Group Commit

I've been working on the patch to enhance our group commit behavior. The
patch is a dirty hack at the moment, but I'm settled on the algorithm
I'm going to use and I know the issues involved.

Here's the patch as it is if you want to try it out:
http://community.enterprisedb.com/groupcommit-pghead-2.patch

but it needs a rewrite before being accepted. It'll only work on systems
that use sysv semaphores, I needed to add a function to acquire a
semaphore with timeout and I only did it for sysv_sema.c for now.

What are the chances of getting this in 8.3, assuming that I rewrite and
submit a patch within the next week or two?

Algorithm
---------

Instead of starting a WAL flush immediately after a commit record is
inserted, we wait a while to give other backends a chance to finish
their transactions and have them flushed by the same fsync call. There's
two things we can control: how many commits to wait for (commit group
size), and for how long (timeout).

We try to estimate the optimal commit group size. The estimate is

commit group size = (# of commit records flushed + # of commit records
arrived while fsyncing).

This is a relatively simple estimate that works reasonably well with
very short transactions, and the timeout limits the damage when the
estimate is not working.

There's a lot more factors we could take into account in the estimate,
for example:
- # of backends and their states (affects how many are likely to commit
soon)
- amount of WAL written since last XLogFlush (affects the duration of fsync)
- when exaclty the commit records arrive (we don't want to wait 10 ms to
get one more commit record in, when an fsync takes 11 ms)

but I wanted to keep this simple for now.

The timeout is currently hard-coded at 1 ms. I wanted to keep it short
compared to the time it takes to fsync (somewhere in the 5-15 ms
depending on hardware), to limit the damage when the algorithm isn't
getting the estimate right. We could also vary the timeout, but I'm not
sure how to calculate the optimal value and the real granularity will
depend on the system anyhow.

Implementation
--------------

To count the # of commits since last XLogFlush, I added a new
XLogCtlCommit struct in shared memory:

typedef struct XLogCtlCommit
{
slock_t commit_lock; /* protects the struct */
int commitCount; /* # of commit records inserted since
XLogFlush */
int groupSize; /* current commit group size */
XLogRecPtr lastCommitPtr; /* location of the latest commit record */
PGPROC *waiter; /* process to signal when groupSize is
reached */
} XLogCtlCommit;

Whenever a commit record is inserted in XLogInsert, commitCount is
incremented and lastCommitPtr is updated.
When it reaches groupSize, the waiter-process is woken up.

In XLogFlush, after acquiring WALWriteLock, we wait until groupSize is
reached (or timeout expires) before doing the flush.

Instead of the current logic to flush as much WAL as possible, we flush
up to the last commit record. Flushing any more wouldn't save us an
fsync later on, but might make the current fsync take longer. By doing
that, we avoid the conditional acquire of the WALInsertLock that's in
there currently. We make note of commitCount before starting the fsync;
that's the # of commit records that arrived in time so that the fsync
will flush them. Let's call that value "intime".

After the fsync is finished, we update the groupSize for the next round.
The new groupSize is the current commitCount after the fsync, IOW the
number of commit records arrived after the previous XLogFlush, including
the time it took to do the fsync. We update the commitCount by
decrementing it by "intime".

Now we're ready for the next round, and we can release WALWriteLock.

WALWriteLock
------------

The above would work nicely, except that a normal lwlock doesn't play
nicely. You can release and reacquire a lightwait lock in the same time
slice even when there's other backends queuing for the lock, effectively
cutting the queue.

Here's what sometimes happens, with 2 clients:

Client 1 Client 2
do work do work
insert commit record insert commit record
acquire WALWriteLock
try to acquire WALWriteLock, blocks
fsync
release WALWriteLock
begin new transaction
do work
insert commit record
reacquire WALWriteLock
wait for 2nd commit to arrive

Client 1 will eventually time out and commit just its own commit record.
Client 2 should be released immediately after client 1 releases the
WALWriteLock. It only needs to observe that its commit record has
already been flushed and doesn't need to do anything.

To fix the above, and other race conditions like that, we need a
specialized WALWriteLock that orders the waiters by the commit record
XLogRecPtrs. WALWriteLockRelease wakes up all waiters that have their
commit record already flushed. They will just fall through without
acquiring the lock.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com