On 23.06.2011 18:42, Robert Haas wrote:
ProcArrayLock looks like a tougher nut to crack - there's simply no
way, with the system we have right now, that you can take a snapshot
without locking the list of running processes. I'm not sure what to
do about that, but we're probably going to have to come up with
something, because it seems clear that once we eliminate the lock
manager LWLock contention, this is a major bottleneck.
ProcArrayLock is currently held for a relatively long period of time
when a snapshot is taken, especially if there's a lot of backends. There
are three operations to the procarray:
1. Advertising a new xid belonging to my backend.
2. Ending a transaction.
3. Getting a snapshot.
Advertising a new xid is currently done without a lock, assuming that
setting an xid in shared memory is atomic. To end a transaction, you
acquire ProcArrayLock in exclusive mode. To get a snapshot, you acquire
ProcArrayLock in shared mode, and scan the whole procarray.
I wonder if it would be a better tradeoff to keep a "materialized"
snapshot in shared memory that's updated every time a new xid is
assigned or a transaction ends. Getting a snapshot would become much
cheaper, as you could just memcpy the ready-built snapshot from shared
memory into local memory.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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