Re: Way to check whether a particular block is on the shared_buffer? - Mailing list pgsql-hackers

> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org
> [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Robert Haas
> Sent: Saturday, February 13, 2016 1:46 PM
> To: Kaigai Kouhei(海外 浩平)
> Cc: Jim Nasby; pgsql-hackers@postgresql.org; Amit Langote
> Subject: Re: [HACKERS] Way to check whether a particular block is on the
> shared_buffer?
> 
> On Thu, Feb 11, 2016 at 9:05 PM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> > Hmm. In my experience, it is often not a productive discussion whether
> > a feature is niche or commodity. So, let me change the viewpoint.
> >
> > We may utilize OS-level locking mechanism here.
> >
> > Even though it depends on filesystem implementation under the VFS,
> > we may use inode->i_mutex lock that shall be acquired during the buffer
> > copy from user to kernel, at least, on a few major filesystems; ext4,
> > xfs and btrfs in my research. As well, the modified NVMe SSD driver can
> > acquire the inode->i_mutex lock during P2P DMA transfer.
> >
> > Once we can consider the OS buffer is updated atomically by the lock,
> > we don't need to worry about corrupted pages, but still needs to pay
> > attention to the scenario when updated buffer page is moved to GPU.
> >
> > In this case, PD_ALL_VISIBLE may give us a hint. GPU side has no MVCC
> > infrastructure, so I intend to move all-visible pages only.
> > If someone updates the buffer concurrently, then write out the page
> > including invisible tuples, PD_ALL_VISIBLE flag shall be cleared because
> > updated tuples should not be visible to the transaction which issued
> > P2P DMA.
> >
> > Once GPU met a page with !PD_ALL_VISIBLE, it can return an error status
> > that indicates CPU to retry this page again. In this case, this page is
> > likely loaded to the shared buffer already, so retry penalty is not so
> > much.
> >
> > I'll try to investigate the implementation in this way.
> > Please correct me, if I misunderstand something (especially, treatment
> > of PD_ALL_VISIBLE).
> 
> I suppose there's no theoretical reason why the buffer couldn't go
> from all-visible to not-all-visible and back to all-visible again all
> during the time you are copying it.
>
The backend process that is copying the data to GPU has a transaction
in-progress (= not committed). Is it possible to get the updated buffer
page back to the all-visible state again?
I expect that in-progress transactions works as a blocker for backing
to all-visible. Right?

> Honestly, I think trying to access buffers without going through
> shared_buffers is likely to be very hard to make correct and probably
> a loser.
>
No challenge, no outcome. ;-)

> Copying the data into shared_buffers and then to the GPU is,
> doubtless, at least somewhat slower.  But I kind of doubt that it's
> enough slower to make up for all of the problems you're going to have
> with the approach you've chosen.
>
Honestly, I'm still uncertain whether it works well as I expects.
However, scan workload on the table larger than main memory is
headache for PG-Strom, so I'd like to try ideas we can implement.

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>