Thread: Re: BgBufferSync(): clarification about reusable_buffers variable

Hi Marcel,


On Sat, Feb 8, 2025 at 6:24 AM M vd H <mvdholst@gmail.com> wrote:
>
> I have been reading the source code of the BgWriter, and there is some
> code in BgBufferSync() that I don't fully understand.
>
> In BgBufferSync(), we have the following code:
>
>     while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
>     {
>         int            sync_state = SyncOneBuffer(next_to_clean, true,
>                                                wb_context);
>
>         if (++next_to_clean >= NBuffers)
>         {
>             next_to_clean = 0;
>             next_passes++;
>         }
>         num_to_scan--;
>
>         if (sync_state & BUF_WRITTEN)
>         {
>             reusable_buffers++;
>             if (++num_written >= bgwriter_lru_maxpages)
>             {
>                 PendingBgWriterStats.maxwritten_clean++;
>                 break;
>             }
>         }
>         else if (sync_state & BUF_REUSABLE)
>             reusable_buffers++;
>     }
>
>
> In SyncOneBuffer(), we lock the bufHdr and then check if both the
> refcount and usage_count are zero. If so, we mark the return value as
> BUF_REUSABLE.
> My understanding is that this means that the buffer could be reused
> when am empty shared buffer is needed by a backend. However, in the
> code above, we seem to track these in the reusable_buffers variable.
> But that variable is always incremented when the buffer was written in
> SyncOneBuffer() even though that buffer might have a non-zero refcount
> or non-zero usage_count.

I also think tha reusable_buffers keep track of the number of reusable
buffers.  BgBufferSync() calls SyncOneBuffer() with skip_recently_used
= true. In that case, if SyncOneBuffer() finds the buffer with
refcount or usage_count non-zero, it just unlocks the header and
returns. Hence when called from BgBufferSync(), SyncOneBuffer() would
write a buffer only when it is not used. Hence the result would be 0
or BUF_REUSABLE or BUF_REUSABLE | BUF_WRITTEN. It can never be just
BUF_WRITTEN.

I guess, a patch like the one attached will be more readable and clear.

I ran pgbench for 5 minutes with this patch applied and didn't see the
Assert failing. But I don't think that's a good enough test to cover
all scenarios.

--
Best Wishes,
Ashutosh Bapat