Re: [HACKERS] SSI modularity questions
On 29.06.2011 00:33, Kevin Grittner wrote: Heikki Linnakangas wrote: On 28.06.2011 20:47, Kevin Grittner wrote: Hmm, the calls in question are the ones in heapgettup() and heapgettup_pagemode(), which are subroutines of heap_getnext(). heap_getnext() is only used in sequential scans, so it seems safe to remove those calls. I haven't found anything to the contrary, if I understand correctly, Dan found the same, and all the tests pass without them. Here's a patch to remove them. This makes the recently-added rs_relpredicatelocked boolean field unnecessary, so that's removed in this patch, too. Thanks, committed. I also moved the PredicateLockRelation() call to heap_beginscan(), per earlier discussion. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
Heikki Linnakangas heikki.linnakan...@enterprisedb.com wrote: On 29.06.2011 00:33, Kevin Grittner wrote: Heikki Linnakangas wrote: On 28.06.2011 20:47, Kevin Grittner wrote: Hmm, the calls in question are the ones in heapgettup() and heapgettup_pagemode(), which are subroutines of heap_getnext(). heap_getnext() is only used in sequential scans, so it seems safe to remove those calls. I haven't found anything to the contrary, if I understand correctly, Dan found the same, and all the tests pass without them. Here's a patch to remove them. This makes the recently-added rs_relpredicatelocked boolean field unnecessary, so that's removed in this patch, too. Thanks, committed. I also moved the PredicateLockRelation() call to heap_beginscan(), per earlier discussion. Thanks! Before we leave the subject of modularity, do you think the entire else clause dealing with the lossy bitmaps should be a heapam.c function called from nodeBitmapHeapscan.c? With the move of the PredicateLockRelation() call you mention above, that leaves this as the only place in the executor which references SSI, and it also is the only place in the executor to call PageGetMaxOffsetNumber() and OffsetNumberNext(), which seem like AM things. The logic seems somewhat similar to heap_hot_search_buffer() and such a function would take roughly the same parameters. On the other hand, it's obviously not a bug, so maybe that's something to put on a list to look at later. -Kevin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
On 27.06.2011 21:23, Kevin Grittner wrote: There are two outstanding patches for SSI which involve questions about modularity. In particular, they involve calls to predicate locking and conflict detection from executor source files rather than AM source files (where most such calls exist). (1) Dan submitted this patch: http://archives.postgresql.org/message-id/20110622045850.gn83...@csail.mit.edu which is a very safe and very simple patch to improve performance on sequential heap scans at the serializable transaction isolation level. The location of the code being modified raised questions about modularity. There is a reasonably clear place to which it could be moved in the heap AM, but because it would acquire a predicate lock during node setup, it would get a lock on the heap even if the node was never used, which could be a performance regression in some cases. The bigger question is if those calls are needed at all (http://archives.postgresql.org/message-id/4e072ea9.3030...@enterprisedb.com). I'm uneasy about changing them this late in the release cycle, but I don't feel good about leaving useless clutter in place just because we're late in the release cycle either. More importantly, if locking the whole relation in a seqscan is not just a performance optimization, but is actually required for correctness, it's important that we make the code and comments to reflect that or someone will break it in the future. (2) In reviewing the above, Heikki noticed that there was a second place in the executor that SSI calls were needed but missing. I submitted a patch here: http://archives.postgresql.org/message-id/4e07550f02250003e...@gw.wicourts.gov I wonder, though, whether the section of code which I needed to modify should be moved to a new function in heapam.c on modularity grounds. If these two places were moved, there would be no SSI calls from any source file in the executor subdirectory. Same here, we might not need those PredicateLockTuple calls in bitmap heap scan at all. Can you check my logic, and verify if those PredicateLockTuple() calls are needed? -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
Heikki Linnakangas wrote: On 27.06.2011 21:23, Kevin Grittner wrote: The bigger question is if those calls are needed at all ( http://archives.postgresql.org/message-id/4e072ea9.3030...@enterprisedb.com ). Ah, I didn't properly grasp your concerns the first time I read that. The heap relation lock for a seqscan is indeed required for correctness and has been there all along. The rs_relpredicatelocked flag was added in response to this: http://archives.postgresql.org/pgsql-hackers/2011-01/msg00730.php I'm uneasy about changing them this late in the release cycle, but I don't feel good about leaving useless clutter in place just because we're late in the release cycle either. More importantly, if locking the whole relation in a seqscan is not just a performance optimization, but is actually required for correctness, it's important that we make the code and comments to reflect that or someone will break it in the future. OK, if that isn't clear in the comments, we should definitely make it clear. Basically, the predicate locking strategy is as follows: (1) We're only concerned with read/write dependencies, also know as rw-conflicts. This is where two transactions overlap (each gets its snapshot before the other commits, so neither can see the work of the other), and one does a read which doesn't see the write of the other due only to the timing. (2) For rw-conflicts where the read follows the write, the predicate locks don't come into play -- we use the MVCC data in the heap tuples directly. (3) Heap tuples are locked so that updates or deletes by an overlapping transaction of the tuple which has been read can be detected as a rw-conflict. Keep in mind that access for such a delete or update may not go through the same index on which the conflicting read occurred. It might use a different index or a seqscan. These may be promoted to page or heap relation locks to control the shared space used by predicate locks, but the concept is the same -- we're locking actual tuples read, not any gaps. (4) Index ranges are locked to detect inserts or updates which create heap tuples which would have been read by an overlapping transaction if they had existed and been visible at the time of the index scan. The entire goal of locks on indexes is to lock the gaps where a scan *didn't* find anything; we only care about conflicting index tuple inserts. (5) When a heap scan is executed, there is no index gap to lock to cover the predicate involved, so we need to acquire a heap relation lock -- any insert to the relation by an overlapping transaction is a rw-conflict. While these *look* just like tuple locks which got promoted, their purpose is entirely different. Like index locks, they are for detecting inserts into the gaps. [Light bulb goes on over head: in some future release, perhaps it would be worth differentiating between the two uses of heap relation locks, to reduce the frequency of false positives. A couple bit flags in the lock structure might do it.] So, the heap relation lock is clearly needed for the seqscan. There is room for performance improvement there in skipping the tuple lock attempt when we're in a seqscan, which will always be a no-op when it finds the heap relation lock after a hash table lookup. But you are also questioning whether the predicate locking of the tuples where rs_relpredicatelocked is tested can be removed entirely, rather than conditioned on the boolean. The question is: can the code be reached on something other than a seqscan of the heap, and can this happen for a non-temporary, non-system table using a MVCC snapshot? I've been trying to work backward to all the spots which call these functions, directly or indirectly to determine that. That's obviously not trivial or easy work, and I fear that unless someone more familiar with the code than I can weigh in on that question for any particular PredicateLockTuple() call, I would rather leave the calls alone for 9.1 and sort this out in 9.2. I'm confident that they don't do any damage where they are; it's a matter of very marginal performance benefit (skipping a call to a fast return) and code tidiness (not making unnecessary calls). I can, with confidence, now answer my own previous question about moving the calls outside the influence of HeapKeyTest(): it's not necessary. The rows currently excluded won't be seen by the caller, so they don't fit under the needs of (3) above, and if (4) or (5) aren't covered where they need to be, locking a few extra rows won't help at all. So we can drop that issue. (2) In reviewing the above, Heikki noticed that there was a second place in the executor that SSI calls were needed but missing. I submitted a patch here: http://archives.postgresql.org/message-id/4e07550f02250003e...@gw.wicourts.gov I wonder, though, whether the section of code which I needed to modify should be moved to a new function in heapam.c on modularity
Re: [HACKERS] SSI modularity questions
On 28.06.2011 20:47, Kevin Grittner wrote: (3) Heap tuples are locked so that updates or deletes by an overlapping transaction of the tuple which has been read can be detected as a rw-conflict. Keep in mind that access for such a delete or update may not go through the same index on which the conflicting read occurred. It might use a different index or a seqscan. These may be promoted to page or heap relation locks to control the shared space used by predicate locks, but the concept is the same -- we're locking actual tuples read, not any gaps. Ok, that's what I was missing. So the predicate locks on heap tuples are necessary. Thanks for explaining this again. So, the heap relation lock is clearly needed for the seqscan. There is room for performance improvement there in skipping the tuple lock attempt when we're in a seqscan, which will always be a no-op when it finds the heap relation lock after a hash table lookup. But you are also questioning whether the predicate locking of the tuples where rs_relpredicatelocked is tested can be removed entirely, rather than conditioned on the boolean. The question is: can the code be reached on something other than a seqscan of the heap, and can this happen for a non-temporary, non-system table using a MVCC snapshot? I've been trying to work backward to all the spots which call these functions, directly or indirectly to determine that. That's obviously not trivial or easy work, and I fear that unless someone more familiar with the code than I can weigh in on that question for any particular PredicateLockTuple() call, I would rather leave the calls alone for 9.1 and sort this out in 9.2. I'm confident that they don't do any damage where they are; it's a matter of very marginal performance benefit (skipping a call to a fast return) and code tidiness (not making unnecessary calls). Hmm, the calls in question are the ones in heapgettup() and heapgettup_pagemode(), which are subroutines of heap_getnext(). heap_getnext() is only used in sequential scans, so it seems safe to remove those calls. (2) In reviewing the above, Heikki noticed that there was a second place in the executor that SSI calls were needed but missing. I submitted a patch here: http://archives.postgresql.org/message-id/4e07550f02250003e...@gw.wicourts.gov I wonder, though, whether the section of code which I needed to modify should be moved to a new function in heapam.c on modularity grounds. If these two places were moved, there would be no SSI calls from any source file in the executor subdirectory. Same here, we might not need those PredicateLockTuple calls in bitmap heap scan at all. Can you check my logic, and verify if those PredicateLockTuple() calls are needed? These sure look like they are needed per point (3) above. Yep. I would like to add a test involving a lossy bitmap scan. How many rows are normally needed to force a bitmap scan to be lossy? The size of bitmaps is controlled by work_mem, so you can set work_mem very small to cause them to become lossy earlier. Off the top of my head I don't have any guesstimate on how many rows you need. What's the easiest way to check whether a plan is going to use (or is using) a lossy bitmap scan? Good question. There doesn't seem to be anything in the EXPLAIN ANALYZE output to show that, so I think you'll have to resort to adding some elog()s in the right places. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
On Tue, Jun 28, 2011 at 1:47 PM, Kevin Grittner kevin.gritt...@wicourts.gov wrote: (5) When a heap scan is executed, there is no index gap to lock to cover the predicate involved, so we need to acquire a heap relation lock -- any insert to the relation by an overlapping transaction is a rw-conflict. While these *look* just like tuple locks which got promoted, their purpose is entirely different. Like index locks, they are for detecting inserts into the gaps. [Light bulb goes on over head: in some future release, perhaps it would be worth differentiating between the two uses of heap relation locks, to reduce the frequency of false positives. A couple bit flags in the lock structure might do it.] You know, it just occurred to me while reading this email that you're using the term predicate lock in a way that is totally different from what I learned in school. What I was taught is that the word predicate in predicate lock is like the word tuple in tuple lock or the word relation in relation lock - that is, it describes *the thing being locked*. In other words, you are essentially doing: LOCK TABLE foo WHERE i = 1; I think that what you're calling the predicate lock manager should really be called the siread lock manager, and all of the places where you are predicate locking a tuple should really be siread locking the tuple. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
2011/6/28, Robert Haas robertmh...@gmail.com: You know, it just occurred to me while reading this email that you're using the term predicate lock in a way that is totally different from what I learned in school. What I was taught is that the word predicate in predicate lock is like the word tuple in tuple lock or the word relation in relation lock - that is, it describes *the thing being locked*. In other words, you are essentially doing: LOCK TABLE foo WHERE i = 1; I think that what you're calling the predicate lock manager should really be called the siread lock manager, and all of the places where you are predicate locking a tuple should really be siread locking the tuple. The predicate in the full table case is: any tuple in this table (including tuples that don't exist yet, otherwise it wouldn't be a predicate). The predicate in the index case is: any tuple that would be returned by so-and-such index scan (idem regarding tuples that don't exist yet, hence locking the gaps). The lock semantics (i.e., how conflicts between it and other locks are defined and treated) are siread. The thing that it applies to is a predicate. (I.e., PostgreSQL before SSI already supported some rather trivial kind of predicate lock: the full table lock.) Conclusion: I don't see the problem :-). Nicolas -- A. Because it breaks the logical sequence of discussion. Q. Why is top posting bad? -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] SSI modularity questions
Heikki Linnakangas wrote: On 28.06.2011 20:47, Kevin Grittner wrote: Hmm, the calls in question are the ones in heapgettup() and heapgettup_pagemode(), which are subroutines of heap_getnext(). heap_getnext() is only used in sequential scans, so it seems safe to remove those calls. I haven't found anything to the contrary, if I understand correctly, Dan found the same, and all the tests pass without them. Here's a patch to remove them. This makes the recently-added rs_relpredicatelocked boolean field unnecessary, so that's removed in this patch, too. I would like to add a test involving a lossy bitmap scan. How many rows are normally needed to force a bitmap scan to be lossy? The size of bitmaps is controlled by work_mem, so you can set work_mem very small to cause them to become lossy earlier. Off the top of my head I don't have any guesstimate on how many rows you need. What's the easiest way to check whether a plan is going to use (or is using) a lossy bitmap scan? Good question. There doesn't seem to be anything in the EXPLAIN ANALYZE output to show that, so I think you'll have to resort to adding some elog()s in the right places. OK, thanks. -Kevin ssi-seqscan-cleanup.patch Description: Binary data -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] SSI modularity questions
There are two outstanding patches for SSI which involve questions about modularity. In particular, they involve calls to predicate locking and conflict detection from executor source files rather than AM source files (where most such calls exist). (1) Dan submitted this patch: http://archives.postgresql.org/message-id/20110622045850.gn83...@csail.mit.edu which is a very safe and very simple patch to improve performance on sequential heap scans at the serializable transaction isolation level. The location of the code being modified raised questions about modularity. There is a reasonably clear place to which it could be moved in the heap AM, but because it would acquire a predicate lock during node setup, it would get a lock on the heap even if the node was never used, which could be a performance regression in some cases. (2) In reviewing the above, Heikki noticed that there was a second place in the executor that SSI calls were needed but missing. I submitted a patch here: http://archives.postgresql.org/message-id/4e07550f02250003e...@gw.wicourts.gov I wonder, though, whether the section of code which I needed to modify should be moved to a new function in heapam.c on modularity grounds. If these two places were moved, there would be no SSI calls from any source file in the executor subdirectory. Should these be moved before beta3? -Kevin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
