Re: make async slave to wait for lsn to be replayed

On 23 January 2017 at 11:56, Ivan Kartyshov <i(dot)kartyshov(at)postgrespro(dot)ru> wrote:
> Thank you for reviews and suggested improvements.
> I rewrote patch to make it more stable.
>
> Changes
> =======
> I've made a few changes:
> 1) WAITLSN now doesn`t depend on snapshot
> 2) Check current replayed LSN rather than in xact_redo_commit
> 3) Add syntax WAITLSN_INFINITE '0/693FF800' - for infinite wait and
> WAITLSN_NO_WAIT '0/693FF800' for check if LSN was replayed as you
> advised.
> 4) Reduce the count of loops with GUCs (WalRcvForceReply() which in 9.5
> doesn`t exist).
> 5) Optimize loop that set latches.
> 6) Add two GUCs that helps us to configure influence on StartupXLOG:
> count_waitlsn (denominator to check not each LSN)
> interval_waitlsn (Interval in milliseconds to additional LSN check)
>
> Feedback
> ========
> On 09/15/2016 05:41 AM, Thomas Munro wrote:
>>
>> You hold a spinlock in one arbitrary slot, but that
>> doesn't seem sufficient: another backend may also read it, compute a
>> new value and then write it, while holding a different spin lock. Or
>> am I missing something?
>
>
> We acquire an individual spinlock on each member of array, so you cannot
> compute new value and write it concurrently.
>
> Tested
> ======
> We have been tested it on different servers and OS`s, in different cases and
> workloads. New version is nearly as fast as vanilla on primary and bring
> tiny influence on standby performance.
>
> Hardware:
> 144 Intel Cores with HT
> 3TB RAM
> all data on ramdisk
> primary + hotstandby on the same node.
>
> A dataset was created with "pgbench -i -s 1000" command. For each round of
> test we pause replay on standby, make 1000000 transaction on primary with
> pgbench, start replay on standby and measure replication gap disappearing
> time under different standby workload. The workload was "WAITLSN
> ('Very/FarLSN', 1000ms timeout)" followed by "select abalance from
> pgbench_accounts there aid = random_aid;"
> For vanilla 1000ms timeout was enforced on pgbench side by -R option.
> GUC waitlsn parameters was adopted for 1000ms timeout on standby with 35000
> tps rate on primary.
> interval_waitlsn = 500 (ms)
> count_waitlsn = 30000
>
> On 200 clients, slave caching up master as vanilla without significant
> delay.
> On 500 clients, slave caching up master 3% slower then vanilla.
> On 1000 clients, 12% slower.
> On 5000 clients, 3 time slower because it far above our hardware ability.
>
> How to use it
> ==========
> WAITLSN ‘LSN’ [, timeout in ms];
> WAITLSN_INFINITE ‘LSN’;
> WAITLSN_NO_WAIT ‘LSN’;
>
> #Wait until LSN 0/303EC60 will be replayed, or 10 second passed.
> WAITLSN ‘0/303EC60’, 10000;
>
> #Or same without timeout.
> WAITLSN ‘0/303EC60’;
> orfile:///home/vis/Downloads/waitlsn_10dev_v2.patch
> WAITLSN_INFINITE '0/693FF800';
>
> #To check if LSN is replayed can be used.
> WAITLSN_NO_WAIT '0/693FF800';
>
> Notice: WAITLSN will release on PostmasterDeath or Interruption events
> if they come earlier then target LSN or timeout.
>
>
> Thank you for reading, will be glad to get your feedback.

Could you please rebase your patch as it no longer applies cleanly.

Thanks

Thom