until it is released, but then
>> returns false without grabbing the lock. This is used in XLogFlush(), so
>> that when the lock is acquired, the backend flushes the WAL, but if it's
>> not, the backend first checks the current flush location before retrying.
>>
>> Original patch and benchmarking by Peter Geoghegan and Simon Riggs, although
>> this patch as committed ended up being very different from that.
>>
>> (Heikki Linnakangas)
>>
>> Is that commit message inaccurate?
>
> I think the commit message is accurate, other than saying
> WALInsertLock where it meant WALWriteLock.
Sorry, wrong number of negations. "I think the commit message is
accurate, other than"
Cheers,
Jeff