Re: WIP: WAL prefetch (another approach)

On 2/12/21 5:46 AM, Andres Freund wrote:
> Hi,
>
> On 2021-02-12 00:42:04 +0100, Tomas Vondra wrote:
>> Yeah, that's a good point. I think it'd make sense to keep track of recent
>> FPIs and skip prefetching such blocks. But how exactly should we implement
>> that, how many blocks do we need to track? If you get an FPI, how long
>> should we skip prefetching of that block?
>>
>> I don't think the history needs to be very long, for two reasons. Firstly,
>> the usual pattern is that we have FPI + several changes for that block
>> shortly after it. Secondly, maintenance_io_concurrency limits this naturally
>> - after crossing that, redo should place the FPI into shared buffers,
>> allowing us to skip the prefetch.
>>
>> So I think using maintenance_io_concurrency is sufficient. We might track
>> more buffers to allow skipping prefetches of blocks that were evicted from
>> shared buffers, but that seems like an overkill.
>>
>> However, maintenance_io_concurrency can be quite high, so just a simple
>> queue is not very suitable - searching it linearly for each block would be
>> too expensive. But I think we can use a simple hash table, tracking
>> (relfilenode, block, LSN), over-sized to minimize collisions.
>>
>> Imagine it's a simple array with (2 * maintenance_io_concurrency) elements,
>> and whenever we prefetch a block or find an FPI, we simply add the block to
>> the array as determined by hash(relfilenode, block)
>>
>> hashtable[hash(...)] = {relfilenode, block, LSN}
>>
>> and then when deciding whether to prefetch a block, we look at that one
>> position. If the (relfilenode, block) match, we check the LSN and skip the
>> prefetch if it's sufficiently recent. Otherwise we prefetch.
>
> I'm a bit doubtful this is really needed at this point. Yes, the
> prefetching will do a buffer table lookup - but it's a lookup that
> already happens today. And the patch already avoids doing a second
> lookup after prefetching (by optimistically caching the last Buffer id,
> and re-checking).
>
> I think there's potential for some significant optimization going
> forward, but I think it's basically optimization over what we're doing
> today. As this is already a nontrivial patch, I'd argue for doing so
> separately.
>

I agree with treating this as an improvement - it's not something that
needs to be solved in the first verson. OTOH I think Stephen has a point
that just skipping FPIs like we do now has limited effect, because the
WAL usually contains additional changes to the same block.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company