Improve the generation memory allocator

Enterprise / PostgreSQL - David Rowley [] - 4 April 2022 08:53 UTC

Here we make a series of improvements to the generation memory allocator, namely:

1. Allow generation contexts to have a minimum, initial and maximum block sizes. The standard allocator allows this already but when the generation context was added, it only allowed fixed-sized blocks. The problem with fixed-sized blocks is that it's difficult to choose how large to make the blocks. If the chosen size is too small then we'd end up with a large number of blocks and a large number of malloc calls. If the block size is made too large, then memory is wasted.

2. Add support for "keeper" blocks. This is a special block that is allocated along with the context itself but is never freed. Instead, when the last chunk in the keeper block is freed, we simply mark the block as empty to allow new allocations to make use of it.

3. Add facility to "recycle" newly empty blocks instead of freeing them and having to later malloc an entire new block again. We do this by recording a single GenerationBlock which has become empty of any chunks. When we run out of space in the current block, we check to see if there is a "freeblock" and use that if it contains enough space for the allocation.

Author: David Rowley, Tomas Vondra

1b0d9aa4f7 Improve the generation memory allocator
src/backend/access/gist/gistvacuum.c | 6 +
src/backend/replication/logical/reorderbuffer.c | 7 +
src/backend/utils/mmgr/generation.c | 385 +++++++++++++++++++-----
src/include/utils/memutils.h | 4 +-
4 files changed, 325 insertions(+), 77 deletions(-)


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