Re: Way to check whether a particular block is on the shared_buffer?

On 1/31/16 7:38 PM, Kouhei Kaigai wrote:
> I'm under investigation of SSD-to-GPU direct feature on top of
> the custom-scan interface. It intends to load a bunch of data
> blocks on NVMe-SSD to GPU RAM using P2P DMA, prior to the data
> loading onto CPU/RAM, to preprocess the data to be filtered out.
> It only makes sense if the target blocks are not loaded to the
> CPU/RAM yet, because SSD device is essentially slower than RAM.
> So, I like to have a reliable way to check the latest status of
> the shared buffer, to kwon whether a particular block is already
> loaded or not.

That completely ignores the OS cache though... wouldn't that be a major
issue?

To answer your direct question, I'm no expert, but I haven't seen any
functions that do exactly what you want. You'd have to pull relevant
bits from ReadBuffer_*. Or maybe a better method would just be to call
BufTableLookup() without any locks and if you get a result > -1 just
call the relevant ReadBuffer function. Sometimes you'll end up calling
ReadBuffer even though the buffer isn't in shared buffers, but I would
think that would be a rare occurrence.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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