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Replace the fixed-size array of fast-path locks with arrays, sized on startup based on max_locks_per_transaction. This allows using fast-path locking for workloads that need more locks. The fast-path locking introduced in 9.2 allowed each backend to acquire a small number (16) of weak relation locks cheaply. If a backend needs to hold more locks, it has to insert them into the shared lock table. This is considerably more expensive, and may be subject to contention (especially on many-core systems). The limit of 16 fast-path locks was always rather low, because we have to lock all relations - not just tables, but also indexes, views, etc. For planning we need to lock all relations that might be used in the plan, not just those that actually get used in the final plan. So even with rather simple queries and schemas, we often need significantly more than 16 locks. As partitioning gets used more widely, and the number of partitions increases, this limit is trivial to hit. Complex queries may easily use hundreds or even thousands of locks. For workloads doing a lot of I/O this is not noticeable, but for workloads accessing only data in RAM, the access to the shared lock table may be a serious issue. This commit removes the hard-coded limit of the number of fast-path locks. Instead, the size of the fast-path arrays is calculated at startup, and can be set much higher than the original 16-lock limit. The overall fast-path locking protocol remains unchanged. The variable-sized fast-path arrays can no longer be part of PGPROC, but are allocated as a separate chunk of shared memory and then references from the PGPROC entries. The fast-path slots are organized as a 16-way set associative cache. You can imagine it as a hash table of 16-slot "groups". Each relation is mapped to exactly one group using hash(relid), and the group is then processed using linear search, just like the original fast-path cache. With only 16 entries this is cheap, with good locality. Treating this as a simple hash table with open addressing would not be efficient, especially once the hash table gets almost full. The usual remedy is to grow the table, but we can't do that here easily. The access would also be more random, with worse locality. The fast-path arrays are sized using the max_locks_per_transaction GUC. We try to have enough capacity for the number of locks specified in the GUC, using the traditional 2^n formula, with an upper limit of 1024 lock groups (i.e. 16k locks). The default value of max_locks_per_transaction is 64, which means those instances will have 64 fast-path slots. The main purpose of the max_locks_per_transaction GUC is to size the shared lock table. It is often set to the "average" number of locks needed by backends, with some backends using significantly more locks. This should not be a major issue, however. Some backens may have to insert locks into the shared lock table, but there can't be too many of them, limiting the contention. The only solution is to increase the GUC, even if the shared lock table already has sufficient capacity. That is not free, especially in terms of memory usage (the shared lock table entries are fairly large). It should only happen on machines with plenty of memory, though. In the future we may consider a separate GUC for the number of fast-path slots, but let's try without one first. Reviewed-by: Robert Haas, Jakub Wartak Discussion: https://postgr.es/m/510b887e-c0ce-4a0c-a17a-2c6abb8d9a5c@enterprisedb.com
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.