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

> -----Original Message-----
> From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
> Sent: Wednesday, February 10, 2016 1:58 AM
> To: Kaigai Kouhei(海外 浩平)
> Cc: Jim Nasby; pgsql-hackers(at)postgresql(dot)org; Amit Langote
> Subject: ##freemail## Re: [HACKERS] Way to check whether a particular block is
> on the shared_buffer?
>
> On Sun, Feb 7, 2016 at 9:49 PM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> > On the other hands, it also became clear we have to guarantee OS buffer
> > or storage block must not be updated partially during the P2P DMA.
> > My motivation is a potential utilization of P2P DMA of SSD-to-GPU to
> > filter out unnecessary rows and columns prior to loading to CPU/RAM.
> > It needs to ensure PostgreSQL does not write out buffers to OS buffers
> > to avoid unexpected data corruption.
> >
> > What I want to achieve is suspend of buffer write towards a particular
> > (relnode, forknum, blocknum) pair for a short time, by completion of
> > data processing by GPU (or other external devices).
> > In addition, it is preferable being workable regardless of the choice
> > of storage manager, even if we may have multiple options on top of the
> > pluggable smgr in the future.
>
> It seems like you just need to take an exclusive content lock on the
> buffer, or maybe a shared content lock would be sufficient.
>
Unfortunately, it was not sufficient.

Due to the assumption, the buffer page to be suspended does not exist
when a backend process issues a series P2P DMA command. (If block would
be already loaded to the shared buffer, it don't need to issue P2P DMA,
but just use usual memory<->device DMA because RAM is much faster than
SSD.)
It knows the pair of (rel,fork,block), but no BufferDesc of this block
exists. Thus, it cannot acquire locks in BufferDesc structure.

Even if the block does not exist at this point, concurrent process may
load the same page. BufferDesc of this page shall be assigned at this
point, however, here is no chance to lock something in BufferDesc for
the process which issues P2P DMA command.

It is the reason why I assume the suspend/resume mechanism shall take
a pair of (rel,fork,block) as identifier of the target block.

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>