Re: Checkpointer write combining - Mailing list pgsql-hackers

On Sep 12, 2025, at 07:11, Melanie Plageman <melanieplageman@gmail.com> wrote:

On Wed, Sep 10, 2025 at 4:24 AM Chao Li <li.evan.chao@gmail.com> wrote:

Thanks for the review!

For any of your feedback that I simply implemented, I omitted an
inline comment about it. Those changes are included in the attached
v6. My inline replies below are only for feedback requiring more
discussion.

On Sep 10, 2025, at 01:55, Melanie Plageman <melanieplageman@gmail.com> wrote:

2 - 0001
```
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c

+ if (XLogNeedsFlush(lsn))
+ {
+ /*
+ * Remove the dirty buffer from the ring; necessary to prevent an
+ * infinite loop if all ring members are dirty.
+ */
+ strategy->buffers[strategy->current] = InvalidBuffer;
+ return true;
+ }

- return true;
+ return false;
}
```

We can do:
```
        If (!XLogNeedsFlush(lan))
           Return false

       /* Remove the dirty buffer ….
        */
      Return true;
}
```

This would make the order of evaluation the same as master but I
actually prefer it this way because then we only take the buffer
header spinlock if there is a chance we will reject the buffer (e.g.
we don't need to examine it for strategies except BAS_BULKREAD)

10 - 0004
```
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c

+ limit = Min(max_batch_size, limit);
```

Do we need to check max_batch_size should be less than (MAX_IO_COMBINE_LIMIT-1)? Because BufWriteBatch.bufdescs is defined with length of MAX_IO_COMBINE_LIMIT, and the first place has been used to store “start”.

I assert that in StrategyMaxWriteBatchSize(). io_combine_limit is not
allowed to exceed MAX_IO_COMBINE_LIMIT, so it shouldn't happen anyway,
since we are capping ourselves at io_combine_limit. Or is that your
point?