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Large chunks (those too large for any palloc freelist) are managed as separate blocks. Formerly, realloc'ing or pfree'ing such a chunk required O(N) time in a context with N blocks, since we had to traipse down the singly-linked block list to locate the block's predecessor before we could fix the list links. This can result in O(N^2) runtime in situations where large numbers of such chunks are manipulated within one context. Cases like that were not foreseen in the original design of aset.c, and indeed didn't arise until fairly recently. But such problems can now occur in reorderbuffer.c and in hash joining, both of which make repeated large requests without scaling up their request size as they do so, and which will free their requests in not-necessarily-LIFO order. To fix, change the block list from singly-linked to doubly-linked. This adds another 4 or 8 bytes to ALLOC_BLOCKHDRSZ, but that doesn't seem like unacceptable overhead, since aset.c's blocks are normally 8K or more, and never less than 1K in current practice. In passing, get rid of some redundant AllocChunkGetPointer() calls in AllocSetRealloc (the compiler might be smart enough to optimize these away anyway, but no need to assume that) and improve AllocSetCheck's checking of block header fields. Back-patch to 9.4 where reorderbuffer.c appeared. We could take this further back, but currently there's no evidence that it would be useful. Discussion: https://postgr.es/m/CAMkU=1x1hvue1XYrZoWk_omG0Ja5nBvTdvgrOeVkkeqs71CV8g@mail.gmail.com |
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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. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.