Re: SLRU optimization - configurable buffer pool and partitioning the SLRU lock

On Thu, Feb 1, 2024 at 4:34 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, Feb 1, 2024 at 4:12 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > On Thu, Feb 1, 2024 at 3:44 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> > Okay.
> > >
> > > While I have your attention -- if you could give a look to the 0001
> > > patch I posted, I would appreciate it.
> > >
> >
> > I will look into it.  Thanks.
>
> Some quick observations,
>
> Do we need below two write barriers at the end of the function?
> because the next instruction is separated by the function boundary
>
> @@ -766,14 +766,11 @@ StartupCLOG(void)
>   ..
> - XactCtl->shared->latest_page_number = pageno;
> -
> - LWLockRelease(XactSLRULock);
> + pg_atomic_init_u64(&XactCtl->shared->latest_page_number, pageno);
> + pg_write_barrier();
>  }
>
> /*
>   * Initialize member's idea of the latest page number.
>   */
>   pageno = MXOffsetToMemberPage(offset);
> - MultiXactMemberCtl->shared->latest_page_number = pageno;
> + pg_atomic_init_u64(&MultiXactMemberCtl->shared->latest_page_number,
> +    pageno);
> +
> + pg_write_barrier();
>  }
>

I have checked the patch and it looks fine to me other than the above
question related to memory barrier usage one more question about the
same, basically below to instances 1 and 2 look similar but in 1 you
are not using the memory write_barrier whereas in 2 you are using the
write_barrier, why is it so?  I mean why the reordering can not happen
in 1 and it may happen in 2?

1.
+ pg_atomic_write_u64(&CommitTsCtl->shared->latest_page_number,
+ trunc->pageno);

  SimpleLruTruncate(CommitTsCtl, trunc->pageno);

vs
2.

  - shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&shared->latest_page_number, pageno);
+ pg_write_barrier();

  /* update the stats counter of zeroed pages */
  pgstat_count_slru_page_zeroed(shared->slru_stats_idx);



--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com