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

On Thu, Feb 1, 2024 at 4:34 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Thu, Feb 1, 2024 at 4:12 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Feb 1, 2024 at 3:44 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)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