Re: Support for N synchronous standby servers - take 2 - Mailing list pgsql-hackers

On Wed, Apr 6, 2016 at 4:08 PM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Wed, Apr 6, 2016 at 3:29 PM, Fujii Masao <masao.fujii@gmail.com> wrote:
>> On Wed, Apr 6, 2016 at 2:18 PM, Kyotaro HORIGUCHI
>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>>> At Tue, 5 Apr 2016 20:17:21 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in
<CAHGQGwE8_F79BUpC5TmJ7aazXU=Uju0VznFCCKDK57-wNpHV-g@mail.gmail.com>
>>>> >> list_member_int() performs the loop internally. So I'm not sure how much
>>>> >> adding extra list_member_int() here can optimize this processing.
>>>> >> Another idea is to make SyncRepGetSyncStandby() check whether I'm sync
>>>> >> standby or not. In this idea, without adding extra loop, we can exit earilier
>>>> >> in the case where I'm not a sync standby. Does this make sense?
>>>> >
>>>> > The list_member_int() is also performed in the "(snip)" part. So
>>>> > SyncRepGetSyncStandbys() returning am_sync seems making sense.
>>>> >
>>>> > sync_standbys = SyncRepGetSyncStandbys(am_sync);
>>>> >
>>>> > /*
>>>> >  *  Quick exit if I am not synchronous or there's not
>>>> >  *  enough synchronous standbys
>>>> >  * /
>>>> > if (!*am_sync || list_length(sync_standbys) < SyncRepConfig->num_sync)
>>>> > {
>>>> >   list_free(sync_standbys);
>>>> >   return false;
>>>> > }
>>>>
>>>> Thanks for the comment! I changed SyncRepGetSyncStandbys() so that
>>>> it checks whether we're managing a sync standby or not.
>>>> Attached is the updated version of the patch. I also applied several
>>>> review comments to the patch.
>>>
>>> It still does list_member_int but it can be gotten rid of as the
>>> attached patch.
>>
>> Thanks for the review!
>>
>>>
>>> regards,
>>>
>>> diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
>>> index 9b2137a..6998bb8 100644
>>> --- a/src/backend/replication/syncrep.c
>>> +++ b/src/backend/replication/syncrep.c
>>> @@ -590,6 +590,10 @@ SyncRepGetSyncStandbys(bool *am_sync)
>>>                 if (XLogRecPtrIsInvalid(walsnd->flush))
>>>                         continue;
>>>
>>> +               /* Notify myself as 'synchonized' if I am */
>>> +               if (am_sync != NULL && walsnd == MyWalSnd)
>>> +                       *am_sync = true;
>>> +
>>>                 /*
>>>                  * If the priority is equal to 1, consider this standby as sync
>>>                  * and append it to the result. Otherwise append this standby
>>> @@ -598,8 +602,6 @@ SyncRepGetSyncStandbys(bool *am_sync)
>>>                 if (this_priority == 1)
>>>                 {
>>>                         result = lappend_int(result, i);
>>> -                       if (am_sync != NULL && walsnd == MyWalSnd)
>>> -                               *am_sync = true;
>>>                         if (list_length(result) == SyncRepConfig->num_sync)
>>>                         {
>>>                                 list_free(pending);
>>> @@ -630,9 +632,6 @@ SyncRepGetSyncStandbys(bool *am_sync)
>>>         {
>>>                 bool            needfree = (result != NIL && pending != NIL);
>>>
>>> -               if (am_sync != NULL && !(*am_sync))
>>> -                       *am_sync = list_member_int(pending, MyWalSnd->slotno);
>>> -
>>>                 result = list_concat(result, pending);
>>>                 if (needfree)
>>>                         pfree(pending);
>>> @@ -640,6 +639,13 @@ SyncRepGetSyncStandbys(bool *am_sync)
>>>         }
>>>
>>>         /*
>>> +        * The pending list contains eventually potentially-synchronized standbys
>>> +        * and this walsender may be one of them. So once reset am_sync.
>>> +        */
>>> +       if (am_sync != NULL)
>>> +               *am_sync = false;
>>> +
>>> +       /*
>>
>> This code seems wrong in the case where this walsender is in the result list.
>> So I adopted another logic. Attached is the updated version of the patch.
>
> To be honest, this is a nice patch that we have here, and it received
> a fair amount of work. I have been playing with it a bit but I could
> not break it.
>
> Here are few things I have noticed:

Thanks for the review!

> +   for (i = 0; i < max_wal_senders; i++)
> +   {
> +       walsnd = &WalSndCtl->walsnds[i];
> No volatile pointer to prevent code reordering?

Yes. Since spin lock is not taken there, volatile is necessary.

>   */
>  typedef struct WalSnd
>  {
> +   int     slotno;         /* index of this slot in WalSnd array */
>     pid_t       pid;            /* this walsender's process id, or 0 */
> slotno is used nowhere.

Yep. Attached is the updated version of the patch.

> I'll grab the tests and look at them.

Many thanks!

Regards,

--
Fujii Masao