| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, sdn(at)amazon(dot)com, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
| Subject: | Re: Refactoring the checkpointer's fsync request queue |
| Date: | 2018-11-16 15:05:44 |
| Message-ID: | CA+TgmoZRNHDKSnG35mPufKKgVXSPAYrQo5nHhD_ZzBzg50cTAQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Nov 14, 2018 at 4:49 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2018-11-14 16:36:49 -0500, Robert Haas wrote:
> > But how do you make reading that counter atomic with the open() itself?
>
> I don't see why it has to be. As long as the "fd generation" assignment
> happens before fsync (and writes secondarily), there ought not to be any
> further need for synchronizity?
If the goal is to have the FD that is opened first end up in the
checkpointer's table, grabbing a counter backwards does not achieve
it, because there's a race.
S1: open FD
S2: open FD
S2: local_counter = shared_counter++
S1: local_counter = shared_counter++
Now S1 was opened first but has a higher shared counter value than S2
which was opened later. Does that matter? Beats me! I just work
here...
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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