Re: index prefetching - Mailing list pgsql-hackers

On 8/13/25 23:37, Andres Freund wrote:
> Hi,
> 
> On 2025-08-13 23:07:07 +0200, Tomas Vondra wrote:
>> On 8/13/25 16:44, Andres Freund wrote:
>>> On 2025-08-13 14:15:37 +0200, Tomas Vondra wrote:
>>>> In fact, I believe this is about io_method. I initially didn't see the
>>>> difference you described, and then I realized I set io_method=sync to
>>>> make it easier to track the block access. And if I change io_method to
>>>> worker, I get different stats, that also change between runs.
>>>>
>>>> With "sync" I always get this (after a restart):
>>>>
>>>>    Buffers: shared hit=7435 read=52801
>>>>
>>>> while with "worker" I get this:
>>>>
>>>>    Buffers: shared hit=4879 read=52801
>>>>    Buffers: shared hit=5151 read=52801
>>>>    Buffers: shared hit=4978 read=52801
>>>>
>>>> So not only it changes run to tun, it also does not add up to 60236.
>>>
>>> This is reproducible on master? If so, how?
>>>
>>>
>>>> I vaguely recall I ran into this some time ago during AIO benchmarking,
>>>> and IIRC it's due to how StartReadBuffersImpl() may behave differently
>>>> depending on I/O started earlier. It only calls PinBufferForBlock() in
>>>> some cases, and PinBufferForBlock() is what updates the hits.
>>>
>>> Hm, I don't immediately see an issue there. The only case we don't call
>>> PinBufferForBlock() is if we already have pinned the relevant buffer in a
>>> prior call to StartReadBuffersImpl().
>>>
>>>
>>> If this happens only with the prefetching patch applied, is is possible that
>>> what happens here is that we occasionally re-request buffers that already in
>>> the process of being read in? That would only happen with a read stream and
>>> io_method != sync (since with sync we won't read ahead). If we have to start
>>> reading in a buffer that's already undergoing IO we wait for the IO to
>>> complete and count that access as a hit:
>>>
>>>     /*
>>>      * Check if we can start IO on the first to-be-read buffer.
>>>      *
>>>      * If an I/O is already in progress in another backend, we want to wait
>>>      * for the outcome: either done, or something went wrong and we will
>>>      * retry.
>>>      */
>>>     if (!ReadBuffersCanStartIO(buffers[nblocks_done], false))
>>>     {
>>> ...
>>>         /*
>>>          * Report and track this as a 'hit' for this backend, even though it
>>>          * must have started out as a miss in PinBufferForBlock(). The other
>>>          * backend will track this as a 'read'.
>>>          */
>>> ...
>>>         if (persistence == RELPERSISTENCE_TEMP)
>>>             pgBufferUsage.local_blks_hit += 1;
>>>         else
>>>             pgBufferUsage.shared_blks_hit += 1;
>>> ...
>>>
>>>
>>
>> I think it has to be this. It only happens with io_method != sync, and
>> only with effective_io_concurrency > 1. At first I was wondering why I
>> can't reproduce this for seqscan/bitmapscan, but then I realized those
>> plans never visit the same block repeatedly - indexscans do that. It's
>> also not surprising it's timing-sensitive, as it likely depends on how
>> fast the worker happens to start/complete requests.
>>
>> What would be a good way to "prove" it really is this?
> 
> I'd just comment out those stats increments and then check if the stats are
> stable afterwards.
> 

I tried that, but it's not enough - the buffer hits gets lower, but
remains variable. It stabilizes only if I comment out the increment in
PinBufferForBlock() too. At which point it gets to 0, of course ...

regards

-- 
Tomas Vondra