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

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:
+   for (i = 0; i < max_wal_senders; i++)
+   {
+       walsnd = &WalSndCtl->walsnds[i];
No volatile pointer to prevent code reordering?
 */typedef struct WalSnd{
+   int     slotno;         /* index of this slot in WalSnd array */   pid_t       pid;            /* this walsender's
processid, or 0 */
 
slotno is used nowhere.

I'll grab the tests and look at them.
-- 
Michael