Here's what mytherin wrote, ...we are allocating and freeing buffers repeatedly. Despite freeing more buffers than we allocate...

So, I assume, the context is, DuckDB allocates x buffers, frees x - m buffers at some point later, then allocates n buffers where n <<<< m, and yet malloc fails.

In the GitHub thread mytherin linked to above, Alexey Milovidov, ClickHouse CTO, points out that ClickHouse uses jemalloc and makes for a better choice than glibc malloc given the issue with fragmentation. It is likely that DuckDB switches to jemalloc, too.