Re: Using per-transaction memory contexts for storing decoded tuples

On Fri, Sep 20, 2024 at 5:13 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
>
> On Fri, 20 Sept 2024 at 05:03, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > I've done other benchmarking tests while changing the memory block
> > sizes from 8kB to 8MB. I measured the execution time of logical
> > decoding of one transaction that inserted 10M rows. I set
> > logical_decoding_work_mem large enough to avoid spilling behavior. In
> > this scenario, we allocate many memory chunks while decoding the
> > transaction and resulting in calling more malloc() in smaller memory
> > block sizes. Here are results (an average of 3 executions):
>
> I was interested in seeing the memory consumption with the test that
> was causing an OOM due to the GenerationBlock fragmentation.
>

+1. That test will be helpful.

> > The fact that we're using rb->size and txn->size to choose the
> > transactions to evict could make this idea less attractive. Even if we
> > defragment transactions, rb->size and txn->size don't change.
> > Therefore, it doesn't mean we can avoid evicting transactions due to
> > full of logical_decoding_work_mem, but just mean the amount of
> > allocated memory might have been reduced.
>
> I had roughly imagined that you'd add an extra field to store
> txn->size when the last fragmentation was done and only defrag the
> transaction when the ReorderBufferTXN->size is, say, double the last
> size when the changes were last defragmented. Of course, if the first
> defragmentation was enough to drop MemoryContextMemAllocated() below
> the concerning threshold above logical_decoding_work_mem then further
> defrags wouldn't be done, at least, until such times as the
> MemoryContextMemAllocated() became concerning again. If you didn't
> want to a full Size variable for the defragmentation size, you could
> just store a uint8 to store which power of 2 ReorderBufferTXN->size
> was when it was last defragmented. There are plenty of holds in that
> struct that could be filled without enlarging the struct.
>
> In general, it's a bit annoying to have to code around this
> GenerationContext fragmentation issue.
>

Right, and I am also slightly afraid that this may not cause some
regression in other cases where defrag wouldn't help.

--
With Regards,
Amit Kapila.