Can you please review the new patch of the extension with implemented
force variable.
On Tue, 11 Jul 2023 at 18:08, Palak Chaturvedi
<chaturvedipalak1911(at)gmail(dot)com> wrote:
>
> Hey Nitin,
> >Will
> >there be a scenario where the buffer is dirty and its reference count
> >is zero?
> There might be a buffer that has been dirtied but is not pinned or
> being used currently by a process. So checking the refcount and then
> dirty buffers helps.
> >First, The TryInvalidateBuffer() tries to flush the buffer if it is
> dirty and then tries to invalidate it if it meets the requirement.
> Instead of directly doing this can we provide an option to the caller
> to mention whether to invalidate the dirty buffers or not.
> Yes that can be implemented with a default value of force. Will
> implement it in the next patch.
>
> On Wed, 5 Jul 2023 at 17:53, Nitin Jadhav <nitinjadhavpostgres(at)gmail(dot)com> wrote:
> >
> > +1 for the idea. It's going to be more useful to test and understand
> > the buffer management of PostgreSQL and it can be used to explicitly
> > free up the buffers if there are any such requirements.
> >
> > I had a quick look over the patch. Following are the comments.
> >
> > First, The TryInvalidateBuffer() tries to flush the buffer if it is
> > dirty and then tries to invalidate it if it meets the requirement.
> > Instead of directly doing this can we provide an option to the caller
> > to mention whether to invalidate the dirty buffers or not. For
> > example, TryInvalidateBuffer(Buffer bufnum, bool force), if the force
> > is set to FALSE, then ignore invalidating dirty buffers. Otherwise,
> > flush the dirty buffer and try to invalidate.
> >
> > Second, In TryInvalidateBuffer(), it first checks if the reference
> > count is greater than zero and then checks for dirty buffers. Will
> > there be a scenario where the buffer is dirty and its reference count
> > is zero? Can you please provide more information on this or adjust the
> > code accordingly.
> >
> > > +/*
> > > +Try Invalidating a buffer using bufnum.
> > > +If the buffer is invalid, the function returns false.
> > > +The function checks for dirty buffer and flushes the dirty buffer before invalidating.
> > > +If the buffer is still dirty it returns false.
> > > +*/
> > > +bool
> >
> > The star(*) and space are missing here. Please refer to the style of
> > function comments and change accordingly.
> >
> > Thanks & Regards,
> > Nitin Jadhav
> >
> > On Fri, Jun 30, 2023 at 4:17 PM Palak Chaturvedi
> > <chaturvedipalak1911(at)gmail(dot)com> wrote:
> > >
> > > I hope this email finds you well. I am excited to share that I have
> > > extended the functionality of the `pg_buffercache` extension by
> > > implementing buffer invalidation capability, as requested by some
> > > PostgreSQL contributors for improved testing scenarios.
> > >
> > > This marks my first time submitting a patch to pgsql-hackers, and I am
> > > eager to receive your expert feedback on the changes made. Your
> > > insights are invaluable, and any review or comments you provide will
> > > be greatly appreciated.
> > >
> > > The primary objective of this enhancement is to enable explicit buffer
> > > invalidation within the `pg_buffercache` extension. By doing so, we
> > > can simulate scenarios where buffers are invalidated and observe the
> > > resulting behavior in PostgreSQL.
> > >
> > > As part of this patch, a new function or mechanism has been introduced
> > > to facilitate buffer invalidation. I would like to hear your thoughts
> > > on whether this approach provides a good user interface for this
> > > functionality. Additionally, I seek your evaluation of the buffer
> > > locking protocol employed in the extension to ensure its correctness
> > > and efficiency.
> > >
> > > Please note that I plan to add comprehensive documentation once the
> > > details of this enhancement are agreed upon. This documentation will
> > > serve as a valuable resource for users and contributors alike. I
> > > believe that your expertise will help uncover any potential issues and
> > > opportunities for further improvement.
> > >
> > > I have attached the patch file to this email for your convenience.
> > > Your valuable time and consideration in reviewing this extension are
> > > sincerely appreciated.
> > >
> > > Thank you for your continued support and guidance. I am looking
> > > forward to your feedback and collaboration in enhancing the PostgreSQL
> > > ecosystem.
> > >
> > > The working of the extension:
> > >
> > > 1. Creating the extension pg_buffercache and then call select query on
> > > a table and note the buffer to be cleared.
> > > pgbench=# create extension pg_buffercache;
> > > CREATE EXTENSION
> > > pgbench=# select count(*) from pgbench_accounts;
> > > count
> > > --------
> > > 100000
> > > (1 row)
> > >
> > > pgbench=# SELECT *
> > > FROM pg_buffercache
> > > WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
> > > bufferid | relfilenode | reltablespace | reldatabase | relforknumber
> > > | relblocknumber | isdirty | usagecount | pinning_backends
> > > ----------+-------------+---------------+-------------+---------------+----------------+---------+------------+------------------
> > > 233 | 16397 | 1663 | 16384 | 0
> > > | 0 | f | 1 | 0
> > > 234 | 16397 | 1663 | 16384 | 0
> > > | 1 | f | 1 | 0
> > > 235 | 16397 | 1663 | 16384 | 0
> > > | 2 | f | 1 | 0
> > > 236 | 16397 | 1663 | 16384 | 0
> > > | 3 | f | 1 | 0
> > > 237 | 16397 | 1663 | 16384 | 0
> > > | 4 | f | 1 | 0
> > >
> > >
> > > 2. Clearing a single buffer by entering the bufferid.
> > > pgbench=# SELECT count(*)
> > > FROM pg_buffercache
> > > WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
> > > count
> > > -------
> > > 1660
> > > (1 row)
> > >
> > > pgbench=# select pg_buffercache_invalidate(233);
> > > pg_buffercache_invalidate
> > > ---------------------------
> > > t
> > > (1 row)
> > >
> > > pgbench=# SELECT count(*)
> > > FROM pg_buffercache
> > > WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
> > > count
> > > -------
> > > 1659
> > > (1 row)
> > >
> > > 3. Clearing the entire buffer for a relation using the function.
> > > pgbench=# SELECT count(*)
> > > FROM pg_buffercache
> > > WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
> > > count
> > > -------
> > > 1659
> > > (1 row)
> > >
> > > pgbench=# select count(pg_buffercache_invalidate(bufferid)) from
> > > pg_buffercache where relfilenode =
> > > pg_relation_filenode('pgbench_accounts'::regclass);
> > > count
> > > -------
> > > 1659
> > > (1 row)
> > >
> > > pgbench=# SELECT count(*)
> > > FROM pg_buffercache
> > > WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
> > > count
> > > -------
> > > 0
> > > (1 row)
> > >
> > >
> > > Best regards,
> > > Palak