Re: Support for N synchronous standby servers - take 2

On Wed, Apr 6, 2016 at 3:29 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Wed, Apr 6, 2016 at 2:18 PM, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> At Tue, 5 Apr 2016 20:17:21 +0900, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote in <CAHGQGwE8_F79BUpC5TmJ7aazXU=Uju0VznFCCKDK57-wNpHV-g(at)mail(dot)gmail(dot)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 process id, or 0 */
slotno is used nowhere.

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