if we have more than twice our starting worker pool, and have had no
tasks when checking the queue for multiple rounds, send a job telling
the system to retire a worker. eventually we'll get down to about 2x
the starting pool size if we stay idle.
We don't need a condition variable for a thing with a single waiter
which waits only once, and a data structure which only one side ever
modifies. That's a closable channel.
Two issues: First, there was a race condition because we were never
using the mutex for anything but the condvar broadcast, second, there
was no reason for the afterCurrentTx to need to maintain the list since
we already know where in the list we are when we are waking it up.
afterCurrentTx still wants to run with the db lock held, because
the degenerate case (no outstanding Tx) means that it will be running
with it held already. That's for another commit.
We need to update db.PageMap after we write the db, but before
we truncate the WAL, so new transactions don't pick up the old
PageMap and then get a truncated WAL.
Also, checkpoint should not abort if there's txs -- that's okay now.
When a qcx is a write, every Tx under it closes immediately, thus
invalidating all returned data. Thus, if you do a Not() inside a Store(),
you're doing a difference on an existence row and some other row
call... and both of those rows were run, individually, as separate
transactions that got invalidated the moment they were fetched. Oops.
This gets us to being able to run reads during a checkpoint, but
now we have to wait for new reads to end before we can release
the write lock, etc.
This is actually slightly slower, but if we could get ONE more step,
we could allow new writes during that phase, to a different WAL,
if we had a different WAL to write to.
PageMap uses "WALID", which is a WAL page ID relative to the "base" ID of the
WAL, rather than the wal page count you'd get just reading the file. So everything
it reports has a fixed offset at any given time. I think this may be left
over from a point where there were partial checkpoints. Anyway, the net
outcome is that each new transaction was getting different page IDs, but
the actual WAL pages did not always reflect that. Each checkpoint increases
the offset. This might imply that we can start having problems after
4 billion pages written even if most of them were redundant?
Anyway, with that fixed, this seems to work. I think.
We change nothing substantive here, except that there's a window
between when a write transaction updates the root pages and when
it removes itself from the db tx list and possibly causes a checkpoint
where it's not holding the db lock.
The issue here is that we want to be able to *keep* the lock but
still return, so no one else can start transactions, but the specific
Rollback or Commit that removed the last outstanding transaction
doesn't block forever. This will, later, allow us to exercise
finer-grained control over when we allow transactions. This is
a separate commit so we can run the test suite against it, and
verify that this part in particular didn't break anything.
Discovered test was running slightly strange and spending an unreasonable
amount of time on rand.Intn(), possibly because we weren't caching the
value used as the loop condition. Tweaked that, also made the pool a
bit different. Now it takes ~50 seconds for benchtime 100x, and produces
a profile with a TON of time spent waiting on sleeps (expected) and
the condition variable for waiting on checkpoints (the thing we want
to measure, really).
Note also the commented-out debug printf in checkpoint, there as a
reference. This is interesting because it turns out that MOST of checkpoint
writes is not actually writing new pages in most cases.
The actual "pages in WAL : pages in map" ratio is typically around 30:1
apparently. This would likely be different in cases where we were
updating existing data, though.
This is scratch space to prep for an actual work. The final
results will likely be different.
