Kristian Waagan wrote:
On 03.08.2010 19:28, Mike Matrigali wrote:
Hi Mike,
Thank you for the feedback. See my comments below, especially the one
regarding flushLogOnCommit.
>>
Kristian Waagan wrote:
Hi,
When working on an experiment for automatic index statistics
(re)generation, I was exposed to the Derby transaction API.
Dan filed an issue [1] suggesting to clean up this API, and I can
give my +1 to that :) In places the comments and actual usage aren't
in sync, and missing functionality of the lcc
(LanguageConnectionContext) can be obtained by working directly on
the tc (TransactionController). One such example is nested read-write
user transactions, which doesn't seem to be supported through the lcc
(although the lcc API suggests so), but is used in some places by
working with the tc.
I tried to use a nested read-write user transaction to write index
statistics to the data dictionary, and was surprised to find that the
changes were lost even though I committed the transaction (they
survive if I do a proper shutdown). Turns out Derby uses the concept
of transaction contexts, and the following are defined in XactFactory:
USER_CONTEXT_ID
NESTED_READONLY_USER_CONTEXT_ID
NESTED_UPDATE_USER_CONTEXT_ID
INTERNAL_CONTEXT_ID
NTT_CONTEXT_ID
Now, the XactFactory also has this method:
/**
Decide if a transaction of this contextId needs to flush the
log when
it commits
*/
public boolean flushLogOnCommit(String contextName)
{
//
// if this is a user transaction, flush the log
// if this is an internal or nested top transaction, do not
// flush, let it age out.
// return (contextName == USER_CONTEXT_ID ||
contextName.equals(USER_CONTEXT_ID));
}
Most of this code is rather old, so I haven't found much history. My
questions:
1) Is using a nested read-write user transaction simply wrong in
this case?
(nested because I want to release the locks on the data
dictionary as soon as possible) There is the problem with locks not
being compatible - see below. I think this is what mamta kept running
into in what she tried. usually it is likely not to be a problem but
if user happens to have accumulated some system catalog locks then
there are issues (and this case I think comes up in a bunch of our
tests). I also see an issue with some user with a big table complaining
when his simple query takes a long time waiting on the stats during
compile.
Although the current prototype code is very crude, here's a brief
description:
o work is queued by a thread compiling a statement. After queuing the
thread continues its normal work; compiling the statement and
potentially executing it.
need to catch the case and somehow stop multiple threads from all
queuing the same statistics work.
o generating the statistics is done by a separate thread created
on-demand (in it's own transaction);
was creating the context for this hard? I believe mamta was having
problems with things like user password, encryption values, roles, ...
- if there is no thread, one is created
- if there is more work when the thread finishes the current unit
of work, it will continue with the next item in the queue
- if there is no more work, the thread dies
o work is scheduled based on tables; all indexes are regenerated, not
individual ones
o writing the stats to the system tables is done by a user thread
compiling a statement
how does this last part work (the writing to the system tables), if the
work stat generation work is async.
In the current prototype, there has to be a mechanism for a user thread
to detect that there are new statistics to be written. One issue right
now, is that this happens at a time where the statement is already
optimized (thus loosing out on the new statistics). I'm not sure if this
is a problem or not, the new stats will be picked up the next time the
statement is compiled.
Ultimately the solution I think would work best is some generic way
to queue background work from the language layer, similar to what can
be done from the storage layer to the daemon thread. This would avoid
making a query wait while a full scan of the entire table is done
to rebuild statistics. A separate thread avoids a lot of the deadlock
issues of a nested user thread.
Yes, this is what I have tried to do, although the "background daemon"
is very specific.
The issues to solve would be:
o how to make sure only correct permissions are enforced on the
background work. I think it would be best at least in first
implementation if it was only possible for internal systems to queue
to this background runner.
Ignored for now, the worker can only generate statistics and the work is
queued from within GenericStatement.
o should the work survive a shutdown? It would be simple enough to
track the work in a table, but is it worth it.
Do you mean the work queue?
If so, I feel that it isn't necessary, as work is queued as determined
by the logic "detecting" stale/missing stats (lots of work to do here I
think).
Maybe saving intermediate results can be useful for huge tables, but
then we have to handle the issue of stale intermediate results too...
o I don't think we should add another thread always to handle this
background work as we already get complaints about the existing
1 thread per db. Best would be some system that could add the
and delete the thread as needed - maybe even add more than one
thread if it could determine it was appropriate on the hardware - or
maybe based on some configuration param.
This is what is done in the prototype, but I haven't looked into
configuration. One concern is that index regeneration will poison the
page cache, but avoiding this is probably a big thing. I think we may
also have to tune how much resources (CPU, IO) are used for this
background activity.
yes, this is the challenging part, but I believe it can be broken down
into manageable chunks. If you can get the queueing stuff to work, I
think that is a big step.
A reasonable bit of work might be to look at the stat generation part
again. Logically it is not really necesary to rebuild the indexes
to get the new stats, it just works that way now. I would imagine code
to do this would be about a weeks work at most, if the existing logic is
used. The existing logic counts on piping all the rows through the
sorter and then getting called back for each row in order, new code
could just scan the index in order instead. All the stats per index can
be generated by code doing a single scan of an existing index.
Rebuilding the index does have the added benefit of reclaiming all
non-used space in the index.
2) Is the method flushLogOnCommit doing the right thing?
I believe flushLogOnCommit is doing the right thing for the current
usage of nested user transactions. The issue is one of performance. In
the current usage these transactions are only used for internal things
where it is ok for either the transaction work to be backed out or
committed, even if the internal transaction commits. All the work
based on the internal transaction is logged after the internal
transaction. So if this transaction is lost then it must be that all
subsequent work is also lost.
Well, the updates to the data dictionary are lost even though I have
committed the parent transaction. They survive a shutdown if I do a
proper shutdown. I cannot explain this, maybe I have a severe bug in the
prototype (I did pretty much copy the code from InsertResultSet though).
This seems wrong. If you commit the nested user transaction and do a
subsequent real commit of the parent transaction there should be a force
to the log at user commit time. Is it likely your user transaction
never does any writes separate from what is done in the nested user
transaction - this may be an edge case that never happens today, maybe
the parent transaction is read only and does not know that it needs to
force log on commit in this case - I am not sure. The fact that it
sounds easy for you to repro this leads me to think there is a bug there
somewhere, because it works when
If I were debugging this I would dump the transaction log
in the 2 cases and see if anything jumps out different. If you need
the properties for this let me know.
What it is doing is avoiding a synchonous write on the log for each
internal transaction. This not only benefits by avoiding the write, but
it is likely that it will increase the "group" of log records that will
be served by the subsequent real user transaction commit. I believe the
usual usage for this read/write transaction is the update of the "block"
of numbers used for generated keys. So the expectation is that usually
the parent user transaction will commit soon.
Okay, so the nested transaction implementation is pretty much tailed to
fit work related to identity columns?
I guess all contexts except USER_CONTEXT_ID are considered internal.
Historically the "real" internal transactions came first - and these are
used heavily by the raw store for things like btree splits and various
reclaim space operations. These really would make the system suffer if
we did hard sync on commit, and they are all generated by user
transactions that are doing real updates so a subsequent commit in the
log is very likely.
Next came read only user nested transactions and they are used for
compile time read only locking so that we can give up locks mid user
transaction. Since they are read-only log commit behaviour does not matter.
And then read-write nested user transactions came and are used for
identity column system cat update. their locks are not compatible so
they do cause problems, so if you can avoid them it is best. The code
expects them to be used to nest work, commit it, and then return to
main user thread. Using it in other ways may not work - I don't know.
If you new code really needs nested read/write transactions it may be
reasonable to do real force on these types of internal transactions, but
we should not do it on ntt and raw store internal transactions.
The prototype uses a nested transaction slightly different - it would be
best if the work done by the nested transaction would be synced. No real
harm is done by loosing the updates though, Derby just have to do the
work again (may affect performance).
I haven't checked yet, but it is also important to know if the update
locks of the nested user transaction is incompatible with the parent
user transaction (to avoid deadlock when using NO_WAIT).
The locks are not compatible. See following documentation in
TransactionController.java!getNestedUserTransaction()
* <p>
* The locks in the child transaction of a readOnly nested user
transaction
* will be compatible with the locks of the parent transaction. The
* locks in the child transaction of a non-readOnly nested user
transaction
* will NOT be compatible with those of the parent transaction -
this is
* necessary for correct recovery behavior.
* <p>
Thanks.
